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Review of Northeast Fishery Stock Assessments (1998)

Chapter: General Review of Northeast Groundfish Stock Assessments

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Suggested Citation:"General Review of Northeast Groundfish Stock Assessments." National Research Council. 1998. Review of Northeast Fishery Stock Assessments. Washington, DC: The National Academies Press. doi: 10.17226/6067.
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2
GENERAL REVIEW OF NORTHEAST GROUNDFISH STOCK ASSESSMENTS

But that dread of something after death, the undiscover'd country from whose bourn no traveler returns, puzzles the will and makes us rather bear the ills we have than fly to others that we know not of.

William Shakespeare, Hamlet

Stock assessment is the science of data collection, analysis, and modeling that provides the basis for prudent, sustainable exploitation of fishery resources. It includes the provision of scientific advice about management strategies used to exploit fish stocks and the integration of science and scientific advice into the management process. In particular, the feedback between stock assessment and fisheries management has to be included to manage fisheries effectively.

A recent National Research Council (NRC) report Improving Fish Stock Assessments (NRC, 1998) reviewed the state of existing knowledge about stock assessment and made ten recommendations to improve the process (Box 2.1). The first recommendation is that a complete stock assessment should include five major topics: stock definition; data; assessment model; policy evaluation; and communication of results to managers and stakeholders. A checklist of items that should be in a stock assessment is given in Box 2.2. These recommendations provide a benchmark against which fishery stock assessments can be measured. The committee considered the framework of recommendations from the earlier NRC report presented in Boxes 2.1 and 2.2.

The approach of this committee was to examine the Northeast groundfish stock assessments against well-defined standards of quality. This investigation was a multistage process:

  1. The committee first examined the data collection protocols and assessment models used. In particular, the following issues were evaluated: whether appropriate data were collected; whether stock assessments could be replicated; whether alternative models were used or should have been used; and whether forecasts of future populations were appropriate. In this chapter, the general results of this examination are presented, which are of interest to a general audience. Recommendations regarding technical details of stock assessments of interest to specialists are given in Chapter 3.

  2. The committee then compared the assessments against approaches used around the world. The idea behind this comparison was to determine whether other stock assessment processes were qualitatively better than those for the Northeast fishery.

  3. The committee evaluated the Northeast groundfish stock assessments against the NRC (1998) recommendations for improving stock assessment. Given that the earlier report was in press and had not been seen by stock assessment scientists, ours was a particularly severe test of the Northeast fishery assessments, which did not have the NRC guidelines.

Suggested Citation:"General Review of Northeast Groundfish Stock Assessments." National Research Council. 1998. Review of Northeast Fishery Stock Assessments. Washington, DC: The National Academies Press. doi: 10.17226/6067.
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Box 2.1: Recommendations of the NRC 1998 Report, Improving Fish Stock Assessments

  1. Stock assessment scientists should conduct complete assessments using a checklist such as given in Box 2.2. Scientists from state and federal governments and from independent fisheries commissions should continue to conduct fish stock assessments with periodic peer review.

  2. At the minimum, at least one reliable abundance index should be available for each stock. Fishery-independent surveys offer the best choice for achieving a reliable index if designed well with respect to location, timing, sampling gear, and other statistical survey design considerations.

  3. Because there are often problems with the data used in assessments, a variety of different assessment models should be applied to the same data; new methods may have to be developed to evaluate the results of such procedures. The different views provided by different models should improve the quality of assessment results. Greater attention should also be devoted to including independent estimates of natural mortality in assessment models.

  4. The committee recommended that fish stock assessments include realistic measures of the uncertainty in the output variables whenever feasible. Although a simple model can be a useful management tool, more complex models are needed to better quantify all the unknown aspects of the system and to address the long-term consequences of specific decision rules adequately. Implementation of this recommendation could follow the methods discussed in Chapter 3 (of the earlier report.)

  5. Precautionary management procedures should include management tools specific to the species managed, such as threshold biomass levels, size limits, gear restrictions, and area closures (for sedentary species).

  6. Assessment methods and harvesting strategies have to be evaluated simultaneously to determine their ability to achieve management goals. Ideally, this involves implementing them both in simulations of future stock trajectories. For complex assessment methods, this may prove very computationally intensive, and an alternative is to simulate only the decision rule while making realistic assumptions about the uncertainty of future assessments. Simulation models should be realistic and should encompass a wide range of possible stock responses to management and natural fluctuations consistent with historical experience. The performance of alternative methods and decision rules should be evaluated using several criteria, including the distribution of yield and the probabilities of exceeding management thresholds.

  7. NMFS (National Marine Fisheries Service) and other bodies responsible for fishery management should support the development of new techniques for stock assessment that are robust to incomplete, ambiguous, and variable data and to the effects of environmental fluctuations on fisheries.

  8. The committee recommended that NMFS conduct (at reasonable intervals) in-depth, independent peer review of its fishery management methods to include (1) the survey sampling methods used in the collection of fishery and fishery-independent data, (2) stock assessment procedures, and (3) management and risk management strategies.

  9. The committee recommended that a standardized and formalized data collection protocol be established for commercial fisheries data nationwide. The committee further recommends that a complete review of methods for collection of data from commercial fisheries be conducted by an independent panel of experts.

  10. NMFS and other bodies that conduct fish stock assessments should ensure a steady supply of well-trained stock assessment scientists to conduct actual assessments and carry out associated research. NMFS should encourage partnerships among universities, government laboratories, and industry for their mutual benefit. This can be accomplished by exchanging personnel and ideas and by providing funding for continuing education at the graduate, postdoctoral, and professional levels, including elements such as cooperative research projects and specialized courses, workshops, and symposia.

SOURCE: NRC, 1998.  

Suggested Citation:"General Review of Northeast Groundfish Stock Assessments." National Research Council. 1998. Review of Northeast Fishery Stock Assessments. Washington, DC: The National Academies Press. doi: 10.17226/6067.
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Box 2.2 Checklist for Conducting or Reviewing Stock Assessments  

Step

Important Considerations

1.0 Stock Definition

What is spatial definition of a ''stock"?

Should the assessment be spatially structured or assumed to be spatially homogeneous?

Stock Structure

Choose single-species or multispecies assessment?

Single or multispecies

Use tagging, microconstituents, genetics, and/or morphometrics to define stock structure?

2.0 Data

 

 

2.1 Removals

 

Catch

Are removals included in the assessment?

Discarding

Are biases and sampling design documented?

Fishing-induced mortality

 

2.2 Indices of abundance

For all indices, consider whether an index is absolute or relative, sampling design, standardization, linearity between index and population abundance, what portion of stock is indexed (spawning stock, vulnerable biomass).

