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Improving the Collection, Management, and Use of Marine Fisheries Data (2000)

Chapter: Summer Flounder: Review and Insights

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Suggested Citation:"Summer Flounder: Review and Insights." National Research Council. 2000. Improving the Collection, Management, and Use of Marine Fisheries Data. Washington, DC: The National Academies Press. doi: 10.17226/9969.
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2

Summer Flounder: Review and Insights

INTRODUCTION

The committee examined the summer flounder assessments with two objectives in mind. The first was to address specific issues raised by Congress and stakeholders regarding the quality and nature of these assessments and to gain a direct understanding of the significance these issues have for managing summer flounder. The second objective was to provide examples to illustrate the broader issues associated with the collection and use of fisheries data that affect the science and management of other stocks.

To focus these tasks, the committee elicited comments from representatives of the commercial and recreational fishing communities and environmental advocates concerning data collection and assessment methods applied to summer flounder. The committee reviewed the summer flounder data and the NMFS assessments and conducted its own independent assessments (Appendix C and Appendix D) and an examination of modeling assumptions (this chapter) to generate its findings.

The committee provided opportunities at its two Washington, D.C. meetings for public input on summer flounder issues. Although more general issues were raised at the committee's third meeting in Seattle, Washington, some of the concerns raised there also had relevance to summer flounder. Commercial and recreational fishermen, scientists from the National Marine Fisheries Service (NMFS), representatives from the Mid-Atlantic Fishery Management Council, representatives from the Atlantic, Gulf, and Pacific States Marine Fisheries Commissions, and representatives from environmental groups made presentations to the committee at these meetings.

Commercial fishermen expressed a variety of views that indicated a general lack of trust in the management of summer flounder and other marine fisheries. Many recreational fishermen and their representatives were invited to the committee's meetings. Unfortunately, the committee was unable to elicit significant input from this sector regarding the summer flounder assessments or other data issues. Although the issues raised by stakeholders are not all resolved in this report, the report does explore a number of the more significant and persistent issues in the hope that NMFS and the regional councils will take steps to fix problems and correct misperceptions.

The committee's public sessions revealed

Suggested Citation:"Summer Flounder: Review and Insights." National Research Council. 2000. Improving the Collection, Management, and Use of Marine Fisheries Data. Washington, DC: The National Academies Press. doi: 10.17226/9969.
×

many perceptions of some commercial fishermen that affect their beliefs about fisheries data and stock assessments.

  • Many fishermen believe that fisheries science and fisheries assessments are not objective scientific exercises; instead, they reflect political agendas.

  • There are several steps between a stock assessment and the final management advice. Many fishermen believe that precaution is built into each step of the assessment process, so that the final results are too conservative. Fishermen advocate that scientists should provide completely objective advice and that managers should build in precaution only at the final management stage.

  • Regulations change too frequently and before it has been determined whether management objectives have been achieved. This is especially problematic in cases that result in changes in gear regulations, because fishermen need to undertake the expense of replacing or modifying their gear with each change. The annual changes in the mesh size requirements for summer flounder were noted as an example.

  • The fishing gear and survey techniques used by NMFS in its fishery-independent surveys have remained relatively constant over time. Fishermen no longer use the same kind of gear and consequently believe that NMFS ' use of outdated fishing gear and fishing practices for surveys result in stock assessments that estimate smaller populations than the industry believes are realistic.

  • Many fishermen question a number of assumptions about the biology and population dynamics used in specific stock assessments (e.g., in the case of summer flounder, this includes the assumed level of natural mortality and the assumption of constant selectivity for the older age groups).

Whether or not these concerns are valid from a scientific perspective, management of marine fisheries will be more difficult if these perceptions of fishermen are not addressed.

Because the National Research Council (NRC) was requested by Congress to focus on summer flounder, the committee examined the 1996 and 1999 summer flounder assessments and makes a number of recommendations in this chapter for improving assessments. It should be recognized, however, that NMFS cannot afford to conduct similar investigations for all the species it manages. Therefore, Chapter 4 highlights only those recommendations that should be applied broadly in fisheries data collection, management, and use in the United States. It is the responsibility of NMFS and the Mid-Atlantic and New England Fishery Management Councils to determine whether the steps recommended for summer flounder are a priority given the value of summer flounder compared to other fisheries. The lack of certainty in assumptions described below may hinder the summer flounder stock from recovering to its 1980s peak, although environmental factors may also be important deterrents to full stock recovery.

SUMMER FLOUNDER ASSESSMENT ISSUES

Here we examine the facts about summer flounder, determine the merits of the concerns, and recommend to Congress, NMFS, and the councils ways to correct problems and misperceptions. The committee investigated questions related to (1) the biology and population dynamics of summer flounder; (2) summer flounder sampling; and (3) the information content of the model and model assumptions currently in use (Box 2-1). In some cases, data were not available for the committee to investigate the issues.

Some of the concerns about the scientific assessment and management of summer flounder are specific and are dealt with in the following sections. Others result from the fishing industry's lack of trust in and an alienation from the management process. Such alienation suggests that there is a wider problem of cooperation and outreach, which will be addressed in Chapter 4.

Suggested Citation:"Summer Flounder: Review and Insights." National Research Council. 2000. Improving the Collection, Management, and Use of Marine Fisheries Data. Washington, DC: The National Academies Press. doi: 10.17226/9969.
×

BOX 2-1

Concerns About Summer Flounder Assessments Investigated by the Committee

  1. Questions related to the biology and population dynamics of summer flounder

  1. Do the summer flounder in waters north of Cape Hatteras comprise a unit stock of fish?

  2. What natural mortality rate is appropriate to use in summer flounder assessment models?

  3. Are there differences between the growth and mortality of male and female summer flounder (sexual dimorphism) and, if so, how do the differences affect the assessments?

  1. Questions related to summer flounder sampling

  1. What are the appropriate survey and commercial catchabilities for summer flounder?

  2. Do problems with determining the age of summer flounder discredit age-based assessments?

  3. Are effort data used appropriately and are the effects of effort changes incorporated properly?

  4. Is the observer program for summer flounder adequate?

  5. Can and should state surveys be standardized?

  6. Is the catch from recreational fishing estimated properly?

  7. Can precision of data be increased?

  1. Questions related to the information content of the model and model assumptions currently in use

What information does each model structure require and how do these requirements relate to the information in the data? Additional things to consider when answering this question are (1) the weight the model gives to the different sources of information that come into it, (2) how uncertainty is expressed in the assessment output provided to managers, and (3) the level of precision managers can expect with the data currently available.

QUESTIONS RELATED TO THE BIOLOGY AND POPULATION DYNAMICS OF SUMMER FLOUNDER

Summer flounder (Paralichthys dentatus; also called fluke) are found primarily from Cape Fear, North Carolina, to Cape Cod, Massachusetts (see Figure C-1) on a variety of seafloor substrates, including mud and sand (Able and Kaiser, 1994; Packer and Hoff, 1999). Summer flounder can reach a length of about 30 inches and a weight of some 20 pounds, and the maximum age is thought to be approximately 20 years. In a pattern typical for adults of many species of estuary-dependent fish, adult summer flounder move inshore in the spring and summer to feed in the bays and sounds and migrate offshore to spawn in fall and overwinter at the shelf break (Able and Fahay, 1998). Summer flounder appear to migrate northeast along the continental shelf as they grow larger. Considered lie-in-wait predators, they eat a large variety of prey, usually smaller fish, crabs, and shrimp.

Commercial catches of summer flounder along the Atlantic Coast have fluctuated between 2 and 18 million kg over the past 50 years. Recreational catch has been tracked only since 1981 and during this period, catch has fluctuated between

Suggested Citation:"Summer Flounder: Review and Insights." National Research Council. 2000. Improving the Collection, Management, and Use of Marine Fisheries Data. Washington, DC: The National Academies Press. doi: 10.17226/9969.
×

FIGURE 2-1 Commercial catch since 1950 and recreational catch since 1981 (earlier recreational data are unavailable) from Maine to North Carolina.

1.4 and 12.7 million kg annually. Amendment 2 to the Fishery Management Plan for summer flounder, approved in 1992, allocated 60 percent of the total allowable catch (TAC) to commercial fishermen and 40 percent to recreational fishermen. However, recreational catch has equaled commercial catches in several years and exceeded commercial catch in 1997 and 1998 (Figure 2-1), 69% over the recreational TAC. The proportion of fish in the population that are three or more years old has increased from 4 percent in 1993 to 43 percent in 1998 (Terceiro, 1999).

Do the summer flounder found in waters north of Cape Hatteras comprise a unit stock of fish?

National Standard 3 of the Magnuson-Stevens Act (Sec 30 [a][3]) states “to the extent practicable, an individual stock of fish shall be managed as a unit throughout its range, and interrelated stocks of fish shall be managed as a unit or in close coordination.” For ideal management, a species' distribution would be split into a set of discrete stocks, each of which spawns separately, has similar growth and mortality characteristics, and is fished by a different set of fishermen. This ideal is rarely achieved and the reality is that stock management areas have to be large, often comprising several separate, but usually overlapping, breeding populations and nursery areas. There are obvious administrative advantages of this approach and scientifically there may be advantages to combining sub-stocks rather than having to estimate migration rates among them. However, problems may result from combining a number of sub-stocks, potentially spanning a large gradient of latitude. The most important problem is that a combined stock may appear to be in reasonable shape even though some component is being fished to extinction.

Combining sub-stocks may also hinder stock assessments if growth, recruitment, natural mortality, or fishing mortality differ substantially among the sub-stocks of a combined stock. This may result in difficulties of estimating average weights at age or mortality rates. Thus, the adequacy of stock definition should be questioned in every assessment (see Appendix D of NRC, 1998a). In the case of summer flounder, Able and Kaiser (1994) published an extensive review of the species' life history. Three possible interpretations of the number and range of summer flounder stocks were presented:

Suggested Citation:"Summer Flounder: Review and Insights." National Research Council. 2000. Improving the Collection, Management, and Use of Marine Fisheries Data. Washington, DC: The National Academies Press. doi: 10.17226/9969.
×
  1. Two major stocks: Middle Atlantic Bight and South Atlantic Bight

  2. Two major stocks, both in the Middle Atlantic Bight

  3. Three stocks: Middle Atlantic Bight, South Atlantic Bight, and trans-Cape Hatteras

The committee was not able to determine which of these interpretations is correct, but provides comments that may be useful for multi-stock management in this context.

Fogarty et al. (1983) used morphological characteristics and discriminant analysis to explore summer flounder stock structure. They found significant morphological differences among New York-New Jersey, Mid-Atlantic, and Cape Hatteras stocks, but were uncertain whether these differences had any biological significance. Although Jones and Quattro (1999) identified some genetic differences between summer flounder from Massachusetts and Rhode Island, it is unlikely that these are different stocks of fish that should be managed separately. Jones and Quattro pointed out that stock identification should be based not only on genetic data but should also consider life history attributes and morphological characteristics.

The existence of genetic, morphological, or other differences does not automatically mean that different populations should be treated as separate stocks for management purposes because (1) morphological or genetic differences may not translate into significant ecological differences and may simply reflect short-term or small-scale fluctuations in population structure and ultimate survival of a species; (2) the differences may not be related to factors that are important in stock management; and (3) the value of the fishery may not justify the cost of treating the stocks separately. Conversely, it is possible for populations to be genetically and morphologically identical throughout a vast geographic range and yet it may be preferable to manage them as separate stocks because of differences in growth, reproduction, and mortality. Such differences can be driven by differences in water temperature, productivity of phytoplankton and zooplankton, abundance of natural predators, fishing pressure, and other factors.

What are the implications for stock assessments and management if the unit stock assumption is wrong? If separate stocks are fished differently, the fishing mortality rate estimated for the combined stock will not be applicable to the separate stocks. Very intensive fishing of a small stock may have little impact on the overall fishing mortality. For example, the Arcto-Norwegian cod population has two components, one that feeds in the Barents Sea and one in the Bear Island-Svalbard area. Both components apparently spawn together on the Norwegian coast, mostly at Lofoten (it is not known whether they interbreed). Typically, the Barents Sea component is twice the size of the Svalbard component. Hence, if the former were not fished and the latter fished to extinction, the combined fishing mortality rate might be 0.3, which might seem reasonable, but the Svalbard stock component would have been eliminated. This is more or less how the North Sea herring collapsed, as separate units of a stock over its entire range were successively fished out. This scenario is more likely with schooling fish because fishermen can maintain high catch rates on declining, contracting populations rather than move to more abundant stock components. A similar situation exists with the various species of salmon on the west coast of North America. Species that are listed as endangered in California, Oregon, and Washington are abundant in Alaska. Thus, if the mortality of all salmon stocks is calculated together, the resulting number is not alarming. However, the number also does not reflect the true situation of the health of the salmon stocks. Northern cod provide another example (Rose et al., 2000).

If, on the other hand, a unit stock splits into two separately assessed stocks without accounting for movement between them, differential migration between the components will distort estimates of fishing mortality and population size. For example, some cod spawned at the southwest corner of Iceland drift as larvae to Greenland, where there

Suggested Citation:"Summer Flounder: Review and Insights." National Research Council. 2000. Improving the Collection, Management, and Use of Marine Fisheries Data. Washington, DC: The National Academies Press. doi: 10.17226/9969.
×

is a separate fishery. They return as spawning adults to Iceland, where the extra numbers accruing to the Icelandic stock distort the fishing mortality rates downward if this factor is not taken into account (Shepherd and Pope, 1993).

Committee Investigations

The committee could not determine whether the summer flounder population should be considered multiple stocks. NMFS told the committee that “no meristic or morphometric data have been or are being regularly collected that would enable separation of the historical landings in potential stock components.” However, any fishery that spans a large range might include multiple stocks.

Actions Needed

Testing the validity of the unit stock assumption requires an analysis of growth rates, fecundity, mortality, and other population factors among sub-areas. NMFS should conduct separate assessments of the stock if there are indications of different population factors (e.g., a difference in size-at-age between the northern and southern ends of the range), using whatever data are available specific to sub-areas of the range. Some relevant data may be available from the states, but a lack of standardization of their methods and differences in the timing and scope of their surveys may make such data inappropriate for reliable comparisons. The decision about implementing a split stock assessment depends on the benefit of doing so and the ability of the councils to use advice based on such assessments.

What natural mortality rate is appropriate to use in summer flounder assessment models?

Natural mortality rate (M) is the rate at which fish are removed from the population for reasons other than fishing activity, for example, predation, disease, and permanent emigration. In most cases, M is assumed to be the difference between total mortality and fishing mortality. For many temperate fish stocks, including summer flounder, stock assessment scientists assume that M is constant over time and at different ages of fish after they are large enough to be caught in the fishery and unaffected by fish population density.1 In the case of summer flounder, M is set at a level of 0.2 per year on the fished ages of the stock; this level is commonly, though perhaps too pervasively, used for many fisheries worldwide. The meaning of this M value is that if a year-class of the population were not fished, its numbers would decline exponentially so that year-class number N(t) is given by the formula N(t) = N(t0)e(−0.2*(t-t0)), where t is the current age in years and t0 the initial age. For example, this formula predicts that after one year has elapsed, 18 percent of the year class would have died, after 5 years, 63 percent would have died, and that after 10 years, 86 percent would have died of causes other than fishing. For fished populations, fishing mortality rate (F) and natural mortality rate (M) are generally viewed as additive. Both F and M have the units of year−1, but can also be expressed as instantaneous rates. In practice, fishing mortality is often the dominant cause of death at ages after fish have become large enough to be caught in the fishery.

Errors in the assumption about the level of natural mortality lead to some well-understood effects on the resulting assessment. For example, errors in the estimate of M lead to approximately equivalent and opposite errors in the estimates of F. Thus, if M is underestimated by 0.1, F will be overestimated by approximately 0.1, and F targets for future catches will be lower than they should be and harder to achieve. Thus, an incorrect choice of the natural mortality rate moves the estimates of current and target fishing levels in a direction opposite to the error in M. A lower natural mortality rate causes the estimated yield-

1  

Mortality of young-of-the-year fish, prior to their recruitment, is not included in the M used for management.

Suggested Citation:"Summer Flounder: Review and Insights." National Research Council. 2000. Improving the Collection, Management, and Use of Marine Fisheries Data. Washington, DC: The National Academies Press. doi: 10.17226/9969.
×

FIGURE 2-2 Summer flounder yield per recruit for three levels of natural mortality. F and M have the units of year−1.

per-recruit curve2 to steepen and peak at a lower level of fishing mortality rate and a higher M causes the curve to flatten and peak at higher levels of fishing mortality rate (Figure 2-2).

An error in M may cause an error in the opposite direction for calculated biological reference points such as Fmax and F0.1 (see Box 1-2 for definitions), which are often targets for management. However, under certain circumstances, an error in M may cause an error in the same direction for biological reference points, such as Fmed (Sissenwine and Shepherd, 1987; Jakobsen, 1992). Thus, Fmed may lead to management advice that moves F in the correct direction even if natural mortality is estimated incorrectly. Because it is managed to an Fmax limit reference point, summer flounder management is affected by the choice of M.

The preceding discussion assumes that natural mortality rate is constant with fish age and through time. If M changes with age, the estimate of F also is affected differentially by age. If M changes through time for fish of specific size, perhaps as a result of changes in the abundance of or susceptibility to predators, the effects may be variable and subtle. One effect of a period of unaccounted-for increased natural mortality is to underestimate the number of young fish expected to have entered the fishery. This is particularly the case if natural mortality increases on fish of post-recruit ages. Such effects are sometimes investigated using multispecies assessment models by accounting for predation mortality. Research on fisheries in other areas of the world suggests that predation mortality may be quite high and variable on the younger and smaller sizes of fish (Sogard, 1997). However, predation mortality estimates for North Sea flounder species are generally low (ICES, 1997).

Multispecies models, in theory, allow estimation of natural mortality for prey species directly because they quantify M by observing predation deaths through stomach content data from predators. Multispecies models of the Georges Bank ecosystem have been constructed in the past, but have not been used to estimate predation

2  

Such curves describe the expected yield from an individual fish (a single recruit) over its lifetime.

Suggested Citation:"Summer Flounder: Review and Insights." National Research Council. 2000. Improving the Collection, Management, and Use of Marine Fisheries Data. Washington, DC: The National Academies Press. doi: 10.17226/9969.
×

mortality of summer flounder.3 Although a number of methods exist for estimating natural mortality rate indirectly, few are reliable at this time because estimates of M are confounded with estimates of the proportion of fish vulnerable to fishing at each age.

Tagging experiments in which the initial mortality and tag loss rates on fish of catchable ages are well known and low provide another, more direct means of estimating M. The combination of knowing how many fish were released and the full time series of annual recaptures allows estimation of natural mortality rate (Hamre, 1980). Tagging data can also be used to estimate the dependence of M on fish age and size (Hampton, 1991). In fisheries characterized by a substantial mixed use (e.g., recreational and commercial harvest of summer flounder), each sector is likely to have a different reporting rate (typically recreational anglers are more likely to return tags) and each must be estimated properly. When tagging studies are combined with port sampling and intercept surveys wherein tags are obtained by the surveyor, both natural and fishing mortality can be estimated even in mixed-use fisheries (Brooks et al., 1998).

Committee Investigations

Because natural mortality rate is one of the more uncertain parameters used in a stock assessment, the committee investigated the effect of using a standard Laurec-Shepherd tuning of the virtual population analysis (VPA) of 1996 data (which gives results similar to the 25 th Stock Assessment Workshop assessment for the same data [NEFSC, 1997]). The biological reference points and future TACs and spawning stock biomass (SSB) estimates were then calculated on the assumption that the 1997 catch was 7,162 metric tons. Calculations were also made under the assumption that fishing mortality remained at the status quo level (the 1996 level) in 1997. These factors were then recalculated assuming a natural mortality rate of 0.1 (instead of M = 0.2) at all steps of the calculation (Table 2-1).

Points to note from this analysis are that Fmax, F0.1, and F20% are lower for M = 0.1 than for M = 0.2. However, Fmed and the estimate of the current level of fishing mortality F1996 are estimated to be higher with M = 0.1 than M = 0.2. These specific results for summer flounder show how the various biological reference points change in response to a change in M, in line with the general theoretical expectations discussed earlier in this section. With respect to the predictions, the level of fishing mortality needed to take the TAC in 1997 was very similar in both cases, but represented a somewhat larger relative reduction in fishing in the M = 0.1 case (0.45 to 0.14), because the current level of fishing mortality appears higher. For M = 0.1, the equivalent catch in 1998 at Fmax is much smaller. This reflects the need to reduce fishing mortality to a lower target level. Spawning stock biomass is similar in both runs in 1997 but is higher for M = 0.1 in 1998. This is due to the larger reduction in fishing mortality rate.

Catches and spawning stock biomass achieved in 1997, applying the same mortality rate as in 1996 (the status quo level), have similar results for both M values. Thus, these specific results for summer flounder also support the general conclusion that natural mortality estimates do not affect estimates of TACs much when M is held constant from one year to the next. It is clear that the correct value of natural mortality rate does influence the biological reference points and the likely target catch rates. A choice of a natural mortality of 0.1 would lead to more restrictive management decisions than one of 0.2, while one of 0.3 (not shown) would require lower reductions. M is a parameter that should be esti-

3  

The Northeast Stock Assessment Workshop in Fall 1995 (SAW 20) examined methods of Hoenig (1983), Pauly (1980), and Fmax = 3/M (e.g., Anthony, 1982) and chose to continue to use M = 0.2 for all ages for summer flounder. In stomach content analyses, young summer flounder do not seem to be preyed on heavily by other species, with the exception that young summer flounder are occasionally found in the stomachs of striped bass and dogfish (M. Terceiro, NMFS, personal communication, 1999).

Suggested Citation:"Summer Flounder: Review and Insights." National Research Council. 2000. Improving the Collection, Management, and Use of Marine Fisheries Data. Washington, DC: The National Academies Press. doi: 10.17226/9969.
×

TABLE 2-1 Effects of Changes in Assumed Natural Mortality Rate on Biological Reference Points, Estimates of Fishing Mortality, and Predicted Future Catch and Spawning Stock Biomass (SSB)

 

M = 0.2

M = 0.1

Biological Reference Points (1996)

   

F0.1

0.15

0.09

Fmax

0.24

0.14

F20%

0.28

0.17

Fmed

1.48

1.77

Estimated fishing mortality in 1996a

   

F1996

0.95

1.03

Predictions for 1997 under status quob

   

F1997

0.95

1.03

Catch1997 (mt)

12,248

12,585

SSB1997(mt)

12,263

11,403

Predictions for 1997 under TAC of 7,162 mt

   

F1997

0.44

0.45

Catch1997 (mt)

7,162

7,162

SSB1997 (mt)

16,419

16,077

Predictions given 1997 TAC of 7,162 mt and F=Fmax in 1998

   

F1998

0.24

0.14

Catch1998 (mt)

4,541

2,898

SSB1998 (mt)

32,677

37,204

NOTE: SSB = spawning stock biomass; TAC = total allowable catch. See Box 1-2 for other definitions. The assumed value of M not only affects estimates of fishing mortality, survivorship, and biomass, it also affects biological reference point estimates. Thus, the consequences of assuming an alternative M must be assessed in the context of all the estimates influenced by M and should be examined on a time scale appropriate to the life span of the fish.

aF1996 = the estimated fishing mortality for fully recruited summer flounder in 1996. This is the arithmetic mean of the fishing mortality over ages two, three, and four.

b The status quo condition assumes that fishing mortality stays at the 1996 level (F = 0.95).

mated correctly unless a management method less sensitive to its value (e.g., using Fmed) is used.

What is the correct level of natural mortality for summer flounder? As with other species, the natural mortality rates of flounder species are difficult to estimate and often have been assigned values below 0.2. However, the M value for summer flounder has been estimated using a number of indirect approaches in previous stock assessment workshop (SAW) assessments (NEFSC, 1997) and the SAW concluded that M = 0.2 is a reasonable working assumption.

Actions Needed

Given that Fmax is a basis for management plans, and that it depends on the value assumed for M, it is essential to obtain an independent estimate of M for summer flounder. A well designed and carefully executed tagging program is an excellent method for estimating M. Such a tagging program has the added advantage of also providing estimates of F and of movement rates between different parts of the range of the population. Tagging programs are time consuming and

Suggested Citation:"Summer Flounder: Review and Insights." National Research Council. 2000. Improving the Collection, Management, and Use of Marine Fisheries Data. Washington, DC: The National Academies Press. doi: 10.17226/9969.
×

difficult to conduct and require full cooperation of all stakeholders in the fishery to be effective.

Emigration of fish from the surveyed area can significantly confound tagging studies, so estimates of emigration should be obtained. Relatively large-scale attempts to estimate emigration for summer flounder of the Chesapeake Bay region are described in the unpublished dissertation by Desfosse (1995). The benefits of such an experiment (unless results from a suitable previous experiment exist) would take a number of years to become available. In addition, the cost associated with a large-scale tagging study should be balanced with the benefit of obtaining information on this component of population dynamics relative to other, more pressing data needs, such as the vulnerability of older fish to fishing pressure across the management area and over time. Multispecies models that estimate predation mortality from stomach content analyses is an approach to estimating the importance of predation as a component of natural mortality. However, unless these kinds of data are already available, a new intensive (and expensive) sampling program would have to be implemented to sample the stomach contents of animals that prey on summer flounder. Such a sampling program may not be cost effective. It might also be possible to estimate M internally in the model, using Bayesian or other methods, as has been done in previous assessments for other species.

Are there differences between the growth and mortality of male and female summer flounder (sexual dimorphism) and, if so, how do the differences affect the assessment?

Fish size generally increases with age up to some upper biological limit for the species. This limit may vary with environmental conditions, differs from species to species, and may differ within a species, for example, between sexes.

For a number of flatfish species, males grow at a faster rate than females, but often to a smaller maximum size. For such species, the largest fish are almost always females; for example, female North Sea plaice typically grow to a greater length and weight than do males (Rijnsdorp and Ibelings, 1989). Spring and fall surveys show a 20 percent larger size-at-age among female summer flounder at five years (NOAA, 1992). Eldridge (1962), cited in NMFS (1981, p. 15) also notes a 20 percent greater size in females than males; this difference increased to 30 percent by age 8. Differences in growth between sexes may also lead to different mortality rates at age between sexes if mortality is related to size. This difference may make it difficult to ascertain the full impact on the stock from data for both sexes combined. The sex of summer flounder can be determined by port samplers for commercial catch and by survey scientists from survey catches, and otoliths and scales can be collected to determine the ages for the same fish. Incorporating sex differences in growth and selectivity at age may be possible. If information collected from surveys can be used to identify sex-specific age and size characteristics that can be applied to commercial and recreational landing data, the assessment may be able to account for differences resulting from the sexual dimorphism.

Committee Investigations

The committee did not have suitable data to calculate the effect of ignoring sexual differences in growth. However, mortality at age should be greater for females than for males, until both are equally well retained by trawl nets in commercial fisheries and hooks in recreational fisheries. In turn, this greater mortality for females would lower the estimated spawning potential of the stock, decrease the effective spawning stock biomass, and increase the effective fishing mortality.

Actions Needed

As discussed in the next section, differences in catchability at different ages can affect stock assessments and the resulting estimates of fishing mortality and biomass. This is analogous to the multi-stock problem discussed in the previ-

Suggested Citation:"Summer Flounder: Review and Insights." National Research Council. 2000. Improving the Collection, Management, and Use of Marine Fisheries Data. Washington, DC: The National Academies Press. doi: 10.17226/9969.
×

ous section, where one stock is male and the other female. If catchability differs with size (and thus between sexes), this will influence the evaluation of stock status. NMFS should determine whether there is enough difference in size at age and/or catchability at age between the sexes to warrant concern about the spawning potential of the stock. Currently, sex is not determined for catch samples from the summer flounder commercial and recreational fisheries. As a result, these data alone do not provide sufficient information to account for differences in growth between males and females in the assessment. It is possible to determine the sex of some fish species in catch samples when fish are gutted at sampling time, or when identifiable external morphological characteristics are available. Apparently this is not the case for summer flounder. It may be possible, however, to use survey information to distinguish gender-related differences since sex, age, and length are determined for survey samples. Use of this information could improve assessment results that are influenced by sexual dimorphism. If such data become available, NMFS should conduct catch-at-age or catch-at-length analyses that explicitly account for sex differences in size-at-age and selectivity. Such data could be applied retrospectively, assuming constant size-at-age by sex for years when observations are not available.