Catch per unit effort (CPUE)

What portions of fleet should be included and how should data be standardized? How are zero catches treated? What assumptions are made about abundance in areas not fished? Spatial mapping of CPUE is especially informative.

Gear surveys (trawl, longline, pot)

Is gear saturation a problem? Does survey design cover the entire range of the stock? How is gear selectivity assessed?

Acoustic surveys

Validate species mix and target strength.

Egg surveys

Estimate egg mortality, towpath of nets, and fecundity of females.

Line transect, strip counting

 

2.3 Age, size, and sex-structure information

Consider sample design, sample size, high-grading selectivity, and ageing errors.

Catch at age

 

Weight at age

 

Maturity at age  

 

Size at age

 

Age-specific reproductive information

 

Suggested Citation:"General Review of Northeast Groundfish Stock Assessments." National Research Council. 1998. Review of Northeast Fishery Stock Assessments. Washington, DC: The National Academies Press. doi: 10.17226/6067.
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2.4 Tagging data

Consider both tag loss and shedding and tag return rates. Was population uniformly tagged or were samples recovered?

2.5 Environmental data

How should such data be used in the assessment? What are the dangers of searching databases for correlates?

2.6 Fishery information  

Are people familiar with the fishery, who have spent time on fishing boats, consulted and involved in discussions of the value of different data sources?

3.0 Assessment Model

 

3.1 Age-, size-, length-, or sex-structured model?  

Are alternative structures considered?

3.2 Spatially explicit or not?

 

3.3 Key model parameters  

 

Natural mortality

Vulnerability

Fishing mortality

Catchability  

Are these parameters assumed to be constant or are they estimated? If they are estimated, are prior distributions assumed? Are they assumed to be time invariant?

Recruitment  

Is a relationship between spawning stock and recruitment assumed? If so, what variance is allowed? Is depensation considered as a possibility? Are environmentally driven reductions (or increases) in recruitment considered?

3.4 Statistical formulation

 

What process errors?

What observation errors?

What likelihood distributions?  

If the model is in the form of weighted sum of squares, how are terms weighted? If the model is in the form of maximum likelihood, are variances estimated or assumed known?

3.5 Evaluation of uncertainty

 

Asymptotic estimates of variance

Likelihood profile

Bootstrapping

Bayes posteriors

How is uncertainty in model parameters or between alternative models calculated? What is actually presented, a distribution or only confidence bounds?

3.6 Retrospective evaluation

Are retrospective patterns evaluated and presented?

Suggested Citation:"General Review of Northeast Groundfish Stock Assessments." National Research Council. 1998. Review of Northeast Fishery Stock Assessments. Washington, DC: The National Academies Press. doi: 10.17226/6067.
×

4.0 Policy Evaluation

 

4.1 Alternative hypotheses  

What alternatives are considered: parameters for a single model or different structural models?

How are the alternative hypotheses weighted?

What assumptions are used regarding future recruitment, environmental changes, stochasticity, and other factors?

 

Is the relationship between spawners and recruits considered? If so, do future projections include autocorrelation and depensation?

4.2 Alternative actions  

What alternative harvest strategies are considered?

What tactics are assumed to be used in implementation?

How do future actions reflect potential changes in future population size?

Is implementation error considered?

Are errors autocorrelated?

How does implementation error relate to uncertainty in the assessment model?

4.3 Performance indicators  

What is the real "objective" of the fishery? What are the best indicators of performance? What is the time frame for biological, social, and economic indices? How is "risk" measured? Are standardized reference points appropriate? Has overfishing been defined formally?

5.0 Presentation of Results  

How are uncertainties in parameters and model structure presented? Can decision tables be used to summarize uncertainty and consequences? Is there explicit consideration of the trade-off between different performance indicators?

 

Do the decisionmakers have a good understanding of the real uncertainty in the assessment and the trade-offs involved in making a policy choice?

SOURCE: NRC, 1998.  

 

Suggested Citation:"General Review of Northeast Groundfish Stock Assessments." National Research Council. 1998. Review of Northeast Fishery Stock Assessments. Washington, DC: The National Academies Press. doi: 10.17226/6067.
×
  1. Finally, the committee interpreted the stock assessments to determine the appropriate scientific advice to be drawn from them. In particular, it determined stock size and condition, whether exploitation rates were high, whether current regulations have reduced fishing mortality, whether lower fishing mortality would diminish the yield obtained from a fixed amount of recruitment, and whether there appeared to be a relationship between recruitment and spawning biomass or between recruitment and fishing mortality. These factors are routinely examined in providing scientific advice about acceptable catch levels, as explained below.

INVESTIGATION OF NORTHEAST FISHERY STOCK ASSESSMENTS

For the most part, the Stock Assessment Review Committee (SARC) and auxiliary reports contained information about the five major topics in stock assessment (Box 2.2): stock definition; data; assessment model; policy evaluation; and presentation of results to managers. The more detailed recommendations follow:

  • Stock definition: Stock identification issues have been considered in the stock assessments, leading to independent assessments for Georges Bank cod, haddock, and yellowtail flounder; Gulf of Maine cod; and southern New England yellowtail flounder; as well as for some 50 other stocks in the general area. It should be noted that the stock boundaries for the U.S. and Canadian assessments are different because the Canadians estimate the biomass of fish inhabiting their waters and set a total allowable catch (TAC) as a proportion of this biomass. U.S. assessments cover the stock in both U.S. and Canadian waters to provide information on total stock. As a consequence, the information in U.S. and Canadian assessments is complementary, not contradictory, and scientists from each country participate in both assessments. The committee notes that better information on genetics and migrating behavior of these populations is needed in order to establish causal mechanisms for changes in stock size by area

  • Data: The National Marine Fisheries Service (NMFS) assessments contain information and documentation of a variety of data, including landings, discards, logbook information, age and other biological sampling information, survey indices of abundance, and various analyses of the data to provide standardization. There are problems with aspects of data collection (see Chapter 3), but these problems are identified (see Appendix D). It was beyond the scope of the committee to conduct an in-depth review of the raw data.

  • Assessment model: The ADAPT assessment model used in the assessments is documented, and in the case of Gulf of Maine cod, an alternative analysis was conducted using concepts developed by Fournier and Archibald (1982) and others (Ianelli, 1997; see NRC, 1998 for additional information). The main source of variability considered is in survey indices of abundance. The committee accepted the use of ADAPT as the primary assessment model but provided comments on its features and alternatives in Chapter 3. Methodology for projecting the future population under alternative scenarios is documented and allows for variation in recruitment and starting abundance. The committee examined spawner-recruit data from the five stocks (see the section "Status of the Five Stocks", (pp. 40-60) and Appendix F) and noted that a wide variety of models could be fitted to the data and that alternative interpretations of what will happen with future recruitment are valid.