QUESTIONS RELATED TO SUMMER FLOUNDER SAMPLING

A common concern voiced by fishermen is that the gear used for sampling fish is antiquated and thus survey results are biased. This section addresses some of the concerns about the gear and methods used in the fishery-independent surveys.

What are the appropriate survey and commercial catchabilities of summer flounder?

Commercial fishermen believe that NEFSC surveys fail to fish appropriately in the survey strata areas and depth where the industry believes large summer flounder are found. Presentations to the committee by commercial fishermen suggested that large numbers of large summer flounder used to be taken on the portion of Georges Bank near the U.S.-Canada border (the Hague Line). This distribution of larger summer flounder is supported by some data from the American Littoral Society tagging program, the dissertation by Desfosse (1995), and Rountree (1994). Fishermen claim that even within the strata fished, the randomized stations tend to miss the sites where aggregations of large summer flounder are found. They also argue that the research gear (roller-rigged)4 and the method for using it (including 30-minute tows) are unsuited to catching the larger, faster swimming summer flounder. Fishermen believe that the groundfish surveys, particularly the fall survey, do not adjust for flounder migrations, so the surveys do not adequately sample the population. Thus, fishermen conclude that the age distribution observed is unrepresentative of the population. The tow duration issue is important to resolve, particularly because NMFS has considered reducing tow durations to 15 minutes in an attempt to increase the number of tows and thereby the precision of estimates based on the surveys. NMFS has not tested the effect of tow durations on catchability-at-age of summer flounder and other flatfish. However, the catch of flounders larger than 45 cm in length, in both the commercial fishery and surveys, has recently reached its highest point since 1982, according to NMFS.

In considering these criticisms, it is important to understand how commercial and recreational catch-at-age data and survey catch per unit effort (CPUE)-at-age data are used in assessment models to estimate population size and structure. If both were used directly as estimates of population structure, rather than interpreted in the con-

4  

Roller gear is a type of footrope (the rope attached to the bottom front of a trawl net) made of round rubber or steel bobbins interlaced with rubber discs and rigged to roll as the net is pulled across the bottom.

Suggested Citation:"Summer Flounder: Review and Insights." National Research Council. 2000. Improving the Collection, Management, and Use of Marine Fisheries Data. Washington, DC: The National Academies Press. doi: 10.17226/9969.
×

BOX 2-2

Gear Selectivity or Catchability

Selection of a gear is best described by the catchability (q) of the gear (g) at age (a). This is the amount of fishing mortality that a unit measure of fishing activity (e.g., one day's fishing) would generate on age a by gear type g. Catchability has several components that relate to the availability of the fish to the gear (Are fish in the area fished by the gear?), its accessibility to the gear (Does the species' behavior put it in the path of the gear?), and its vulnerability to the gear (Does the gear catch fish if they are encountered?). In the case of trawls, mesh selectivity can be measured by selectivity experiments (e.g., by using trawls with codends covered with finer mesh to estimate the proportion retained by size). Other factors contribute to gear selectivity, such as footrope size, headrope height, tow speed, and tow duration; however, these factors relate to distribution and behavior of fish and are less easy to estimate directly.

Overall catchability can be measured using the results of VPA, provided adequate data are available. Estimates of catchability at age may change over time due to changes in availability or accessibility resulting from different gear uses or changing environmental conditions. When fishery or survey effort or catch rates are used to tune the VPA, it is usual to adopt a null hypothesis that they are constant from year to year. Since various choices of q(a', y, f) for the oldest age a' can give equally plausible interpretations of the data, it is often the practice to set this value to the same value as the average of one or more of the intermediate ages. This assumption implies that the oldest fish have the same catchability as those of the intermediate age. Alternatively, a “dome-shaped” selectivity curve is assumed if younger and older fish are less susceptible to the fishing method than fish of intermediate ages.

text of the stock assessment model, the industry concerns would be valid. However, most models include assumptions about how selective the gear is for different age and size classes. For summer flounder, catch-at-age data are used as direct measures of the fishing-induced mortality at age. Because no measure of effort is associated with landings, no auxiliary data are developed from commercial catch rates. For summer flounder survey CPUE-at-age data, however, an age structure is implied based on the type of selectivity curve assumed. Thus, the important question becomes whether or not the selectivity curves are appropriate.

Models can correct for changes in the proportion of different ages caught through incorporation of an exploitation pattern (see Figure 2-3 for an example) for the fishery and for the survey gear. Exploitation pattern is a function of the gear used (Box 2-2) and of the geographic distribution of fishing effort. For gear such as gillnets, designed to catch a specific size of fish, the exploitation pattern initially increases with size and then declines as bigger fish cannot entrap themselves in the mesh of the net. With such gear types as trawls, which potentially can catch all fish greater than a certain size, the exploitation pattern initially rises and then may plateau. The exploitation pattern, however, may decline at larger sizes if larger fish are better able to escape the gear or the gear is used more intensely in areas where smaller fish live. Thus, if at least some of the older ages are caught and the exploitation pattern can be estimated, assessment models can correct for changes in the proportion of different ages caught.

Suggested Citation:"Summer Flounder: Review and Insights." National Research Council. 2000. Improving the Collection, Management, and Use of Marine Fisheries Data. Washington, DC: The National Academies Press. doi: 10.17226/9969.
×

In practice, the data sets typically collected for use in stock assessments provide little information on the shape of the exploitation pattern. Pope and Shepherd (1982) showed that in simple one-fleet situations the exploitation pattern could not be determined by catch-at-age data alone and their general results indicate that the exploitation pattern remains undetermined despite the existence of additional survey or fleet results. In practice, therefore, assumptions have to be made about the shape of the exploitation pattern. The normal default assumption, and the one that is used in the summer flounder assessments, is that the oldest ages of fish in the survey have the same chance of being caught as younger fish. These assumptions are applied to the consolidated commercial catch in some methods of analysis and to the survey catch rate results in others. Although this is a sensible default assumption, it may be wrong. Some studies of Irish Sea fish stocks have found that egg numbers gave larger estimates of spawning stock size than those obtained by standard assessment models using the usual default assumptions (Horwood, 1993; ICES, 1998). Estimates of spawning stock biomass based on egg surveys can be higher than those based on landings when there is under-reporting. Generally, however, unexpectedly high spawning stock biomass results from older fish being less selected by the gear than younger fish, leading to an incorrect estimation of the proportion of fish of various ages in the population.

A more important question about catchability bias in surveys is not whether the gear is biased but whether it is possible to correct any bias. This requires an estimation of the exploitation pattern of gear. It is difficult to do this directly because data that bear on this point are often unavailable. The limited span of ages sampled by the fishery and various state and federal survey series also adds to this problem. If data on older fish were available, any trend in the catchability of these fish could be indicative of changes in catchability over some of the older ages. Although the use of default values of exploitation pattern for the older ages is perhaps inevitable, given the lack of data, it may lead to underestimation of the size of the spawning stock biomass and an inappropriate estimate of the age structure. Such default assumptions also may lead to overestimation of the status quo fishing mortality; it is apparent from the VPA results that catchability is assumed constant for ages 3, 4, and the 5+ group. Because recreational catch accounts for about half of the total catch, the catchability of the recreational fishery and its effect on population estimates also is important.

Committee Investigations

A concern raised by industry representatives was that both the NEFSC surveys and commercial catch data failed to represent the true age structure of the summer flounder population, by missing large fish offshore. Issues relevant to this potential problem include (1) whether there is evidence that older fish are missed in the assessments and (2) if older fish are missed, how this could affect the assessments.

Is there evidence for a large stock of older fish offshore? Commercial flounder fishermen cite past catches of large fish offshore. They claim that older fish do not enter the assessments because

  1. the trip limits in the flounder fishery since 1992 make it less economical for commercial fishermen to go offshore where the larger fish are found in the winter;

  2. large fish that are caught are missed by the port surveys because they quickly go, unsampled, to the sushi markets in Japan;

  3. NEFSC spring and fall surveys use gear inappropriate to catch flatfish; and

  4. all NEFSC surveys use tows that are too short and too slow to tire and catch larger, stronger flounders.

Commercial fishermen claim that there are older, larger fish at the edge of the continental

Suggested Citation:"Summer Flounder: Review and Insights." National Research Council. 2000. Improving the Collection, Management, and Use of Marine Fisheries Data. Washington, DC: The National Academies Press. doi: 10.17226/9969.
×

shelf, an area seldom fished or sampled. Packer and Hoff (1999), on the other hand, indicate low catches of summer flounder at the shelf edge in their description of essential habitat for this species, although this analysis is based on NEFSC data that the industry views as suspect. Even if a refuge for large flounder is missed by both commercial and survey vessels, one would expect to see these fish in the spawning areas offshore at some point. If they do not migrate to the spawning areas, they are not part of the spawning stock biomass and thus their exclusion from the stock assessment is appropriate. What would constitute evidence to the contrary? The existence of a population of large summer flounder offshore should be reflected by

  1. unexpectedly large numbers of flounder eggs in egg surveys;

  2. catches of large flounder as they migrate seasonally inshore in the spring and back offshore to winter grounds;

  3. logbook and export records of large flounders, even if not represented in port samples; and

  4. large flounders being caught in general recreational fisheries and tournaments (if tournaments take place in areas where large flounders are expected to be caught).

The committee was not able to locate data on all these factors and was given little evidence that an otherwise unassessed population of large summer flounder exists offshore. The one possible exception is the recent rise in the number of citations in Virginia for summer flounder weighing 6 pounds or more (see Figure 2-7), although the fishing effort and locations related to these catches are not reported.

If large flounders are less catchable, how would this affect stock assessments? The proposed inability of the survey to find and catch large flounder can be simulated by altering the assumptions of the standard Laurec-Shepherd VPA tuning to halve the relative fishing mortality on age 4 and older fish relative to the average fishing mortality on age 2-3 fish (Table 2-2).

The main effects of this alternative assumption are:

  1. the estimate of current fishing mortality F1996 is reduced from 0.95 to 0.63 if larger flounder are less catchable (although part of this reduction results from the lower estimate of the fishing mortality on 4-year-olds).

  2. estimates of the biological reference points F0.1, Fmax, and F20% all increase and thus would require less restrictive management under the alternative assumption.

  3. Fmed is reduced in line with F1996 and hence advice based on achieving some specified proportion of Fmed would be more stable. In other words, the adjustment needed in fishing intensity under alternative assumptions is less for Fmed than it is for the other biological reference points.5

  4. the fishing mortality that would have been required to achieve the 1997 TAC is reduced to

5  

The main short-term negative effect on fishermen generated by management is usually associated with the proportion by which fishing mortality, and hence fishing effort and short-term catch, must be reduced. Thus, how critical a factor (such as the right exploitation curve) is to management might be measured by asking how it changes the ratio between the current and desired fishing mortality rates. For example, reducing fishing mortality to Fmax under the standard assumptions requires a reduction from F = 0.44 to F = 0.24, a 45 percent reduction. If the “alternative F for 4+ is half of F for ages 1-3” hypothesis is correct, a reduction from F = 0.31 to F = 0.30, only 3 percent, is required. Therefore, assumptions about catchability affect target F values, which affect the difficulty of implementing management to control fishing mortality. By contrast, managing at Fmed would apparently allow F to increase from 0.44 to 1.48 in the first case, a 236 percent increase, and from 0.31 to 1.17 in the second case, a 277 percent increase. This example illustrates that Fmed seems to be more robust under uncertainty than Fmax and F0.1, though the committee regards Fmed as a threshold reference point rather than a target reference point.

Suggested Citation:"Summer Flounder: Review and Insights." National Research Council. 2000. Improving the Collection, Management, and Use of Marine Fisheries Data. Washington, DC: The National Academies Press. doi: 10.17226/9969.
×

TABLE 2-2 Effects of Changes in Assumed Fishing Mortality Rate at Age on Biological Reference Points and Predictions of Fishing Mortality (F), Catch, and Spawning Stock Biomass (SSB)

 

F Constant at All Ages

F for Ages 4+ is Half That for Ages 1-3

Biological Reference Points (1996)

   

F 0.1

0.15

0.18

F max

0.24

0.30

F 20%

0.28

0.30

F med

1.48

1.17

Estimated fishing mortality in 1996

   

F 1996

0.95

0.63

Predictions for 1997 under status quo

   

F 1997

0.95

0.63

Catch1997 (mt)

12,248

12,445

SSB1997 (mt)

12,263

17,181

Predictions for 1997 under TAC of 7,162 mt

   

F 1997

0.44

0.31

Catch1997 (mt)

7,162

7,162

SSB1997 (mt)

16,419

21,425

Predictions given 1997 TAC of 7,162 mt and F = F max in 1998

   

F 1998

0.24

0.30

Catch1998 (mt)

4,541

6,950

SSB1998 (mt)

32,677

33,289

NOTE: SSB = spawning stock biomass; TAC = total allowable catch. See Box 1-2 for other definitions.

0.31. Although lower in absolute terms than the 0.44 level of the standard run, a proportionally similar reduction in fishing is still required to achieve half the current exploitation level (the 1997 TAC at status quo fishing mortality is similar for both runs).

  • the 1997 spawning biomass is higher for the new assumption, both at the 1997 TAC level of fishing and at status quo.

  • since the fishing mortality rate required to catch the 1997 TAC (0.31) under the new assumption is close to the new estimate of F max (0.30), the catch in 1998 under F max is substantially higher with the new assumption. However, since there is less reduction in fishing mortality, the biomass that results is similar in 1998 to the standard run.

In summary, adopting an assumption that old fish are less catchable than young fish leads to a more optimistic view of the stock. It is recognized that F 0.1F max, and F 20% are proportionally more sensitive to current F than is F med. Adopting a different assumption about the exploitation rate of older fish could cause substantial changes to estimates of future TACs under an F max target.