  • Policy evaluation: Alternative hypotheses for policy evaluation are considered to a limited extent, mainly in the pattern of expected recruitment. Alternative actions considered include a range of different fishing mortalities (Fs) without consideration of implementation error (deviations from fishing mortalities due to the dynamic nature of fishing and regulation). The main performance indicator is the statistical distribution of spawning biomass over a 10-year period, which is condensed into a risk measure related to the probability of reaching a rebuilding target set by the Northeast Fishery Management

Suggested Citation:"General Review of Northeast Groundfish Stock Assessments." National Research Council. 1998. Review of Northeast Fishery Stock Assessments. Washington, DC: The National Academies Press. doi: 10.17226/6067.
×

Council (NEFMC). The committee is concerned that uncertainty tends to be underestimated in these projections, with the consequence that probabilities of not reaching the rebuilding targets may be higher than suggested. Consequently, in "Evaluating the Consequences of Alternative Management Actions," the committee gives a rationale for considering a wider range of alternatives and illustrates this approach by reexamining the Georges Bank haddock assessment.

  • Presentation of results: Results are presented in a concise advisory report, an in-depth report of the Stock Assessment Workshop (SAW), and related technical documents and reports (see Appendix C for a list of material provided to the committee). Improvements could be made in all of these areas, but it is clear that considerable documentation and analysis are available.

Replication of Assessment Results

The committee's consultant, Marine Resources Assessment Group (MRAG) Americas Inc., reran the ADAPT models, using workspaces provided by NMFS and was able to replicate the NMFS stock assessment results. The committee did not have the resources to investigate raw data sources, so it utilized NMFS data summaries. In addition, the consultant used a different ADAPT program, frequently employed in Canadian assessments (Gavaris, 1991; Gavaris et al., 1996, Gavaris and VanEeckhaute, 1997), that treats the oldest age group somewhat differently. The results were very similar to the NMFS assessments, with average annual differences in age 1 and total abundance (numbers of fish) being less than 3% for all five stocks.

Evaluating Consequences of Alternative Management Actions

A primary objective of fish stock assessments is to evaluate the possible consequences of alternative management decisions. This evaluation is accomplished by simulating future stock projections under different management options and assessing gains and risks associated with each. The ability to predict future stock responses to management interventions is, in general, very limited, and this fact should be reflected in the simulations. Uncertainty about future stock projections has several sources: (1) uncertainty about the current status of the stock (assessment error); (2) variability of future stock dynamics, such as environmental effects on recruitment (process error); (3) uncertainty about how to model the future dynamics of the population, including recruitment, growth, mortality and other relevant processes (model uncertainty); and (4) errors in the implementation of management strategies (implementation error).

Errors in the implementation of management strategies only compound the inherent biological uncertainties. Managers can only attempt to control fishing mortality indirectly through effort and/or catch regulations or, more directly, by closing grounds to fishing. In either case, the link between regulatory tactics and the resulting fishing mortality is uncertain. One of the problems is that it is difficult to predict how the fishing industry will respond to a given set of regulations. Delays or adjustments in management strategies in the Northeast due to the industry's response have been typical. As discussed in Chapter 4, these problems can sometimes be reduced when stakeholders are involved in developing management strategies early in the process.

Assessments of the Northeast groundfish stocks include an evaluation of the consequences of setting different fishing mortality over a 10-year period (see Appendix D and NEFSC, 1997a). Among the management alternatives considered for the five stocks were to maintain the current fishing mortality and to implement a mortality of F0.1. In addition, the effects of closing down the fishery were evaluated for Gulf of Maine cod, and F = 0.1 was explored for Georges Bank haddock. Overall, the committee believes that these projections have underestimated the uncertainty inherent in predicting future stock responses to different management regulations.

Suggested Citation:"General Review of Northeast Groundfish Stock Assessments." National Research Council. 1998. Review of Northeast Fishery Stock Assessments. Washington, DC: The National Academies Press. doi: 10.17226/6067.
×

NMFS scientists did incorporate uncertainty about the current stock status in simulations. However, as is standard practice in fishery stock assessments, the level of uncertainty was evaluated on the assumption that the assessment model was correct. Especially problematic in the case of Northeast groundfish assessments are the assumptions that natural mortality is fixed and known and that catches-at-age are observed without error. These assumptions about mortality and catch-at-age result in overly optimistic assessments of possible estimation errors and, in turn, an underestimation of the variability of possible stock sizes at the start of the simulations (see Chapter 3). An alternative model that incorporates these two factors (Ianelli, 1997) showed a greater range of uncertainty, and further efforts of this type are desirable.

With respect to uncertainty in the stock dynamics, a wide range of possible stock responses is usually consistent with historical experience and should be considered in the simulations. In many situations (e.g., in the Northeast groundfish complex), a discrete set of alternative scenarios can be identified to characterize possible future trends in recruitment. The projections conducted by NMFS are instead based on a single, "best-fit" stock-recruitment model. A Beverton-Holt model was fitted to the time series of estimates of spawning biomass and recruitment provided by the assessment model, with assessment errors ignored. Residual variability in the stock-recruitment process was incorporated into the simulations, but no uncertainty in the specification of the model was considered.

In general, the results of these projections indicate that stocks, recruitment, and future catches will increase if fishing mortalities are reduced substantially relative to recent high values. Although, in most cases, stock and recruitment time series indicate low recruitment levels on average in recent years when the stocks were depleted, there is no guarantee that these trends would reverse if the stocks recovered. The possibility that historical trends in recruitment were driven by changes in the ecosystem cannot be ruled out. So the alternative that recruitment may not recover to historical high levels when and if stocks rebuild should be considered. Also, an evaluation of risks under high fishing mortality hinges on how future recruitment may be affected if the stock is kept at very low levels. Some stock-recruitment (S-R) plots appear to be consistent with a depensatory relationship, in which the number of recruits produced per unit of spawning biomass decreases as the stock becomes more severely depleted. This depensation alternative should be considered in such cases. An interesting approach that may be used to postulate alternative hypotheses about the relationship between spawning biomass and subsequent recruitment is to compare stock-recruitment patterns across populations of a single species or groups of similar species. These comparisons may be used to evaluate the possibility of depensation (e.g., Liermann and Hilborn, 1997; Myers et al., 1995), the capacity of the stock to recover from low abundance levels (e.g., Myers et al., 1997), or simply to see how parameters estimated for a particular stock fit in relation to other similar stocks.