Figure 2-3 illustrates the change in the commercial selection pattern between the two runs. It also illustrates that the fishing mortality rate value adopted for age 4 (the last true age in the catch-at-age data) has to be applied to age 5 fish and older. Since age 4 fish are typically less than half the weight of the largest fish, this may be a doubtful assumption. This suggests that including more ages in the assessment could be helpful (see Figure 2-4).

Suggested Citation:"Summer Flounder: Review and Insights." National Research Council. 2000. Improving the Collection, Management, and Use of Marine Fisheries Data. Washington, DC: The National Academies Press. doi: 10.17226/9969.
×

FIGURE 2-3 Assumptions about summer flounder fishing mortality on ages 0-14.
NOTE: The fishing mortality rate (not selection) of 0.8 on age-4 and older fish stems from the fact that the age-4 mortality is estimated as the average of the mortality rate on ages 1-3 and that on older ages set equal to the age-4 fish. This assumption does not provide a better or worse fit in the Laurec-Shepherd model than a curve with higher or lower mortality on age-4 fish. This is because the model fits are almost completely insensitive to the assumption of how domed the selection is (see Pope and Shepherd, 1982).

The SAW25 report (NEFSC, 1997) provides catch-at-age data that are relatively complete up to age 7. However, the NMFS VPA generally includes data only through age 4. Separable VPAs (Pope and Shepherd, 1982) conducted on these data suggest that selection has not changed through time and that regardless of whether the fishing mortality of 7-year-old fish is set at half, equal to, or 50 percent greater than the value of current fishing mortality on 2-year-olds, the exploitation pattern is relatively flat between ages 2 and 5. Runs of Laurec-Shepherd tuning were made on this data set with an assumption that fishing mortality on age 7 was either (1) equivalent to that on ages 3-6 or (2) was 50 percent of this level (Figure 2-4). These runs both provide estimates of biological reference points that are broadly similar to those when the Laurec-Shepherd is run with constant catchability on ages 0-4 (see Table 2-2). Both the runs, however, give a similar, more pessimistic view of the stock size than the equivalent Laurec-Shepherd runs on ages 0-4. The catch and SSB results from the two runs correspond quite closely, suggesting that adding the extra age classes to the analysis makes the decision about the level of exploitation of older fish less critical.

The more pessimistic view of the stock, given by both runs using the extended data set, occurred because the exploitation pattern remained high on ages 4 and 5 rather than decreasing as it did in the standard Laurec-Shepherd run conducted on ages 0-4 only (Figure 2-4). This result (based on actual catch data) does not support the view that

Suggested Citation:"Summer Flounder: Review and Insights." National Research Council. 2000. Improving the Collection, Management, and Use of Marine Fisheries Data. Washington, DC: The National Academies Press. doi: 10.17226/9969.
×

FIGURE 2-4 Comparison of exploitation pattern of summer flounder from runs including more ages in comparison to the standard runs. The middle line is the standard assessment with F(age 4) set as the average F on ages 1-3 (the standard line of Figure 2-3) and fishing mortality estimated on catch-at-age data up to age 7. The alternate lines show (1) where F(age 7) is set to the average of F on age as 4-6 and (2) where F(age 7) is set to 0.5 of the average F on ages 4-6.

exploitation is less on older ages. It seems that at least ages 4-6 years are exploited at much the same level as ages 2 and 3 years. Moreover, at age 6, the fish are a substantial proportion of their maximum size and weight. These fish may reasonably be expected to behave like even older fish and be equally catchable and may have the same fishing mortality rate. It should be noted that this interpretation relies on the older fish being properly sampled.

Actions Needed

Whether a population of large summer flounder is missed by surveys, recreational fisheries, and commercial fisheries—but contribute significantly to spawning stock biomass and recruitment—is one of the major points of contention between NMFS and commercial fishermen. If this population exists but is not surveyed, not fished, and does not contribute to stock spawning potential, it begs the question of whether it should be included in management considerations. Although the committee found no data to indicate reduced catchability of larger flounders or the presence of a large enough offshore population of large flounders to affect the assessments, NMFS should work with industry on this issue. The data normally used in population abundance assessments, commercial catch-at-age data and commercial and survey catch-rate-at-age data, do not provide information on the exploitation pattern of fish of the oldest ages. Since errors in the assumptions made to cover this deficiency may

Suggested Citation:"Summer Flounder: Review and Insights." National Research Council. 2000. Improving the Collection, Management, and Use of Marine Fisheries Data. Washington, DC: The National Academies Press. doi: 10.17226/9969.
×

have quite important effects on management decisions, it is appropriate to consider what other sources of data could be useful to elucidate the population age structure and exploitation pattern. Tagging is one mechanism for collecting such information and managers will have to decide whether the cost of conducting an extended tagging experiment would be justified by the potential return. The potential return of tagging experiments would include improvements in estimation of exploitation rates at age and thus biological reference points, as shown in Table 2.2, decreasing the uncertainty of management based on biological reference points.

If older fish have a lower fishing mortality rate than is supposed in current assessments, they would have a higher average age than predicted. Thus, tagged fish would continue to be returned for a longer period than would have been predicted under the current assessment. A number of problems exist with tagging experiments, however, such as the need to accumulate the data over a number of years, the need for accurate estimates of tag loss and non-reporting (a common problem with earlier summer flounder tagging studies), and the need for complete mixing of the tagged fish in the population. If an extensive and expensive tagging study were undertaken, cooperation of the fishing community would be essential to make it successful. There are many ways this cooperation could be enlisted. Directly involving some fishermen with the tagging is one example. Commercial fishermen could be enlisted to assist with tagging large numbers of summer flounder offshore, either as paid participants or as an in-kind contribution to NMFS and industry cooperative efforts.

Alternatives to tagging could be used to answer the question about summer flounder catchability directly. One alternative that is faster, though inherently expensive (see Horwood, 1993), is to conduct egg surveys. These are used to estimate the number of eggs spawned and (with knowledge of average fecundity per gram of adult female) the biomass of the spawning stock. Summer flounder eggs are distinguishable from eggs of other species even at early stages, except for some difficulty in distinguishing between summer and windowpane flounder eggs (Berrien and Sibunka, 1999). Egg surveys with usable levels of precision can be difficult for summer flounder because this species ' long spawning period would require multiple survey cruises to sample its eggs adequately. NMFS has not conducted egg surveys specifically for summer flounder, although egg and larval densities are available from the Marine Resources Monitoring, Assessment, and Prediction (MARMAP) program for 1977-1987.

A third (and possibly the least expensive) approach to deal with any catchability-at-age problem is to include more ages in the assessment. Survey data on age classes 5 and older typically are excluded from the analyses for all but the NEFSC winter survey (available only since 1992). This is due in part to the difficulty some assessment procedures have in dealing with sparse data, observed zero values, or missing data, which are more common for older age classes. Committee analyses determined that the assessments can be sensitive to the number of age classes included in assessments. Although this does not resolve the problem of the proportion of the older ages being caught, it may move the problem up to ages that are sufficiently scarce, thus reducing the problem to a negligible level. Therefore, future NMFS assessment activities for summer flounder should include more year classes (up to age 7) to tune the model and decrease the possibility of missing changes in fishing mortality on older ages. This will be particularly important as the summer flounder population recovers and includes a greater proportion of older fish.

Finally, NMFS should conduct studies of catchability at age as a function of tow times, using nets that include video cameras to study fish behavior and determine whether larger fish are more likely to outswim the nets than are smaller fish. Another means to test gear performance would be to compare survey data from cruises using spring roller rig trawl gear versus winter flatfish trawls; both surveys occur at approximately the

Suggested Citation:"Summer Flounder: Review and Insights." National Research Council. 2000. Improving the Collection, Management, and Use of Marine Fisheries Data. Washington, DC: The National Academies Press. doi: 10.17226/9969.
×

same time when fish are not likely to be very mobile. Joint NMFS and industry survey exercises and sampling targeted at possible locations of large flounder should be undertaken. Because the debate about the population structure centers in the offshore area, this area should be targeted for special tagging and egg survey studies, as well as other observations designed to determine if larger, older summer flounder are more abundant offshore than expected.

The significant contribution of the recreational fishery to the fishing mortality rate makes the actual exploitation patterns of recreational anglers and model assumptions about these patterns more important in summer flounder stock assessments. NMFS should examine its assumptions about recreational exploitation patterns and determine whether these assumptions are valid.

Do problems with determining the age of summer flounder discredit age-based assessments?

The age of fish are most commonly determined by counting the number of rings on fish scales or ear bones (otoliths). These often show annual rings similar to those seen in trees. Interpreting the number of rings can be contentious and different readers may assign different ages to the same fish. Typically, scales are more appropriate for ageing younger fish, whereas otoliths are more appropriate for ageing older fish, because scales tend to erode as a fish ages. Operational assessments of summer flounder are based on ageing of scales, because ageing of scales is reliable when stocks are heavily fished and most fish are less than 3 or 4 years old. As the summer flounder population is rebuilt and age structure widens, scales will be increasingly problematic because they underestimate age in older fish and ageing may need to switch to otoliths. In recent years, significant effort has been devoted to developing accurate otolith ageing techniques.

A disagreement existed for several years between the age readers of NMFS' Northeast Fisheries Science Center and those at the North Carolina Division of Marine Fisheries in relation to fish ages based on scales and otoliths. (In 1996, landings in North Carolina accounted for 39 percent of the total commercial landings of summer flounder.) The disagreement on fish ages may have resulted from the problems described above. Groups of age readers from the two labs collaborated in several workshops in the 1990s and agreed in the latest one (in 1999) that summer flounder can be aged reliably using scales, if protocols agreed to by the two groups are followed (Bolz et al., in press). Participants in the 1999 workshop concluded that the majority of ageing disagreements between the two groups resulted from differing interpretations of “marginal scale increments due to highly variable timing of annulus formation and from the interpretation of first-year growth patterns and first annulus selection. It was agreed that the NEFSC and NCDMF [North Carolina Department of Marine Fisheries] age data used in the current assessment are valid for the respective components (NER [New England Region] and North Carolina waters) of the stock and fishery” (Terceiro, 1999). The level of agreement between age readers from NEFSC and the North Carolina Department of Marine Fisheries was 83% in this exercise. The committee is concerned that ages based on scales will not be adequate for older fish as the population recovers and believes that research to improve determinations of age from otoliths should continue. There is some evidence that measurements based on scales underestimate the ages of summer flounder older than five years (C. Jones, unpublished research).

In practice, a certain amount of error in ageing may not create serious differences to fishery assessments, particularly if the errors are consistent from year to year in magnitude and direction; the same ageing errors would occur in the catch-at-age, fecundity-at-age, and weight-at-age data, and the degree of mis-ageing should be similar throughout the range of the species. An effect that may be important is that any error in ageing tends to smear out a strong year-class over the adjacent year-classes. This may make the

Suggested Citation:"Summer Flounder: Review and Insights." National Research Council. 2000. Improving the Collection, Management, and Use of Marine Fisheries Data. Washington, DC: The National Academies Press. doi: 10.17226/9969.
×

TABLE 2-3 Effects of Changes in Assumed Fish Ageing Protocol on Biological Reference Points and Predictions of Fishing Mortality (F), Catch, and Spawning Stock Biomass (SSB)

 

NEFSC and North Carolina Ages Used

NEFSC Ages Used Throughout

Biological Reference Points (1996)

   

F0.1

0.15

0.15

Fmax

0.24

0.25

F20%

0.28

0.28

Fmed

1.48

1.57

Estimated fishing mortality in 1996

   

F1996

0.95

1.33

Predictions for 1997 under status quo

   

F1997

0.95

1.33

Catch1997 (mt)

12,248

11,415

SSB1997 (mt)

12,263

8,542

Predictions for 1997 under TAC of 7,162 mt

   

F1997

0.44

0.62

Catch1997 (mt)

7,162

7,162

SSB1997 (mt)

12,132

16,419

Predictions given 1997 TAC of 7,162 mt and F = Fmax in 1998

   

F1998

0.24

0.25

Catch1998 (mt)

4,541

3,199

SSB1998 (mt)

32,677

30,167

NOTE: SSB = spawning stock biomass; TAC = total allowable catch. See Box 1-2 for other definitions.

understanding of year-class variations more difficult. A useful discussion of the effects of misageing fish can be found in Gulland (1955).

Committee Investigations

To determine whether any age discrepancies between readers from North Carolina and NEFSC could affect the assessment, a Laurec-Shepherd analysis was conducted in which North Carolina-aged catches were replaced by an equivalent number of fish with the age distribution of the rest of the catch (as aged by NEFSC readers). As in previous subsections, the standard run of the Laurec-Shepherd model is compared with the modified run (Table 2-3).

The main points to note from this analysis are that

  • the biological reference points were not very different under the two scenarios.

  • when NEFSC ages are used exclusively, the current estimate of fishing mortality is increased by about a third and the fishing mortality required to catch the 1997 TAC is increased, presumably due to population estimates being decreased.

  • spawning stock biomass estimates are also decreased for the same reason, as is the 1998 Fmax TAC and the 1997 status quo TAC.

  • the estimate of fishing mortality rate is higher using the NEFSC ages only. Since the

Suggested Citation:"Summer Flounder: Review and Insights." National Research Council. 2000. Improving the Collection, Management, and Use of Marine Fisheries Data. Washington, DC: The National Academies Press. doi: 10.17226/9969.
×

value of Fmax is similar in both assessments, the reduction in effort required to meet Fmax is 60 percent (NEFSC ages only) as opposed to 45 percent (NEFSC and North Carolina ages).

It appears that the potential results of the ageing discrepancies are significant in terms of the assessments, so it is fortunate that this issue has been resolved. This finding also points out the importance of ensuring that both groups continue to study and compare ages from both scales and otoliths, as necessary.

Actions Needed

The committee recommends that NEFSC and North Carolina continue efforts to ensure agreement on ages from scales and otoliths, perhaps using tags or oxytetracycline6 marks, if necessary. Given that the disagreements relate to the early rings, an oxytetracycline experiment on young fish raised in culture could provide results in a short period of time. Work is ongoing and should continue until the issue is resolved. Other techniques, such as tagging and retrieving tetracycline-treated wild fish, and using other microchemical methods (Campana et al., 1990; Campana and Jones, 1998), could be pursued to resolve this issue.

Are effort data used appropriately and are the effects of effort changes incorporated properly?

In many fishery assessments, commercial fishing effort data and the associated commercial catch data are the primary means of establishing the trends in fishing mortality rate and population size and hence in establishing the current state of the stock and the current level of TACs. The summer flounder assessment is unusual in this respect in having a number of survey series to provide abundance data, and including no commercial fishing effort data in the VPA tuning. In a few fisheries, recreational effort is significant and is included through Marine Recreational Fisheries Statistics Survey (MRFSS) data. Commercial fishing effort data and the associated catch rates are both based on far more sampling than research vessel data and thus are likely to be less variable. However, because of changes in commercial practice and government regulations, commercial data may exhibit changes through time that, if not corrected, could bias fishery assessments. Namely, a mandatory logbook and trip limits were instituted in the commercial summer flounder fishery in 1992. NMFS believes that these changes invalidate the time series for commercial summer flounder data.

Committee Investigations

For fisheries in which a significant portion of the catch is taken by recreational anglers (e.g., the current summer flounder fishery), effort data can be obtained from MRFSS. Use of these effort data, however, is not straightforward. The data are obtained from telephone queries to angling households, in which the anglers provide information on the number of fishing trips they took, but not the target species or the number of hours they fished. Thus, the telephone survey does not provide directed effort, so directed effort must be estimated through a proportional conversion obtained from on-site interviews at access points, where anglers are asked which fish species they sought and what they caught. The ratio of directed effort to total effort can therefore be obtained from on-site interviews. Because this is a proportion in a given fishing mode and sampling period, the variance can be calculated as the variance of a proportion. Ultimately, the variance of directed effort is calculated as the product of the variance of the ratio and total effort for the fishing mode and sampling period. Total effort data for the Atlantic Coast for all species typically has a proportional standard error of less than 3 percent. When effort is requested for

6  

Oxytetracycline and other chemicals that mark calcium can be injected into a fish on a known date. Any subsequent age increments can be validated with time elapsed since the fish was injected.

Suggested Citation:"Summer Flounder: Review and Insights." National Research Council. 2000. Improving the Collection, Management, and Use of Marine Fisheries Data. Washington, DC: The National Academies Press. doi: 10.17226/9969.
×

specific regions (e.g., mid-Atlantic states), estimates are more variable, but have proportional standard errors still less than 10 percent. Additional increases in resolution increase variability. Effort data are not available on the MRFSS Web site, but are available to state and federal clients upon request.

The committee did not investigate the issue of including effort data in assessments. The committee believes, however, that inclusion of available effort data in stock assessments could help NMFS understand changes in the fishery, and fishermen could better relate what they observe to what the survey observes.

Actions Needed

The problems and benefits of including commercial catch and effort data in summer flounder assessments should be investigated. Comparisons of assessments with commercial catch rates is one way for NMFS to involve industry more closely and to monitor changes in catchability that may be occurring in the fishery. One approach to involve commercial fishermen in a manner more comparable to fishery-independent surveys is the sentinel survey, as used in the Atlantic Canada inshore cod fisheries. In these surveys, fishermen are hired by the government to fish using commercial gear in a systematic fashion to estimate cod abundance. Having the commercial CPUE used in a statistically designed survey to compare in time and space with the surveys is important. This approach provides results that appear valid to both scientists and fishermen.

One of the many examples of a sentinel survey (and probably one of the least contentious) is the 4VsW sentinel longline survey, which has been conducted on the east Scotia Shelf off Nova Scotia by commercial longliners in September-October each year since 1995. Locations are randomly selected from the strata used by the Canadian Department of Fisheries and Oceans' summer survey plus from three inshore strata. At present, the estimates from this survey are used to comment on general trends and compare with the departmental surveys (Mohn et al., 1998). Attempts will be made to include this survey along with the departmental surveys to fit an ADAPT model when five years of data are available in the time series.

Is the observer program for summer flounder adequate?

Observers on commercial fishing vessels can perform several different functions, including estimating bycatch and discard rates, estimating underreporting of landed catch, and assisting in tagging programs. In many fisheries, unwanted fish are discarded to the sea after being caught; they are very often killed by the fishing process, although discard mortality rates (the fraction of fish that die from being caught and discarded) vary by species, gear, and handling techniques. Discarding can occur for a number of reasons that fall into the categories of economic discards and regulatory discards. Economic discards occur when the fish are too small for the market, are damaged, or belong to a species for which there is no market or only a limited market. Regulatory discards occur when fish are required by law to be discarded, such as protected species, fish smaller than the legal minimum size, fish caught out of season, fish caught with the wrong kind of gear, and fish for which the fisherman holds no quota. Globally, the discarding rate for all species combined is thought to be approximately 27 million tons per year or about one-quarter of the world catch (Alverson et al., 1994).

An associated problem is landings that are undisclosed or misreported (i.e., catches that are not reported to the authorities or are misreported in terms of species or where caught) and thus cannot be used in assessments. For fishery assessments, it is important to know how many fish are actually removed from the population by fishing (harvested, discarded, and otherwise killed by gear) each year. This is necessary because undisclosed discards or catch may cause an underestimation of fishing mortality rates, and an

Suggested Citation:"Summer Flounder: Review and Insights." National Research Council. 2000. Improving the Collection, Management, and Use of Marine Fisheries Data. Washington, DC: The National Academies Press. doi: 10.17226/9969.
×

underestimation of the benefits of fisheries management. In general, unreported or misreported discards or catches can affect fishery assessments in complex ways that are difficult to understand. However, some insights can be gained by considering the following simple cases.

  • If half of all fish of all ages caught were discarded or landed, but unreported, estimates of the population size would be wrong by 50 percent but the estimates of fishing mortality and biological reference points would be correct. Catch would be underestimated by 50 percent, but if fishermen continued to catch twice as much as they report, this might not matter in a relative sense.

  • If young fish are caught but not landed or reported as bycatch (due to a size-based limit, for example), no fishing mortality will be ascribed to that age. The benefits of reducing exploitation levels or of increasing the age of first capture (e.g., by increasing mesh size) might be underestimated in this case because some of the mortality that these measures would reduce exist but are not currently accounted for.

  • If discarding or underreporting of all ages suddenly increases, this initially could be misinterpreted as a reduction in fishing mortality and vice versa. An increased tendency to discard or underreport old fish would give an impression of decreased fishing mortality on these ages.

The effects of more complex patterns of discarding, which could result from a restrictive quota system, are difficult to predict. One pattern could be more discards for years when the quota is particularly small or a large year-class of small fish enter the fishery. Another pattern could be an increasing level of discarding, particularly of smaller fish (i.e., as bycatch or discarding of smaller fish replaced by larger fish caught later), through time in response to more restrictive quotas. Such a pattern is of particular concern because, if not recorded, it can cause the stock to appear to be suffering increasing levels of fishing mortality and diminishing population size. This could lead to still tighter quotas being imposed and fishermen responding with even more increased discarding or undisclosed landings. Overall, undetected changes in discard rates and non-reporting rates can cause a downward spiraling negative feedback effect on assessments and fish populations.

In the case of the commercial summer flounder fishery, estimates of discards are available from onboard observers (see Table A10 in NEFSC, 1997) and from on-site interviews for anglers (see Table A14 in NEFSC, 1997). Estimates of commercial discards have been available since 1989 and recreational discards have been estimated since 1982. No estimates of the precision of discards by age are available. NMFS told the committee that “for summer flounder, the discard rates reported in the VTRs [vessel trip reports] look comparable to those observed by sea samplers [i.e., observers], but no in-depth study of possible biases has been conducted to date.”

Committee Investigations

In assessing the importance of discards, the mortality of discards is the most important measure, because although discards may form a large part of the catch of the youngest age, it is possible that this discard mortality of young fish does not constitute a large mortality rate. In the case of summer flounder, 75% of age-0 fish caught are discarded and 38% of age-1 fish caught are discarded. In practice, it is this latter percentage that is of far greater importance because it represents a fishing mortality rate of up to about 0.4 (Figure 2-5); only a small number of age-0 fish are caught in the fishery, so the high discard rate is not as significant to the population. The discard mortality is assumed to be 80 percent for the commercial fishery (based on commercial advice to NMFS) and 10 percent for the recreational fishery.

As mentioned earlier, the observer coverage rate for summer flounder has been less than 1 percent for at least the past two years. Figure 3-2 shows that observer coverage below 10-25 per-

Suggested Citation:"Summer Flounder: Review and Insights." National Research Council. 2000. Improving the Collection, Management, and Use of Marine Fisheries Data. Washington, DC: The National Academies Press. doi: 10.17226/9969.
×

FIGURE 2-5 Discard fishing mortality for age-0 and age-1 fish.

cent can yield significant errors in estimates of catch and bycatch.

Actions Needed

Better data are needed regarding the rates of discarding and misreporting of summer flounder in both the directed fishery and other fisheries. At-sea sampling by observers is required to estimate discarding and may, if observers have the confidence of industry, also provide estimates of undisclosed landings. Observer programs are expensive, but can provide a possible supplement to shore-based sampling of landings. NMFS should investigate the scope and reasons for failures to report discards so that management methods decreasing such discards can be devised (e.g., full retention of catch or the use of temporary closed areas to restrict fishing where young fish are abundant, possibly based on industry information). Most groundfish fisheries in the U.S. Northeast region, including the summer flounder fishery, are observed to monitor interactions of the fisheries with marine mammals, although observer coverage is quite low. NMFS should continue to use the observer programs for marine mammals to observe bycatch and misreporting of catches and discards, while considering augmentation of observations specific to the summer flounder fishery. NMFS should consider increasing observer coverage to 25% or more of commercial summer flounder trips for several years to obtain a better estimate of bycatch discards and misreporting in the summer flounder fishery. Conversely, if the survey vessels use non-commercial trawls designed to capture small fish, the surveys can provide data on year-class strength before the fish have attained sizes susceptible to capture in the commercial fishery. This could provide early warning of potential future declines in the summer flounder population.

Can and should state surveys be standardized?

The summer flounder stock assessment is unusual in that a large number of fishery-independent surveys are available to assess the stock. In addition to the three annual NEFSC seasonal surveys, 9 state surveys were used in the 1996 assessment7, and these contributed to 6 abundance indices for flounder ages 1 and older and 6 abundance indices for age-0 fish. The state surveys use different gear and survey design and cover different time periods (see Appendix C). Many of these surveys have undergone changes in gear, survey design, and purpose over time, but all surveys used in the summer flounder stock assessments, including the NEFSC surveys, are given equal weight in the NMFS ADAPT model. Taken together, these surveys cover most of the area fished for summer flounder, with the NEFSC surveys generally covering the more offshore areas and the state surveys covering the inshore areas. The temporal coverage of the various surveys varies from sampling within one month to combining samples taken over a number of months (see Table C-4).

7  

The Delaware state survey was added in the 1999 stock assessment.

Suggested Citation:"Summer Flounder: Review and Insights." National Research Council. 2000. Improving the Collection, Management, and Use of Marine Fisheries Data. Washington, DC: The National Academies Press. doi: 10.17226/9969.
×
Committee Investigations

Trends from the individual surveys are not coincident (NEFSC, 1997). Results from the committee's analyses indicate that some of the surveys display greater lack of fit (residuals) than others, for example, in their catchability (see Figure D-6) and abundance (see Figure D-11).

The committee is not aware of any evaluation of the relationship between the timing of surveys and the movement of summer flounder from offshore to inshore and back. Such an evaluation would be important for interpreting trends of summer flounder in the individual survey series. Standardizing state surveys could increase the value of the data, advance survey methods, and result in increased technical support for data management and statistical analysis.

Actions Needed

The coastwide summer flounder stock assessment might be improved if the state surveys were standardized and coordinated. The committee recommends that NMFS investigate each federal and state survey of the summer flounder stock, including the temporal and spatial coverage of each survey and the nature and quality of the data produced, how the states use the data, and how standardization could affect state interests. However, the committee recognizes that the surveys may have purposes other than contributing to the coastwide stock assessment and that such purposes may make it difficult to standardize and coordinate state surveys.

Is the catch from recreational fishing estimated properly?

Recreational fishing used to be a minimal source of fishing mortality, but current estimates show that anglers contribute substantially to fishing mortality in several fisheries, including summer flounder. Summer flounder is one of several major flatfish species found along the Atlantic coast of the United States (others include winter flounder, American plaice, yellowtail flounder, and witch flounder); only a few are targeted by recreational anglers. Summer flounder are targeted by marine anglers along the U.S. Atlantic coast, with more than 80 percent of the catch usually being taken in the Mid-Atlantic region (Table 2-4). On average, recreational landings accounted for 37 percent of reported summer flounder landings between 1982 and 1996. Recreational landings have increased in their proportion of total landings since 1996, however, having approached and exceeded 50 percent of the total landings (see Figure 2-1 and Table 3-8). Therefore, accurate estimates of recreational effort and catch are important to stock assessments. Since 1979, NMFS has surveyed marine recreational fishing with the Marine Recreational Fisheries Statistics Survey (MRFSS). The survey is conducted in nearly all the coastal counties in the United States. The survey has undergone methodological and statistical scrutiny over the years and has been modified, as needed, to provide more accurate estimates of recreational catch and effort.