To illustrate how future stock responses to management can change for different stock-recruitment scenarios, the committee conducted a limited set of simulations using Gulf of Maine cod, the most problematic stock, which is still subject to high levels of fishing mortality. Four alternative recruitment scenarios were postulated based on assessment results: (1) recruitment will increase in proportion to stock size if spawning biomass is allowed to increase; (2) recruitment will stay constant on average at the historical mean value independent of stock size; (3) the stock-recruitment relationship shows depensation at very low stock size; and (4) the same stock-recruitment model used in NMFS assessments. In all cases, residual variance was assumed to be uncorrelated from year to year. Further details about the methods and results are provided in the section "Status of the Five Stocks" (pp. 40-60) and Appendix F.

The Beverton-Holt recruitment function did not fit the data particularly well for Gulf of Maine cod, as well as some other stocks, which motivated the use of the depensatory model. Whether the lack of fit in the recruitment data were due to variability related to environmental conditions or to depensation in the spawner-recruitment relationship cannot easily be resolved. In the NMFS analysis, the age at

Suggested Citation:"General Review of Northeast Groundfish Stock Assessments." National Research Council. 1998. Review of Northeast Fishery Stock Assessments. Washington, DC: The National Academies Press. doi: 10.17226/6067.
×

maturity for George Bank cod is assumed to be stable throughout the past 10 years, whereas for Gulf of Maine cod, age at maturity has increased. This type of population response at low stock sizes is contrary to what could be expected: if the carrying capacity of the stocks is constant, one would rather expect a decrease in age at maturity when stocks decline. If the assumed increase in age at maturity were artificial, the spawning biomass would be underestimated in recent years, and the spawner-recruit relationship would have even stronger depensation than estimated. Other hypotheses related to carrying capacity changes also are possible. Understanding possible processes affecting recruitment at small stock sizes would require a more refined formulation of the spawning process, where the sex and age structure of the stock are taken into account as they affect egg production.

The simulations show that reducing fishing mortality to the F0.1 level resulted in increases in stock size in all recruitment scenarios (see the section "Status of the Five Stocks" (pp. 40-60) and Appendix F). Increases were only moderately larger when recruitment was assumed to increase in proportion to spawning biomass than when recruitment was independent of stock size. This occurred because most of the increase in adult biomass is due to a reduction in mortality of recruits, and very little is due to increases in recruitment. Larger gains derived from improved recruitment would be realized later if this scenario is correct. Maintaining fishing mortalities at the current high levels, on the other hand, would have very contrasting effects depending on the stock-recruitment scenario. On one extreme, under the depensatory stock-recruitment relationship, the stock continued to decline and collapsed in all trials after six to nine years. Similar results, although the stocks did not become extinct, were obtained when recruitment was proportional to stock size. On the other extreme, when recruitment was assumed to be independent of spawning biomass, predicted stock size increased only slightly, since average recruitment was set equal to the average of the last 15 years, which is somewhat higher than the most recent recruitment estimates.

Results of this type can be summarized in a decision-analysis table in which the consequences of alternative actions can be evaluated across different recruitment scenarios (Table 2.1). To be even more useful in decisionmaking, such tables should be constructed using actual management tactics that could be employed to implement different target fishing mortalities, rather than using target fishing mortalities themselves.

How conservative management actions should be depends on the probabilities assigned to alternative scenarios. For example, assigning a high probability to the depensatory model would prompt severe restrictions in fishing effort to minimize the possibility of stock collapse. On the other hand, if depensation was considered unlikely and recruitment was assumed to be driven mostly by the environmental conditions, the motivations to rebuild stocks rapidly would not be as strong. In many cases, assigning probabilities is difficult when views contrast on how nature may operate based solely on the stock-recruitment data. Independent observations of similar stocks could, for instance, be used to assess how likely depensation may be (Myers et al., 1995). Also, studies addressing possible links between recruitment and environmental changes may provide some evidence for or against hypotheses involving environmental change. Ecosystem changes have been studied extensively in the area, and these studies could have a more prominent role in assessment, especially in the construction of alternative recruitment scenarios.

Because of the underestimation of uncertainty and the limited set of management options explored, the analysis presented cannot be used to evaluate trade-offs between possible gains and risks under more or less stringent management regulations. The committee recommends that a more comprehensive analysis of the effects of alternative management options incorporate the three main sources of uncertainty discussed above. In addition, socioeconomic factors may be included and would also be subject to similar peer review as the stock assessments.

Suggested Citation:"General Review of Northeast Groundfish Stock Assessments." National Research Council. 1998. Review of Northeast Fishery Stock Assessments. Washington, DC: The National Academies Press. doi: 10.17226/6067.
×

TABLE 2.1 Consequences of Implementing Different Rates of Fishing Mortality Under Alternative Stock-Recruitment (S-R) Scenarios for Gulf of Maine Cod.

 

Fishing Mortality

Recruitment (R) scenarios

F = 0.16 (F0.1)

F = 0.29 (Fmax)

F = 1.04 (10-year mean)

R proportional to stock size

SSB increases by 274 to 1488% Catches increase by 247 to 1315%

SSB increases by 107 to 850% Catches increase by 85 to 801%

SSB continues to decline -66 to -92% Substantial drop in catches -67 to -92

R independent of stock size

SSB increases by 306 to 1240% Catches increase by 288 to 1214%

SSB increases by 175 to 867% Catches increase by 154 to 840%

Slight increase in SSB Slight increase in catches

Depensatory S-R relationship

SSB increases by 365 to 1503% Catches increase by 329 to 1450%

SSB increases by 14 to 568% Catches increase by 2 to 534%

Stock collapses with probability = 1

S-R model used in NMFS projections

SSB increases by 307 to 1204% Catches increase by 272 to 1158%

SSB increases by 142 to 760% Catches increase by 122 to 718%

SSB increases by 142 to 760% Catches increase by 122 to 718%

NOTE: Results are percent change in spawning stock biomass (SSB) and catch at the end of a 10-year projection. Range corresponds to 2.5 and 97.5 percentiles of 1000 trials as a percentage of the median at the start of projections.

COMPARISON WITH ASSESSMENTS AROUND THE WORLD

Input Data

In the Northeast, as is common worldwide, assessments are based on age-structured assessment methods, in this case using a particular age-structured model calibrated to time series of survey catch rates. The data necessary to conduct such analyses are available in the Northeast and are comparable to the data routinely used in stock assessments elsewhere, although Northeast data quantity and quality could be improved. It would be particularly useful to collect additional biological samples, to increase the number of sets made during the surveys, and to improve the reliability of catch-and-effort data, as explained in Chapter 3. Inaccurate landing statistics are a widespread problem in stock assessments around the world that can be particularly acute in TAC-managed fisheries, where the incentives to misreport catches are greater.

Method and Calibration

Several methods can be used in stock assessments that utilize catch-at-age information, and particular methods tend to be associated with specific geographic areas. For example, stocks assessed by the International Council for Exploration of the Sea (ICES) most often are analyzed by the Extended Survivor Analysis method (Anonymous, 1992). Those covered by the International Commission for the Conservation of Atlantic Tunas (ICCAT) are assessed with the ADAPT methodology (Gavaris,

Suggested Citation:"General Review of Northeast Groundfish Stock Assessments." National Research Council. 1998. Review of Northeast Fishery Stock Assessments. Washington, DC: The National Academies Press. doi: 10.17226/6067.
×

1993), whereas stocks in the northeast Pacific are assessed with statistical models developed for several data sources (e.g., Stock Synthesis [Methot, 1989]; CAGEAN [Deriso et al., 1985]). Some of these methods have been evaluated (Patterson and Kirkwood, 1993; NRC, 1998), and the results indicate that their performance is generally comparable. Northeast stock assessments use ADAPT in a standard formulation where stock size is estimated by minimizing the square of the difference between the natural log of predicted stock size index (indices) minus the observed stock size index. Northeast stock assessments use ADAPT in a standard formulation where stock size is estimated by minimizing the sum of squared differences between the natural logarithms of predicted and observed stock size indices.

Analyses Included in the Assessment

The Northeast groundfish stock assessments include all the analyses expected by the committee: regression analysis to standardize catch rates (when CPUE [catch per unit effort] data are available and considered useful), ADAPT analysis to estimate stock size yield, spawner-per-recruit analyses, and stock-recruitment analyses to estimate biological reference points. Short-term projections are made with recruitment estimates when available, and stochastic medium-term projections are made to compare the effects of various fishing mortality rates over a 10-year period (see discussion of assessment models in Chapter 3). Uncertainties are likely greater than indicated in the assessment, but this is a problem shared by most other assessments based on virtual population analysis (VPA). The medium-term projections presented are all based on a Beverton-Holt stock-recruitment model, constrained in some cases so that the number of recruits per unit of spawning biomass did not exceed the median observed value when spawning biomass dropped below the historical minimum. No alternative recruitment scenarios were explored. In addition, the primary management measures used, days at sea and closed areas, are not explicitly taken into account in making the projections.

Provision of Advice

The provision of advice involves the collection of data, data analysis, documentation, peer review of the analyses formulation, and communication of advice. Ways in which these steps are performed around the world vary.

Methods for Regional Stock Assessment

U.S. Northeast Coast

On the U.S. Northeast coast, these steps are performed by SARC, implemented in the region in 1985. The assessments under review are conducted collaboratively by NMFS and Canadian Department of Fisheries and Oceans (DFO) analysts and are peer reviewed in both Canada and the United States. In the United States, a peer review is provided first at working group meetings by federal, state, and academic scientists (see Figure 1.5). A second peer review takes place at the Stock Assessment Workshop (SAW) where draft advice is formulated. The assessments and advice are then presented at a SAW plenary, where the final advice is formulated.

Canadian Maritimes

In the Canadian Maritimes Region, multidisciplinary stock assessment teams, including assessment scientists, oceanographers, and in some cases, individuals from the fishing industry, prepare an assessment. Preparation of the assessment may involve several meetings of each assessment team.

Suggested Citation:"General Review of Northeast Groundfish Stock Assessments." National Research Council. 1998. Review of Northeast Fishery Stock Assessments. Washington, DC: The National Academies Press. doi: 10.17226/6067.
×

These assessments go through a first review generally within each laboratory (Halifax-Dartmouth, Moncton, St. Andrews). A second review takes place during the regional advisory process (RAP) involving federal, provincial, and academic scientists, and possibly fishery managers and harvesters or processors as well (see Figure 1.5). A consensus stock status report is drafted, agreed upon, and presented to the Fisheries Resource Conservation Council (FRCC), whose members, mostly from the fishing industry and academia, have been nominated by the Minister of Fisheries and Oceans. FRCC provides management advice to the Minister.

U.S. West Coast

On the U.S. West Coast, most assessments are done by NMFS analysts and then peer-reviewed by plan development teams (where active) composed of federal (including NMFS), state, and academic scientists. These assessments are then reviewed by Statistical and Scientific Committees (SSCs), also composed of federal, state, and academic scientists, which provide direct testimony to the regional fishery management councils. These councils frequently manage fish stocks with TAC levels, so that related scientific recommendations come in the form of acceptable biological catch (ABC) limits. The councils rarely (if ever) allow their recommended TACs to exceed the ABCs. In contrast, the Northeast fishery assessment advice is more qualitative, presumably because NEFMC does not practice TAC management. Nevertheless, the assessment review and recommendations in the Northeast fishery assessment are comparable to those from the West Coast.

Assessments and Management of Fish Stocks by International Council for the Exploration of the Sea (ICES)

The approach to assessments, advice, and implementation varies considerably within the ICES area. ICES has 19 member nations, conducts marine research in the North Atlantic ocean, and provides scientific advice to member countries. ICES coordinates fish stock assessments and advice through its advisory body, the Advisory Council on Fisheries Management (ACFM). This body in turn works as a review panel, formulating advice based on assessments and draft advice from area-based assessment working groups. In addition, ACFM coordinates the work of methodology-oriented working groups.

Membership on ACFM is mainly through national representation, in addition to some working group chairs and ex officio members. Membership in the working groups is by delegation from various member countries.

ACFM recommendations are passed to whatever party has asked for advice. This may be a country in the ICES area, the European Union, or a commission such as the Northeast Atlantic Fisheries Cooperative, North Atlantic Salmon Commission, or International Baltic Sea Fisheries Commission. Traditionally, this advice has been a response to questions about the size of the TAC for the coming year, but recently the emphasis has moved toward the evaluation of medium-term strategies, including elements of risk analysis.

A noticeable lack of dialogue is evident between advisors at various levels of the process and recipients of the advice. Thus, no forum exists for discussing appropriate management strategies, elucidating appropriate management targets, and so forth. The exchange of information is limited: the stock assessment along with annual TAC advice are given when requested. This situation has caused major problems in the past, ranging from criticism of ACFM for giving explicit advice when no danger signals are present, to severe criticism of ACFM for not providing advice until stocks are well into an overfished state. The present evolution into a process that emphasizes the medium-term consequences of different harvesting strategies is the result of an attempt to alleviate these problems.

Suggested Citation:"General Review of Northeast Groundfish Stock Assessments." National Research Council. 1998. Review of Northeast Fishery Stock Assessments. Washington, DC: The National Academies Press. doi: 10.17226/6067.
×

Apart from concerns about its form, ACFM advice has generally been received without much criticism, or calls for re-evaluations. This is due, in part, to the international nature of the process and to the fact that traditionally all ACFM advice is given as consensus advice.