Because recreational fishing is geographically dispersed, it is expensive to monitor catches where and when they occur. Nonetheless, acceptable precision in catch and effort estimates can be obtained in some fisheries. Typically, precision of estimates of catch rates depends on the number of anglers sampled, the heterogeneity in the ability of anglers to catch fish, and the temporal and spatial heterogeneity of fish abun-

Suggested Citation:"Summer Flounder: Review and Insights." National Research Council. 2000. Improving the Collection, Management, and Use of Marine Fisheries Data. Washington, DC: The National Academies Press. doi: 10.17226/9969.
×

TABLE 2-4 Recreational Harvest of Summer Flounder (in kilograms) Categorized by Region

Year

North Atlantic

(PSE)

Mid-Atlantic

(PSE)

South Atlantic

(PSE)

Total

Total (PSE)

1981

176,503

(34.4)

4,182,492

(8.9)

221,541

(24.4)

4,580,535

(8.3)

1982

1,047,856

(16.2)

6,492,823

(31.7)

743,837

(23.4)

8,284,516

(25)

1983

599,673

(17.2)

11,838,757

(7.6)

265,938

(21.3)

12,704,368

(7.2)

1984

408,288

(15.7)

7,511,218

(8.2)

624,889

(29.4)

8,544,395

(7.6)

1985

2,667,214

(18.8)

4,906,649

(8.7)

533,753

(43.8)

8,112,385

(8.6)

1986

2,667,214

(18.8)

4,906,649

(8.7)

533,753

(43.8)

8,112,385

(8.6)

1987

606,703

(19.7)

4,823,630

(10.1)

116,437

(10.7)

5,546,771

(9.1)

1988

322,817

(11.6)

6,033,755

(4.5)

292,248

(9.2)

6,648,820

(4.1)

1989

143,213

(17.0)

1,160,805

(7.3)

136,504

(13.3)

1,440,522

(6.3)

1990

106,527

(21.3)

1,984,893

(4.6)

240,872

(10.6)

2,332,292

(4.2)

1991

161,371

(14.2)

3,343,168

(4.5)

195,748

(15.8)

3,700,287

(4.2)

1992

195,183

(12.5)

2,939,005

(4.6)

121,236

(8.1)

3,2476,425

(4.3)

1993

250,376

(11.3)

3,543,703

(4.8)

217,473

(7.1)

4,011,552

(4.3)

1994

444,675

(9.3)

3,576,988

(4.4)

218,234

(7.3)

4,239,898

(3.8)

1995

340,106

(10.1)

2,006,706

(5.7)

112,540

(24.0)

2,496,353

(5)

1996

542,131

(9.3)

3,737,110

(4.4)

193,783

(9.5)

4,473,024

(3.9)

1997

480,033

(13.2)

4,736,561

(4.7)

177,155

(8.8)

5,393,750

(4.3)

1998

911,821

(8.5)

4,529,118

(5.4)

239,410

(8.8)

5,680,349

(4.5)

1999

783,824

(10.9)

2,882,943

(5.1)

136,539

(12.6)

3,803,306

(28.6)

SOURCE: www.st.nmfs.gov/stl/recreational/database/quires/catch/time_series.html , accessed 06/12/00.

NOTE: Weights are expressed in kilograms for summer flounder that were landed whole and identifiable (MRFSS catch type A) and weight of fish caught and filleted, released, dead, or given away and not identifiable, but claimed by the angler to be summer flounder (MRFSS catch type B). These data do not include fish that were caught, released, and may have subsequently died. The North Atlantic region includes recreational landings in Maine, New Hampshire, Massachusetts, Rhode Island, and Connecticut. The Mid-Atlantic region includes landings in New York, New Jersey, Delaware, Maryland, and Virginia. The South Atlantic region includes landings in North Carolina, South Carolina, Georgia, and Florida. PSE = proportional standard error, which is the standard error of an estimate as a percentage of the estimate.

dance. MRFSS attempts to improve precision by increasing the number of anglers intercepted and interviewed. This approach works well in fisheries for which the fish is a popular target for anglers, fish distributions are consistent throughout the fishing season, and anglers predominantly use a few access sites. Summer flounder meets these criteria for acceptable precision in catch statistics for the majority of recreational fishing, with the exception of charter and party boats.

The broad habitat use, wide prey preferences, and seasonal migratory patterns of summer flounder make them vulnerable to many of the modes of recreational fishing, including all access modes surveyed by MRFSS (Table 2-5). The largest catch has come from anglers using private and rental boats, followed by party and charter vessels. Anglers using any of the access modes have equal probability of being contacted through the telephone survey of effort. The probability of intercept surveyors encountering anglers, the method of calculation of catch rate, and the precision of the catch rate differ substantially by access mode.

The private/rental access mode is readily sampled, yields interviews from completed trips, and produces catch estimates that are relatively precise; the proportional standard error of the private and rental access catch estimates is less than 10 percent. Hence, the bulk of the fishery should be sampled well.

The party/charter mode is more difficult to sample because access occurs at fewer sites, and at

Suggested Citation:"Summer Flounder: Review and Insights." National Research Council. 2000. Improving the Collection, Management, and Use of Marine Fisheries Data. Washington, DC: The National Academies Press. doi: 10.17226/9969.
×

very specific times. The proportional standard errors of catch estimates from the charter and party component of catch commonly can be greater than 50 percent (Table 2-5). Moreover, the charter and party component is approximately 20 percent of the catch. The imprecision of this access mode contributes significantly to the imprecision in total catch. Additionally, all anglers on a given party/ charter vessel experience the same fishing conditions, implying that data for individuals on a single angling trip may not be statistically independent. To the extent that fishermen on party and charter vessels fish farther offshore than fishermen using private and rental boats, this could affect the MRFSS assumptions. Aside from increasing the number of anglers interviewed, greater precision can be achieved by developing a specific sampling strategy that better suits the charter and party boat fleets. For example, a telephone survey that used a list frame of charter boat operators or charter boat logbooks would be more efficient than the current MRFSS random-digit dialing approach. This access mode can be surveyed with specifically designed sampling that takes advantage of the set schedules of the party/charter fleets. Occasionally, the interviewer rides on the party boat to obtain interviews. MRFSS acknowledges that charter and party boat catch estimates are a weakness of their sampling strategies and plans to address this issue in the future.

Committee Investigations

The committee did not examine the summer flounder data from the MRFSS, beyond that presented in Table 2-4 and Table 2-5.

Actions Needed

The summer flounder fishery can be used to determine the extent to which other sampling approaches, such as logbooks, observers, and access-site-only surveys could increase the accuracy and precision of catch and catch-rate estimates for charter and party boats. Party and charter fleets are difficult to sample for most fisheries, and innovative approaches tested on summer flounder may be transferable to other fisheries. NMFS should proceed with such tests of the alternative sampling approaches mentioned above.

Another method for increasing precision of recreational data is to sample fishermen directly, rather than sampling the general population in coastal counties. Direct sampling approaches use sampling frames based on lists of fishermen who fish on charter and party boats and who possess a marine recreational fishing license, and by recontacting active anglers identified in earlier random-digit dialing (longitudinal sampling). Other ways for identifying the population of fishermen may increase sampling efficiency. NMFS should work with state agencies and recreational groups to better characterize this growing component of many fisheries. A related question, which the committee did not address, is whether the recreational catch is aged correctly. The typical practice in recreational fisheries that have important commercial components is to obtain length data from recreational access-site surveys and to estimate catch at age from age-length keys derived from the commercial catch. Hence, if there are problems with ageing the commercial catch for a species, the same problems will exist for the recreational catch.

Can the precision of data be improved?

Two main approaches can be used to investigate the precision of assessment data sets.

  1. Make detailed investigations of the statistical characteristics (e.g., means and variability characteristics) of the samples taken in each sampling stratum.8 These within-strata statistical characteristics are then combined to give the overall statistical characteristics of the data used in the assessment models.

  2. Make models that explain separately each of the assessment data sets. The models are constructed with restricted sets of parameters, and estimates of the statistical characteristics of the data are obtained from the deviations of the data from the models.

8  

A stratum is a subarea of a sampling area delineated based on factors such as depth, habitat type, stock areas, and management areas.

Suggested Citation:"Summer Flounder: Review and Insights." National Research Council. 2000. Improving the Collection, Management, and Use of Marine Fisheries Data. Washington, DC: The National Academies Press. doi: 10.17226/9969.
×

TABLE 2-5 Recreational Harvest of Summer Flounder (in kilograms) Along the U.S. Coast, Categorized by MRFSS Access Mode

Year

Shorea

(PSE)

Charterb

(PSE)

Party/Charterc

(PSE)

Private/Rentald

(PSE)

Total

Total(PSE)

1981

1,071,014

(26.4)

   

863,184

(15.5)

2,646,337

(8.1)

4,580,535

(50.0)

1982

478,576

(16.9)

   

3,210,890

(63.6)

4,595,050

(7.4)

8,284,516

(87.9)

1983

1,454,854

(38.5)

   

2,294,449

(12.5)

8,955,065

(7.4)

12,704,368

(58.4)

1984

522,469

(15.3)

   

1,452,482

(16.2)

6,569,443

(9.1)

8,544,394

(40.6)

1985

376,621

(21.1)

   

631,049

(19.1)

4,657,614

(12.9)

5,665,284

(53.1)

1986

747,884

(32.4)

6,664

(89.1)

1,137,156

(18.6)

6,220,680

(9.9)

8,112,385

(150.0)

1987

195,216

(19.9)

6

(93.7)

1,101,880

(37.7)

4,249,669

(6.6)

5,546,771

(157.9)

1988

479,496

(9.8)

21

(102.1)

776,599

(9.7)

5,392,703

(4.8)

6,648,819

(129.4)

1989

113,347

(15.9)

79

(62.5)

124,870

(11.2)

1,202,226

(7.3)

1,440,522

(96.9)

1990

149,112

(16.6)

7

(63.3)

264,914

(8.4)

1,918,259

(4.7)

2,332,292

(93.2)

1991

343,043

(15.3)

122

(42.4)

370,028

(8.2)

2,987,103

(4.7)

3,700,287

(70.5)

1992

171,808

(15.1)

53

(47.2)

267,940

(10.2)

2,806,624

(4.7)

3,246,425

(77.2)

1993

183,799

(12.3)

30

(99.2)

626,812

(14.3)

3,200,911

(4.5)

4,011,552

(130.3)

1994

257,073

(8.0)

1,309

(17.4)

512,025

(8.4)

3,469,490

(4.5)

4,239,897

(38.3)

1995

152,180

(10.8)

488

(51.2)

184,529

(16.3)

2,122,155

(5.6)

259,352

(83.9)

1996

121,315

(12.1)

2,132

(42.1)

422,562

(8.8)

3,927,016

(4.3)

4,473,025

(67.3)

1997

163,432

(12.8)

1,034

(39.5)

729,456

(9.7)

4,449,827

(4.9)

5,393,749

(66.9)

1998

246,294

(11.2)

1,088

(29.9)

296,891

(11.8)

5,136,077

(4.9)

5,680,350

(57.8)

1999

178,486

(12.7)

94

92.9

304,421

(11.5)

3,320,303

(5.0)

3,803,304

(112.1)

SOURCE: http://www.st.nmfs.gov/stl/recreational/database/quires/catch/time_series.htm and http://www.st.nmfs.gov/stl/recreational/survey/glossary.html, accessed 06/07/2000.

NOTE: PSE = proportional standard error, which is the standard error of an estimate as a percentage of the estimate. Weights are expressed in kilograms, with the proportional standard error of the harvest values (MRFSS Catch Types A and B1) given in parentheses. These data do not include fish that were caught, released, and may have subsequently died. NMFS cautions that care should be exercised in using MRFSS weight data because weight estimates are minimums and may not reflect the actual total weight landed or harvested.

a Shore fishing includes all direct use of natural shorelines and artificial structures attached to the shore, such as piers, docks, jetties, and breakwaters.

b A charter boat is “a boat operating under charter for a price, time, etc. It is operated by a licensed captain and crew and the participants are part of a pre-formed group of anglers. Thus, charters are usually closed parties, as opposed to the open status of head boats.” These data include catches from charter boats operating in the South Atlantic and Gulf of Mexico regions.

c The harvest values in this column refer to landings from charter boats and party boats in the North Atlantic and Mid-Atlantic regions. A party boat is “a boat on which fishing space and privileges are provided for a fee. The vessel is operated by a licensed captain and crew.”

d Rental and private boats include rentals and private use in the North, Mid-, and South Atlantic regions. Rental boats are boats that are rented without crew and are operated by the renter.

Suggested Citation:"Summer Flounder: Review and Insights." National Research Council. 2000. Improving the Collection, Management, and Use of Marine Fisheries Data. Washington, DC: The National Academies Press. doi: 10.17226/9969.
×

These approaches can be combined, with various degrees of reliance on data and models. The first approach requires access to the elements of the sampling data and a clear knowledge of how the samples are extrapolated to the overall figures. Although this is usually straightforward for survey data, sampling of commercial fisheries often contains gaps that have to be filled with default assumptions (e.g., the catch of a fleet in a given month was not sampled but was assumed to be like that of the same fleet in a different month). Such decisions often have to be based on professional judgement, although cross-validation techniques can be used to calibrate the likely accuracy of gap-filling decisions. These assumptions can be difficult to track in retrospect unless they are very well documented, although some information can be salvaged by interviewing the staff that handled the data.

An example of the second approach has been provided by Shepherd and Nicholson (1991), who developed a simple linear modeling approach that can be applied directly to survey catch rate-at-age data, weight-at-age data, and catch-at-age data. They noted that much of the systematic changes in the catch rate from a survey should be explained by the product of an age factor (to account for differential catchability across ages and the progressive decline of numbers with age due to fishing and natural mortality) and a year-class factor (to account for differences in the sizes of cohorts). This model is readily applied using an analysis of variance approach that fits age and year-class factors to a logarithmic transformation of the survey data. The lack of fit between the model and the data is ascribed to sampling error that is usually taken to have a log-normal distribution. However, a range of alternative sampling distributions can be fitted if a generalized linear model is used.

Weight-at-age data also can be investigated with similar models, again with age factors (to account for growth), year-class factors (to account for systematic changes in growth between year classes), and possibly year factors (to account for changes in growth between years) fitted to log-transformed data. Again, any lack of fit is ascribed to sampling error. The same approach also can be adopted for catch-at-age data using age and year-class factors to explain sources of systematic variation seen in the survey catch rate data and a year factor (to account for annual changes in fishing mortality rate as well as systematic changes through time).