Assessments and Management of Whale Stocks by the International Whaling Commission (IWC)

The guidelines for stock assessment and management for commercially exploitable whale stocks are contained in the Revised Management Scheme, developed mainly by the Scientific Committee (but not adopted by the IWC). The Revised Management Scheme consists of a monitoring and observer scheme and a management procedure. The management procedure is implemented for a given fishery through an extensive computer-based risk analysis and consists of subdividing the ocean basin into subareas and specifying how the catch limit algorithm is to be applied to various subareas.

Catch limits are calculated from historical catches and past and current survey data by way of a Bayesian-like computation. Surveys are conducted regularly, and the abundance estimates are reviewed by the IWC Scientific Committee. These reviews are usually done every five years and if the Scientific Committee agrees that a set of abundance estimates is acceptable as input to the catch limit algorithm, catch limits for the coming five-year period are calculated. If acceptable abundance estimates are not available, the fishery is phased out over 10 years.

More comprehensive stock assessments are carried out when the management procedure is implemented for the stock and when a revised implementation is required (e.g., when area definitions change or alternative management measures are considered).

Compared to the stock assessment carried out under the IWC Revised Management Scheme (and actually undertaken only for Norwegian minke whaling in the northeast Atlantic), the assessment and management of the five groundfish stocks off New England are characterized by: (1) a higher frequency of assessments; (2) less external scrutiny of the assessments; and (3) no clear long-term management context for assessments. The last difference is the most striking. An implementation of the management procedure represents a long-term management strategy that has been put to extensive risk analysis for long-term behavior. For New England groundfish stocks, the assessment results have not been handled within the framework of an explicit long-term strategy. Therefore, no formal long-term risk analyses of future stock development with an emphasis on collapse and continuing yield have been undertaken. However, several economic analyses have been conducted for the region (Edwards and Murawski, 1993; Overholtz et al., 1993).

Precautionary Management Advice

Scientific advice to fisheries managers has become more precautionary as scientists have learned that sustainability of fish resources requires less intense harvesting or other stringent management measures. The extra precaution has become necessary due to uncertainty about the accuracy of assessment inputs and methods, and errors in implementation of management measures (see NRC, 1998, Chapter 4). For example, the most common biological reference point to use as a target in the 1960s and 1970s was the fishing mortality producing maximum sustainable yield (FMSY). In the 1990s, FMSY has become a limit reference point to be avoided with high probability (FAO, 1995). Other common reference points include fishing mortalities related to maximizing yield per recruit (Fmax) and the point at which the marginal increase in yield per recruit is reduced to 10% (F0.1). These also may not be suitably conservative for some stocks with variable recruitment, maturation at relatively old ages, or large implementation errors (Clark, 1993; Mace and Sissenwine, 1993). Since the late 1980s, biological reference points such as fishing mortalities that preserve spawning biomass per recruit at 35 to 45% of

Suggested Citation:"General Review of Northeast Groundfish Stock Assessments." National Research Council. 1998. Review of Northeast Fishery Stock Assessments. Washington, DC: The National Academies Press. doi: 10.17226/6067.
×

the unfished level (denoted F35% for example) are becoming more frequently used, particularly on the west coast of the United States. More recently, techniques which further reduce fishing mortality at low spawning biomass are being considered to further minimize risk of stock collapse (NRC, 1998).

Table 2.2 shows recent estimates of fishing mortality from the stock assessments along with selected biological reference points. F40% is selected to represent a typical fishing mortality with a low probability of stock collapse and F20% is selected because it corresponds to the definition of overfishing used for many stocks by the NEFMC. Recent management measures by the NEFMC have reduced fishing mortality to or slightly below F0.1 for four out of five stocks considered in this report, except Gulf of Maine cod. Furthermore, the values for F0.1 are similar to those for F40%, suggesting that current estimated fishing mortalities are approaching the upper limit of what is considered conservative in the scientific community. Recent management measures by NEFMC have had no effect for Gulf of Maine cod, which remains fished at much above Fmax, which itself is not sustainable in many situations (e.g., Deriso, 1982).

It should be emphasized, though, that scientific advice provided by NMFS and the SARC process has been in the spirit of the precautionary approach. They have recommended that fishing mortality not be allowed to increase for four of the five stocks and that strong management measures be used to sharply reduce fishing mortality for Gulf of Maine cod (NEFSC, 1997b). Nevertheless, for any of the five stocks, there are scientists who would recommend lower fishing mortalities than are currently estimated to occur. For example, the default target fishing mortality for groundfish managed by the North Pacific Fishery Management Council is F40% scaled down by the ratio of current biomass to a target biomass level. For a stock at 1/2 of its target biomass, the default fishing mortality is 1/2 F40%. If this strategy were applied to Georges Bank haddock for example, the recommended fishing mortality would be roughly 1/2 × 0.2 = 0.1, since spawning biomass is at roughly 1/2 of the Council-recommended threshold.

Summary

Internationally, there are several models for the stock assessment process. The assessment process followed in Northeast fishery stock assessment and management appears open and involves steps analogous to those implemented for other fisheries. Critical evaluation of the assessments is undertaken, and the techniques used are comparable to those in other jurisdictions.

TABLE 2.2 Fishing Mortality (F) for the Five Northeast Groundfish Stocks, 1993-1996, F0.1 and Fmax a Yield-per-recruit curve, and F20% and F40% from a Spawning Biomass-per-recruit Curve.

Stock

F(93)

F(94)

F(95)

F(96)

F20%

Fmax

F0.1

F40%

GOM cod

0.9

2.1

1.1

1.0

0.4

0.3

0.15

0.15

GB cod

1.0

1.1

0.4

0.2

0.4

0.3

0.15

0.15

GB haddock

0.5

0.4

0.15

0.2

0.7

>1

0.25

0.2

GB yellowtail flounder

1.0

1.7

0.3

0.1

0.7

0.6

0.25

0.2

SNE yellowtail flounder

0.8

0.8

0.25

0.1

1.4

>1

0.3

0.3

 

SOURCE: NEFSC, 1997a. Fishing mortalities rounded to the nearest 0.05 between 0 and 0.3 and to the nearest 0.1 above 0.3. NOTE: GOM = Gulf of Maine; GB = Georges Bank; SNE = southern New England.

Suggested Citation:"General Review of Northeast Groundfish Stock Assessments." National Research Council. 1998. Review of Northeast Fishery Stock Assessments. Washington, DC: The National Academies Press. doi: 10.17226/6067.
×

GENERAL EVALUATION OF NORTHEAST GROUNDFISH STOCK ASSESSMENTS BASED ON RECOMMENDATIONS FROM IMPROVING FISH STOCK ASSESSMENTS

Concise statements of the 10 recommendations (Box 2.1) from Improving Fish Stock Assessments (NRC, 1998) are italicized and then followed by a qualitative discussion of how well the Northeast fishery stock assessments compared.

  1. Stock assessments should contain the information identified in Box 2.2. As described above, the Northeast fishery stock assessments contain most of this information.

  2. At least one reliable abundance index should be available for each stock. In the Northeast fishery, indices of abundance come from the autumn and spring NMFS surveys and a Canadian spring survey. The amount of survey information is large compared to that available for many other places, and the reliability of the surveys has been ascertained. However, the amount of survey information is not large compared with information available for Canadian East Coast stocks and many stocks assessed in ICES (Pálsson et al., 1989; ICES, 1993). The committee finds that the level of effort for a particular survey may be too low (see Chapter 3).

  3. A variety of assessment models should be used, and independent estimates of mortality (M) should be considered. Alternative assessment models have been used to only a limited degree. Little consideration has been given to independent estimates of mortality. Therefore, stock assessments are somewhat deficient in this regard.

  4. Stock assessments should include realistic measures of uncertainty in output variables, and more complex models should be considered to provide these measures. The alternative assessment of Gulf of Maine cod (Ianelli, 1997) follows this approach. For the other assessments, attempts were made by NMFS to capture the uncertainty in models and projections. However, as described above, the committee believes that uncertainty is understated by NMFS in the present assessments.

  5. Precautionary management measures should include tools specific to the species managed. Although this is not strictly an assessment issue, NMFS and SARC have provided scientific advice for many years specific to the species and area managed with regard to the effects of fishing mortality, size limits, and closed areas. Current management measures include a combination of tools that have apparently led to lower fishing mortality for four of the five stocks considered in this report.

  6. Assessment methods and harvesting strategies should be evaluated simultaneously to determine their ability to achieve management goals . In NMFS assessments, this has been done by making the projections and looking at rebuilding probabilities. The major disconnection in the process is in determining how the management system, which is based on area closures and days at sea limitations, affects the fishing mortality of the stocks. The current assessments evaluate only the effects that different rates of fishing mortality would have on the probability of achieving rebuilding targets. Links between fishing mortality targets and management tactics were not analyzed. In other words, the implementation problem was not addressed in the assessments.

  7. New data and models should be developed that either can reduce uncertainty or are robust to incomplete, variable data or environmental fluctuations. The committee found that NMFS scientists are considering new approaches in their stock assessments and are eager to respond to old problems and new challenges. However, NMFS should further advance the level of its science as suggested in this chapter and in Chapter 3. It was beyond the scope of this committee to conduct an in-depth review of the raw data. There are problems with aspects of data collection (see Chapter 3), but these problems are identified (see Appendix D). It may be worthwhile for NMFS to have an independent audit of the raw data undertaken.

  8. Periodic, independent peer reviews of assessments should be done . The SARC process does provide for peer review and appears to be leading to constructive changes in assessment methodology,

Suggested Citation:"General Review of Northeast Groundfish Stock Assessments." National Research Council. 1998. Review of Northeast Fishery Stock Assessments. Washington, DC: The National Academies Press. doi: 10.17226/6067.
×

data collection, and research. Nevertheless, SARC includes several NMFS personnel, so the review is not strictly independent. Thus, periodic outside review completely independent of NMFS should be implemented. Any regular assessment system and management advice will have a tendency to become more or less entrenched. Therefore, regular exchanges between various entities within the system and between systems are needed. Any well-designed system allows for the injection of new ideas and methods. External peer review should examine not only the stock assessment analyses but also the process of how stock assessment is linked to management concerns.

  1. Further documentation of standardized and formalized data collection systems is needed. In the Northeast fishery assessment system, NMFS has recently documented data collection protocols in the catch, survey, and sampling areas (see Appendix C).

  2. NMFS should be encouraged to form partnerships with universities, government labs, and industry for the exchange of personnel and ideas and to provide funding for continuing education to keep the stock assessment process fresh and invigorating. In the NMFS presentations to the committee and at the public hearing, there were indications that such activities are under way. Communications between NMFS and industry seem to be improving with sharing of personnel on research and commercial vessel operations. Such activities should be strengthened.

In summary, many of the earlier NRC report recommendations are already being addressed by NMFS even though it had not yet seen the report. The current stock assessment process, despite the need for improvements, clearly provides a valid scientific context for understanding fish populations and the effects of fishery management.

STATUS OF THE FIVE STOCKS

The final task of the committee was to interpret the assessments of the status of the five stocks based on information provided by NMFS and DFO and on analytical results from its consultant (discussed earlier; see Appendix F). In the early 1990s, NMFS stock assessments suggested that the five stocks had similar characteristics: low spawning-stock biomass (SSB) relative to 20 years ago and very high fishing mortality rates, with 50-80% of the fish being captured every year. These assessments advised that maintaining the high fishing mortality rates would lead to continued low catches and the likelihood of the total collapse of stocks and catch.

The need for major reductions in fishing pressure is predicated on the following assumptions:

  1. Current stock size is low relative to recent history.

  2. Recent fishing mortality has been high and is not sustainable.

  3. Recent recruitments have been low relative to earlier periods, presumably because of low spawning-stock biomass, presence of depensation, and/or possibly other factors such as environmental changes.

4a. Reducing fishing pressure will allow spawning stocks to rebuild and/or recruitment to increase.

4b. Maintaining high fishing mortality rates has a high probability of leading to continued poor recruitment, making current yields non-sustainable.

Points 4a and 4b are the most important links in the chain. Although the recent high rates of fishing mortality are clearly incompatible with sustainable management, the extent of the biological benefits from reducing fishing pressure will depend on whether or not rebuilding spawning stocks results in higher recruitments.

The committee evaluated the evidence to support these assumptions. Most data and documentation can be found in Appendix D and/or NEFSC (1997a), and accompanying working papers. The committee examined plots of landings, CPUE, spawning-stock biomass, recruitment, and yield per recruit. Different

Suggested Citation:"General Review of Northeast Groundfish Stock Assessments." National Research Council. 1998. Review of Northeast Fishery Stock Assessments. Washington, DC: The National Academies Press. doi: 10.17226/6067.
×

spawner-recruit relationships for the five stocks were produced by the consultant to the committee and show data from ADAPT, fitted Beverton-Holt (B&H) curves produced by NMFS and the consultant, and curves fit using a density-independent model (Figures 2.1 through 2.5; see Appendix F for details).