The second approach is easier to apply in retrospect, but it suffers from the problem that the simple models adopted may not capture all the variation that results from systematic changes (e.g., changes in fishing patterns in catch-at-age data). Such variation may thus be ascribed incorrectly to sampling variation when it is a real signal in the data, inflating statistical variation and giving an impression of greater uncertainty than is the case. Most obviously, catch-at-age data may be affected systematically by changes in the selectivity of the fishing gear or the fishing practice of the commercial and recreational fishermen, or by the changing balance of catch between these users. Some of these changes can be examined by adding terms to the simple models described above, but others may be wrongly ascribed to variation in the data due to sampling variation. This can increase sampling variation and this possibility needs to be considered in the interpretation of the results. With this caveat the approach provides a simple means to examine the quality of the several data sets taken individually.

Committee Investigations

The analysis of design efficiency presented in Appendix C is an example of the detailed approach described above. In this analysis the stratified random designs used in the three NEFSC seasonal surveys in 1995 and the winter survey in 1996 were evaluated. In the first part

Suggested Citation:"Summer Flounder: Review and Insights." National Research Council. 2000. Improving the Collection, Management, and Use of Marine Fisheries Data. Washington, DC: The National Academies Press. doi: 10.17226/9969.
×

of the analysis, the winter, spring, and fall surveys for 1995 were evaluated with respect to the application of the stratified design. A number of the strata in these surveys had only one station allocated to them and hence did not contribute to the variance estimates of the survey indices. In fact, sampling intensity is quite low for all of these surveys given the area covered, especially in the case of the 1995 fall survey, in which 48 out of a total of 56 strata had only one or two tows assigned to them.

Design efficiency is a measure of how much the survey design has contributed to increasing the precision of the survey estimates. In the case of the stratified random design, two components contribute: the strata boundaries and the number of tows allocated to each stratum. The efficiency of the design for the 1995 surveys could not be evaluated because of the occurrence of strata with only one tow. Instead, the winter survey in 1996 was evaluated for each of three major flounder species caught in the survey—summer flounder, winter flounder, and yellowtail flounder. The contribution of the strata component to the imprecision was substantial for all three species, indicating that the current strata boundaries are related to differences in the distribution of the flounder species. However, the allocation of tows to strata was not optimal with respect to precision and, in fact, worked against the advantages contributed by the strata component. Our investigations indicated that the precision of the survey estimates could be improved by basing the tow allocation scheme for the current year's survey on a combination of the previous year 's survey results and the grouping of strata based on species distributions. The committee did not have the time to extend its analysis to the other years of the survey, but believes that such analyses would be very useful.

The analysis of variance model approach was used to examine suitable survey data sets (some only provided data for one age, which is not sufficient for fitting the model), the weight-at-age data, and the overall catch-at-age data used in the summer flounder assessments. Table 2-6 shows the estimate of the standard deviation of the log(e)-transformed data, by age and overall, for the various survey series and for the weight-at-age and catch-at-age data of each data set both by age and overall. The standard deviation of the log-transformed data approximates to the coefficient of variation of the untransformed data series when it has small values, so these figures (if converted to percentages) give a first approximation of the coefficients of variation of the various data sets.

Table 2-6 suggests that the individual surveys are rather variable. In practice they are used collectively to tune the virtual population analysis and indices used are all assigned the same weight.9 Hence, it is some average of their values rather than their individual values that affects the assessment outcome. Table 2-6 shows an attempt to estimate their combined logarithmic standard deviation (calculated as the square root of the average variance from all the surveys). Note that individually, the NEFSC fall and winter surveys are more variable than the combined survey estimate. The weight-at-age data seem to have lower standard deviations of log-transformed data, which is adequate for most assessments.

Note, in particular, on a survey-by-survey basis, that the NEFSC fall and winter survey estimates are less variable than those of the state surveys. However, the combined survey estimates sometimes have half the standard deviations of those given for the NEFSC survey estimates alone. Clearly, some judgment has to be made about the precision, quality, and consistency of the survey estimates before they are averaged into a combined estimate for the assessment. By their nature, these data are susceptible to systematic variations that the simple model used here cannot capture. These variations are particularly likely to affect the catch numbers of

9  

Because of differing precision at different ages, some indices might be more informative at some ages than are other indices. Assessment methods are available to estimate unique weightings among surveys for each age group.

Suggested Citation:"Summer Flounder: Review and Insights." National Research Council. 2000. Improving the Collection, Management, and Use of Marine Fisheries Data. Washington, DC: The National Academies Press. doi: 10.17226/9969.
×

TABLE 2-6 Comparison of Federal and State Survey Precision in Terms of Standard Deviation of Log(e)-Transformed Data

Data Set

Age

         
 

0

1

2

3

4

Overall

Survey catch rate-at-age data

 

NEFSC fall survey

n/a

n/a

0.42

0.54

0.46

0.46

MADMF fall survey

n/a

n/a

n/a

0.77

0.77

0.75

NEFSC winter survey

n/a

0.58

0.55

0.50

n/a

0.51

NJDF survey

n/a

0.82

0.82

n/a

n/a

0.79

CTDEP fall survey

n/a

n/a

0.85

0.60

0.60

0.68

CTDEP spring survey

n/a

n/a

0.67

0.58

0.53

0.58

NEFSC spring survey

n/a

0.73

0.62

0.64

0.65

0.65

MADMF spring survey

n/a

n/a

0.76

0.76

n/a

0.75

Combined survey estimate

n/a

0.41

0.26

0.24

0.27

0.23

Commercial data

 

Weight-at-age data

0.19

0.11

0.11

0.13

0.14

0.14

Catch numbers-at-age data

0.40

0.30

0.24

0.29

0.39

0.32

Catch numbers-at-age dataa (omitting age 0)

n/a

0.30

0.21

0.25

0.34

0.27

NOTE: Cells with “n/a” are for those ages with no available data. CTDEP = Connecticut Department of Environmental Protection; MADMF = Massachusetts Department of Marine Fisheries; NEFSC = Northeast Fisheries Science Center; NJDF = New Jersey Department of Fisheries.

a Omitting the age-0 catch numbers affects the values for older ages because of the way the ANOVA model used in this analysis fits factors for age and year-class to the data. Numbers at age 0 are highly variable and influence the overall estimates of variance for each combination of age and year-class.

the youngest ages. It is noticeable that the apparent coefficient of variation of these data improves if the age-0 fish are omitted from the analysis. It is thus possible that the estimates in the table present a pessimistic view of data quality. It would be worthwhile for NMFS to investigate further the quality of these data.

Actions Needed

The efficiency analysis conducted by the committee was based on only two years of surveys. A similar analysis over a longer time series needs to be done to understand the persistence of the distribution patterns for summer flounder, as well as other species routinely caught by the survey. This information could be used to develop compromise allocation schemes to produce near-optimal results similar to those presented in Appendix C. If spatial patterns are not predictably persistent over time, the application of other methods, such as adaptive allocation techniques (Thompson and Seber, 1996), need to be explored.

Simple tests of the quality of the assessment data suggest that the precision of the weight-at-age data are adequate but that the survey and to a somewhat lesser extent the catch-at-age data could be made more precise. As an initial step, the committee recommends that NMFS routinely calculate the variance associated with its routine catch-at-age data sampling.

The committee recommends that constructions of estimated variance for commercial samples be included in the NMFS computer programs used to estimate quantities from these samples and that assessments of the statistical quality of results be given with the estimates. This will help to ensure that sampling schemes are performing appropriately and that the sampling design is relevant and provides the best value for the sampling effort. Moreover, it produces a value that is helpful in monitoring data

Suggested Citation:"Summer Flounder: Review and Insights." National Research Council. 2000. Improving the Collection, Management, and Use of Marine Fisheries Data. Washington, DC: The National Academies Press. doi: 10.17226/9969.
×

quality, and if necessary, defending appropriate sampling levels and methods. The problem of missing data is particularly acute in the VPA method, but some other methods are better able to deal with missing data, highlighting the usefulness of analyzing data with multiple models.

QUESTIONS RELATED TO THE INFORMATION CONTENT OF THE MODEL AND MODEL ASSUMPTIONS CURRENTLY IN USE

What information does each model structure require and how do these requirements relate to information in the data?

The structure and assumptions used in formulating a population model influence the modeling results in ways that are not always documented by stock assessment scientists and recognized by fishery managers and factored into their management decisions. The choice of a model and assumptions leads to some level of modeling uncertainty, as illustrated through the committee's re-analysis of summer flounder data. Much of the model structure and assumptions are derived from basic beliefs and principles, such as the notion that fish in a certain area are members of a closed population and that they die due to natural causes in proportion to the number present. However, some of the “information” or structure is imposed to provide estimates that cannot be derived from the data or to overcome deficiencies in knowledge about the system. Such imposed structure often includes assumptions that selectivity or catchability is constant through time, that natural mortality is constant and known, and that the same fishing mortality applies over the entire region. Although we know that such assumptions are unlikely to be completely valid, we cannot avoid making assumptions of some kind. We can, nevertheless, explore the sensitivity of our results to the choice of model and model assumptions by exploring the sensitivity of outputs to alternative models, alternative assumptions, and reasonable variations in the input data. Some of the assumptions have been examined earlier in this section. Here we consider how much difference in model formulations (with their typical assumptions) influence the outcome of assessment, including three related questions: (1) What weight does the model give to the different pieces of information that come into it? (2) How is uncertainty expressed in the assessment output provided to managers? and (3) Under what level of precision can managers expect to manage with data currently available?

Committee Investigations

Three alternative assessments of summer flounder—using VPA, ADAPT, and CAGEAN models—were carried out independently by different analysts and were compared with the NMFS 1999 assessment, which was done with an ADAPT model (see Appendix D). The same data were used in these alternative assessments, but the outputs from each of the models differed in response to the assumptions and structure of the model chosen (see Figure D-4 and Figure D-5). The effects of a number of alternative assumptions were explored, including different weightings of the data components and whether the selectivity curve fell or remained constant for older ages.

The most significant difference among model results occurred as a function of the assumed consistency in fishing mortality across years. In the tuned VPA-type models, variations in F were controlled by the degree of shrinkage10 of estimated Fs to the terminal F. In the CAGEANtype model, the variation of fishing mortality was influenced by the assumption of constant selectivity. The ADAPT model apparently does not use shrinkage and could be emulated by the LaurecShepherd VPA model (by eliminating weighting towards recent years and by not using any shrinkage). However, the CAGEAN-type model in its pure form cannot reproduce the NMFS results, because of its different treatment of F, in that it interprets catch-at-age data in terms of F values that depend on an age-varying selectivity (constant through time) and a year-varying fishing intensity (constant over ages)— the so-called separability assumption. The VPA-based Laurec-Shepherd and ADAPT models, by contrast, allow fishing mortality rate to vary more flexibly but the price is that catch-at-age data have to be treated as though they are exact, which they are not (since F is based on a sampling process).

10  

Shrinkage is the term for a procedure that gives some weight to the assumption that fishing mortality rate (and in some models also population numbers) are unchanging through the recent past. Thus, the values at age for the most recent year may be estimated from the values at age of past years, which are known with more confidence due to convergent properties of the VPA.

Suggested Citation:"Summer Flounder: Review and Insights." National Research Council. 2000. Improving the Collection, Management, and Use of Marine Fisheries Data. Washington, DC: The National Academies Press. doi: 10.17226/9969.
×

Model results also differ as a function of each model's assumptions about how the population estimates should be interpreted given the data. For data collected after 1994, a marked decline is seen in the commercial catch numbers of age-0 fish and a smaller decrease in the catch numbers of age-1 fish. In the survey data from years after 1994, the numbers of age-0 and age-1 fish are equal to or greater than the number in preceding years. The models may interpret these changes in one of two ways, depending on the model assumptions. In one sense, the relative number of young fish may be decreasing, as evidenced by the commercial catch-at-age data. This is the interpretation resulting when selectivity and fishing mortality at age are assumed to remain constant (as in the CAGEAN model). Conversely, there may be a shift to a lower selectivity in the fishery for the younger age classes. Such a shift to catching less young (small) fish might be expected from the increase in mesh size and minimum size regulations and seems to be corroborated by the survey's data regarding relative abundance data. The resulting estimates, if there is lower selectivity for young fish in the fishery, would be more optimistic and the overall population size could be interpreted as being larger than if fishing mortality and selectivity at age had not changed. Under the latter assumption, the population is estimated to have held steady or decreased. The actual situation probably falls somewhere between the two extremes, because neither assumption is entirely valid. The risks for the fishery are very different under the two hypotheses, so the consequences of both should be explored.

Some investigation is possible using the wellknown properties of VPA estimates. Fishing mortality rates estimated by this approach for younger ages in earlier years can be regarded as being relatively unbiased by assumptions. Consequently, graphing the ratio of the fishing mortality at each age to the average of the fishing mortality rate on 2- and 3-year-olds can be used to track changes in age-specific catchability. Summer flounder mortality estimates seem to indicate that the catchability of younger fish has decreased since 1982 and to have dropped sharply since 1994 (Figure 2-6). The results for 1995 and 1996 are likely to be fairly robust to assumptions made in 1998, but the 1997-1998 results are still strongly influenced by the assumptions made in that year (as indicated in the discussion above). A second approach uses analysis of variance of catch-at-age data to look for evidence of changing catchability (extending the approach of Pope and Shepherd [1982]), but in the case of summer flounder this analysis was inconclusive.

A final suggested alternative would be to divide catch into the survey catch-per-unit-effort index to derive a measure of effort that should be linearly related to annual fishing mortality, selectivity, and survey catchability, and analyze the variance of these values. In all three approaches, departures from simple linear models can indicate trends that might not otherwise be noted in the nonlinear assessment model results.

A quick example of this method using the total catch-at-age data by year (C[a,y]) divided by the NEFSC spring catch-at-age data for ages 1-3 and their winter survey for the 0 groups by year (U[a,y]) is presented in Table 2-7.

In applying this last method to the summer flounder data, the total catch-at-age data from the fleet (C(a,t) is divided by the NEFSC spring catch-at-age data (U(a,t)) for ages 1-3 and the winter survey for the age-0 groups. The quotient

Suggested Citation:"Summer Flounder: Review and Insights." National Research Council. 2000. Improving the Collection, Management, and Use of Marine Fisheries Data. Washington, DC: The National Academies Press. doi: 10.17226/9969.
×

FIGURE 2-6 Summer flounder age-based selection relative to fishing mortality of age-2 and age-3 fish.

C(a,t)/U(a,t) is approximately s(a)f(t)/q(a), where s(a) and f(t) are the selectivity at age a and full recruitment fishing mortality at year t for the fleet and q(a) is the catchability at age of the survey. Consequently, an ANOVA applied to the log ratio should be able to distinguish a year effect (log(f(t))), and an age effect (log(s(a)/q(a)). As a broad check for interactions, we divide the log ratio matrix into four quadrants (young ages-early years, old ages-early years, young ages-late years, old ages-late years) such that the sum of quadrant 1 and 4 minus the sum of quadrants 2 and 3 equal zero. From this an estimate is derived with one degree of freedom that indicates how selectivity at age in the commercial catch may be changing in earlier versus later years relative to the survey catchability at age, which we assume to be constant (Figure 2-6).

The significant quadrant effect shown in Table 2-7 indicates that there is a systematic shift in s(a)/q(a) through time. Assuming that q(a) in the survey is constant, we must conclude that s(a) is changing. This would indicate that the constant selection hypothesis must be rejected. If the constant selectivity assumption of the CAGEAN model is relaxed to allow selectivity to change progressively through time, the NMFS results are better approximated. This echoes what the ANOVA above indicates—the consistency of selectivity is a key assumption and certainly critical in determining biomass and fishing mortality estimates for the most recent years.

TABLE 2-7 Analysis of Variance Table for Survey Data

Source of Variation

Degrees of Freedom

Sums of Squares

Mean Squares

F-statistic Value

Probability Value

Age (a)

3

59.30

19.77

35.20

0.0000

Year (y)

15

28.08

1.87

3.33

0.0010

Quadrant

1

7.94

7.94

14.14

0.0005

Residual

43

24.14

0.56

   

Total

62

119.46

1.93

   
Suggested Citation:"Summer Flounder: Review and Insights." National Research Council. 2000. Improving the Collection, Management, and Use of Marine Fisheries Data. Washington, DC: The National Academies Press. doi: 10.17226/9969.
×

BOX 2-3

Committee's Conclusions About Assumptions Used in Summer Flounder Assessments and Recommendations for Studies Needed

Using the virtual population analysis model runs, the committee concludes that:

  • the common assumption that the natural mortality rate M = 0.2 is reasonable, given knowledge of natural mortality rates for other flounder species and data from the summer flounder stock. As long as M is not changed from year to year its effects on total allowable catch and spawning stock biomass are small.

  • if older summer flounder are substantially less susceptible to trawl gear used in fisheryindependent surveys than younger fish (NMFS assumes constant catchability over all ages), the fishing mortality rate would be substantially lower than estimated by NMFS (but would still not attain the target for summer flounder) and the total allowable catch and spawning stock biomass would be substantially higher.

  • if the NMFS values of summer flounder ages are correct and the North Carolina values are incorrect, the spawning stock biomass and TAC would be lower than if both are assumed to be correct. Conversely, if the North Carolina values are correct and the NMFS values are incorrect, spawning stock biomass would be higher.

  • the statistical precision of the winter flatfish surveys (including summer flounder) could be increased by allocating sampling effort in a more efficient manner.

Through other analysis and committee discussions, the committee concludes that:

  • data available as part of the stock assessment were not adequate to determine whether the summer flounder population should be managed as a single unit.

  • females are 20 percent larger than males at 5 years of age. This probably would not result in a major difference in the assessments now, because few flounder survive to age 5. However, if the fishery recovers to a broader age distribution, this sexual dimorphism may need to be considered in summer flounder stock assessments.

  • the level of observer coverage in the fishery is probably too low to estimate discards accurately. Accurate estimation of discard levels is important because discard mortality is so high (assumed to be 80% for the commercial fishery) that a discarded fish has almost the same effect on the fishing mortality level as a fish that is landed.

  • it is not apparent whether state surveys are timed and located to coincide with annual movements of summer flounder; it is therefore impossible to determine whether these surveys should be standardized.

  • some components of recreational catch, particularly for the charter and party boat components, are relatively imprecise.

The committee recommends actions in each case that NMFS could take to improve data and/or assumptions used. Priorities for changes in the summer flounder stock assessments and for research related to this species are discussed in the following points (not listed in priority order).

  1. One of the major disagreements between NMFS and commercial fishermen regards whether the methods and locations of NEFSC trawl surveys undersample large summer flounder. Because this is a major area of contention and the committee's simulations show that incorrect assumptions could substantially affect the actual values for fishing mortality and spawning stock biomass, it is important that NMFS and industry work together to resolve this issue. The following steps should be considered:

    1. Use data for fish up to 7 years of age, rather than 4 years, to constrain (tune) the model.

    2. Use tagging studies to determine whether the average life expectancy of fish is greater than the average age of fish caught in surveys, indicating the existence of a greater number of older fish in the population than observed in the surveys.

    3. Conduct sampling surveys of flounder eggs to determine whether the total biomass of spawning females is greater than assumed.

    4. Study whether the ability of survey trawl gear to catch large summer flounder is overestimated, thus underestimating the biomass of these fish.

  2. Conduct catch-at-age or catch-at-length analyses that explicitly account for sex differences in size at age and selectivity at age.

  3. Find a way to include commercial effort data in the stock assessments and use commercial vessels to assist in conducting summer flounder surveys, as exemplified by the Canadian sentinel surveys and adaptive sampling exercises.

  4. Increase observer coverage to improve the accuracy and precision of estimates of bycatch, discard rates, and landings of summer flounder, as well as to assist in tagging programs.

  5. Determine how state surveys are designed, timed, and conducted in relation to summer flounder migrations. Use the findings of this exercise to determine how best to use the state survey data in the summer flounder assessment.

  6. Increase the precision of recreational catch and effort estimates, perhaps by finding ways to better identify participants for inclusion in sampling surveys.

  7. Maximize the precision of trawl surveys relative to sampling objectives, as demonstrated in Appendix C.

  8. Work with fishermen and processors to ensure that larger (sushi market) flounder are properly sampled.

Box 2-3 contains a summary of the committee's findings and recommendations related to analyses of the summer flounder assessments and data.

POSSIBLE IMPROVEMENTS TO THE SUMMER FLOUNDER DATA SETS

Based on the answers to the questions in Box 2-1, a number of actions could be taken to improve data collection. Actions could include

Suggested Citation:"Summer Flounder: Review and Insights." National Research Council. 2000. Improving the Collection, Management, and Use of Marine Fisheries Data. Washington, DC: The National Academies Press. doi: 10.17226/9969.
×

using tournament data, conducting social and economic studies of commercial and recreational fishermen, employing commercial fishermen as collectors, and improving observer program data. Any changes in the sampling intensity should be judged in the context of the contribution of specific data to the overall assessment and management of the species and the value of improving management of summer flounder in relation to the management of other species.

Suggested Citation:"Summer Flounder: Review and Insights." National Research Council. 2000. Improving the Collection, Management, and Use of Marine Fisheries Data. Washington, DC: The National Academies Press. doi: 10.17226/9969.
×

FIGURE 2-7 Virginia saltwater fishing tournament (summer flounder citations). Numbers indicate the minimum weight in pounds for a citation to be awarded.

Suggested Citation:"Summer Flounder: Review and Insights." National Research Council. 2000. Improving the Collection, Management, and Use of Marine Fisheries Data. Washington, DC: The National Academies Press. doi: 10.17226/9969.
×
Tracking and Management of Data Precision

In stock assessment, it is usually a mistake to measure one input (e.g., commercial or survey CPUE) with great precision, while only measuring other inputs approximately. The precision of all inputs, therefore, should be reviewed. Matching precision of inputs can be difficult because inputs often are sampled by different federal and state bodies drawing on different budgets and using different methods. Moreover, sampling for summer flounder is often carried out in conjunction with sampling for other species, with the result that costs are shared and optimization of sampling needs is viewed in terms of the wider program.

To achieve the greatest possible precision of outputs, we suggest that precision targets be set for all sampling schemes and that performance be monitored annually in terms of achieving these targets. If this is done at the same time that data are analyzed, the requirement need not increase the time needed to conduct assessments. Such tracking of precision would assist year-to-year adjustments of data sources to keep them in appropriate balance. A useful exercise would be to conduct a thorough sensitivity analysis to evaluate how a given percentage change in the precision of each of the different inputs is translated by the assessment model into changed precision in the assessment outputs. Coefficients of variation (CVs) between 10 percent and 20 percent for composite estimates (e.g., total catch at age over significant ages, or combined survey indices of abundance) would be appropriate for most important fish stocks, at present, though changing needs and problems could require greater precision. Precision of the subcomponents of such measures could also be set to achieve this target range of CVs. For example, in the case of components of catch-at-age data, this would require a CV that is approximately equal to the overall target CV/(the square root of the catch share) (Pope, 1983). For example, if the target were a 10 percent CV, a fleet sector that caught 25 percent of the catch should require a CV of 20 percent, that is, half the precision.

A Role for Commercial and Recreational Fishermen in Data Collection

The summer flounder fishery clearly demonstrates the need for better cooperation among recreational anglers, commercial fishermen, state agencies, and federal agencies in data collection so that data quality is improved and its credibility is enhanced. There is room for improvement from all sectors. Strengthening the active support of commercial fishermen is particularly important. Chapter 3 and Chapter 4 include discussions of and suggestions about different ways that commercial fishermen could be enlisted to help with surveys and improve the quality of commercial fishery-dependent data.

Summer flounder fishermen related several concerns about data collection methods currently in use. In other fisheries, mistrust between fishermen and scientists has been greatly reduced by their joint participation in data collection efforts: fishermen provide boats and advise on appropriate gear types and deployment procedures and scientists gain “indigenous” familiarity with the fishery and get to explain quality control limitations with which they must be concerned. Summer flounder fishery managers should examine these partnership approaches and test a working model in the summer flounder fishery.

New Sources of Recreational Fishery Data

Actions also could be taken to increase the participation of anglers in collection of summer flounder data. Anglers, who are particularly important in summer flounder population dynamics, participate in at least two types of programs that reward the catch of large fish and keep track of either exact fish weights (tournaments) or weights above a certain pre-set value (citations). Ancilliary data exist in state citation programs, and data from tournaments could provide qualitative trends in fish abundance. Many of the At -

Suggested Citation:"Summer Flounder: Review and Insights." National Research Council. 2000. Improving the Collection, Management, and Use of Marine Fisheries Data. Washington, DC: The National Academies Press. doi: 10.17226/9969.
×

lantic Coast states sponsor citation programs in which anglers are awarded citations for fish that are at or above specific weights. The Virginia program is the best example of this type of data (Figure 2-7). The Virginia Saltwater Fishing Tournament records the number of citations awarded each year to anglers catching trophysize fish of specified weights. Most other state programs are much smaller.

Citation data must be used with care because the effort of obtaining these trophy fish is not included and the weight that defines a trophy fish has changed over the years. Nonetheless, these data do reveal general trends in the abundance of large fish. For summer flounder there was a general increase in the number of trophy citations from 1958 to 1977, even with an increase in minimum citation weight. From 1978 to 1992, the number of citations remained low, despite a lowering of the minimum weight for a citation fish. Since the early 1990s, the number of citations awarded has increased dramatically. The trends in citations reflect, to some extent, the availability of large fish in the population. Although commercial fishermen expressed concern that older fish were potentially missed by NEFSC trawl surveys, some of these fish are apparently distributed near shore, where they are captured in recreational fisheries, primarily in the summer. A comparison of such data with commercial catch data from the same area could be used to crosscheck data sources and assumptions. Tournaments provide another auxiliary source of recreational data, particularly snapshots of the abundance of legal-size fish at particular times and locations.

Suggested Citation:"Summer Flounder: Review and Insights." National Research Council. 2000. Improving the Collection, Management, and Use of Marine Fisheries Data. Washington, DC: The National Academies Press. doi: 10.17226/9969.
×
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Suggested Citation:"Summer Flounder: Review and Insights." National Research Council. 2000. Improving the Collection, Management, and Use of Marine Fisheries Data. Washington, DC: The National Academies Press. doi: 10.17226/9969.
×
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Suggested Citation:"Summer Flounder: Review and Insights." National Research Council. 2000. Improving the Collection, Management, and Use of Marine Fisheries Data. Washington, DC: The National Academies Press. doi: 10.17226/9969.
×
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Suggested Citation:"Summer Flounder: Review and Insights." National Research Council. 2000. Improving the Collection, Management, and Use of Marine Fisheries Data. Washington, DC: The National Academies Press. doi: 10.17226/9969.
×
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Suggested Citation:"Summer Flounder: Review and Insights." National Research Council. 2000. Improving the Collection, Management, and Use of Marine Fisheries Data. Washington, DC: The National Academies Press. doi: 10.17226/9969.
×
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Suggested Citation:"Summer Flounder: Review and Insights." National Research Council. 2000. Improving the Collection, Management, and Use of Marine Fisheries Data. Washington, DC: The National Academies Press. doi: 10.17226/9969.
×
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Suggested Citation:"Summer Flounder: Review and Insights." National Research Council. 2000. Improving the Collection, Management, and Use of Marine Fisheries Data. Washington, DC: The National Academies Press. doi: 10.17226/9969.
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Suggested Citation:"Summer Flounder: Review and Insights." National Research Council. 2000. Improving the Collection, Management, and Use of Marine Fisheries Data. Washington, DC: The National Academies Press. doi: 10.17226/9969.
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Congress has promoted fisheries science for over a century and its involvement in fisheries management took a great leap forward with passage of the Fisheries Conservation and Management Act of 1976. In the past decade, Congress has requested advice from the National Research Council (NRC) on both national issues (e.g., individual fishing quotas and community development quotas) and the assessments related to specific fisheries (Northeast groundfish). This report was produced, in part, in response to another congressional request, this time related to the assessments of the summer flounder stocks along the East Coast of the United States. Following the initial request, the NRC, National Marine Fisheries Service (NMFS), and congressional staff agreed to broaden the study into a more comprehensive review of marine fisheries data collection, management, and use.

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