The committee made a determination of whether a stock had collapsed by examining the historical estimates of spawning biomass and recruitment. If current estimates of spawning biomass were near the low end of historical values and if there had recently been little or no estimated recruitment for a number of years compared to historical estimates, then the committee stated that the stock had collapsed. This definition was used in mind of the Magnuson-Stevens Fishery Conservation and Management Act definition of ''overfished" as being fished down to a level that jeopardizes the capacity of a stock to produce maximum sustainable yield on a continuing basis (16 U.S.C. 1801 et seq.). A stock at a low spawning level combined with low or little recruitment would be in such jeopardy.

To provide supporting evidence for trends in exploitation obtained from the ADAPT assessment model, an exploitation fraction index (EFI) was calculated as the ratio of the catch to the spring survey index of abundance. This index is independent of the assessment model, being based only on data. It is an index because the survey index was used rather than a survey estimate of total abundance.

FIGURE 2.1 Gulf of Maine cod spawning stock biomass (SSB) and recruitment from original VPA 1983-1996. SOURCE: NEFSC, 1997a.

Suggested Citation:"General Review of Northeast Groundfish Stock Assessments." National Research Council. 1998. Review of Northeast Fishery Stock Assessments. Washington, DC: The National Academies Press. doi: 10.17226/6067.
×

FIGURE 2.2 Georges Bank Cod spawning stock biomass (SSB) and recruitment from original VPA 1979-1996. SOURCE: NEFSC, 1997a.

FIGURE 2.3 Georges Bank haddock spawning stock biomass (SSB) and recruitment from original VPA 1965-1996. SOURCE: NEFSC, 1997a.

Suggested Citation:"General Review of Northeast Groundfish Stock Assessments." National Research Council. 1998. Review of Northeast Fishery Stock Assessments. Washington, DC: The National Academies Press. doi: 10.17226/6067.
×

FIGURE 2.4 Georges Bank yellowtail flounder spawning stock biomass (SSB) and recruitment from original VPA 1974-1996. SOURCE: NEFSC, 1997a.

FIGURE 2.5 Southern New England yellowtail flounder spawning stock biomass (SSB) and recruitment from original VPA 1974-1996. SOURCE: NEFSC, 1997a.

Suggested Citation:"General Review of Northeast Groundfish Stock Assessments." National Research Council. 1998. Review of Northeast Fishery Stock Assessments. Washington, DC: The National Academies Press. doi: 10.17226/6067.
×

Gulf of Maine Cod (NEFSC, 1997a; pp. 51-107)

Stock Size and Condition

The survey data, ADAPT output, and CPUE data all suggest that the current stock size is well below that of the 1960s and 1970s (see Appendix D; NEFSC, 1997a, 1997b). Standardized landings per unit effort (LPUE) data in 1995-1996 were about 30% of the 1982-1983 values (NEFSC, 1997a, Figure A3). Survey biomass estimates in 1994-1996 were roughly 25% of the 1960s values (NEFSC, 1997a, Figure A4). The spawning biomass estimate for 1996 is 10,700 tons, compared to 24,500 tons in 1982 (Figure 1.2). ADAPT estimates are not available for years before 1982.

Recent Exploitation Rates

All indicators suggest that the exploitation rate remains high. Fishing effort has increased consistently over time; few fish are now found in the catch or surveys in the age 7+ group; and the current catch is moderately high in historical terms (Figure 2.6), whereas indicators suggest that the stock size is low (Figure 1.2). The current F is estimated to be 1.04 (NEFSC, 1997a; see Appendix D).

Have Current Regulations Reduced Fishing Mortality (F)?

The current assessment suggests that F has not been reduced for the Gulf of Maine. The model-independent Exploitation Fraction Index (EFI) in 1995 and 1996 is of the same magnitude as in previous years, suggesting that there has been no major reduction in fishing mortality (Figure 2.7).

FIGURE 2.6 Commercial landings (metric tons, live) and fishing mortality of Gulf of Maine (GOM) cod (ages 4-5). Based on ADAPT-tuned VPA. SOURCE: NEFSC, 1997a.

Suggested Citation:"General Review of Northeast Groundfish Stock Assessments." National Research Council. 1998. Review of Northeast Fishery Stock Assessments. Washington, DC: The National Academies Press. doi: 10.17226/6067.
×

FIGURE 2.7 The ratio of commercial landings to spring survey index spawning stock biomass (SSB) for Gulf of Maine (GOM) and Georges Bank (GB) cod. SOURCE: NEFSC, 1997a.

Will Yield-Per-Recruit Change with Lower Fishing Mortality (F)?

The yield-per-recruit curve is reasonably flat over all target ranges of F. The estimated increase in yield-per-recruit would be perhaps 15% if F were reduced from the current value of 1.04 to Fmax (F=0.29). There would be associated increases in CPUE and age-range observed in the stock and in the catch. Lower fishing mortality would be likely to decrease the cost of harvesting and would increase spawning biomass per recruit. Spawning biomass per recruit is only about 10% of the unfished level at the current F and would increase to about 25% at Fmax and 40% at F0.1 (which is near F40%).

Will Recruitment Increase with Increasing Spawning Biomass or Decline with Recent High Fishing Mortality (F)s?

The spawner-recruit data for Gulf of Maine cod are inconclusive; therefore, drawing unambiguous conclusions from these data is impossible. NMFS has fit a Beverton-Holt spawner-recruit relationship through the data that suggests relatively constant recruitment over the historical range (1982-1995) of spawning stocks. Estimated recruitments for the last three years are of considerable concern (Figures 2.1,2.8)—they may reflect a depensatory relationship (lower recruit-per-spawner at lower stock biomass; see the section Alternative Projections for Gulf of Maine Cod below), or they may reflect environmentally driven poor recruitment years. There is no evidence from the stock assessment that rebuilding the spawning stock will result in substantial increases in recruitment over the 1982-1995 levels shown in Figures 2.1, 2.8. However, failing to rebuild the spawning stock may result in a drastic stock collapse. Recent low recruitments may be the first signs of recruitment overfishing (with the caveat that recent recruitment estimates are the most uncertain).

Suggested Citation:"General Review of Northeast Groundfish Stock Assessments." National Research Council. 1998. Review of Northeast Fishery Stock Assessments. Washington, DC: The National Academies Press. doi: 10.17226/6067.
×

FIGURE 2.8 Recruitment of Gulf of Maine (GOM) cod at age 2 and Georges Bank (GB) cod at age 1 in millions of fish. SOURCE: NEFSC, 1997a.

Alternative Projections for Gulf of Maine Cod

The committee conducted a limited set of simulations to test the effects of various fishing mortalities on stock projections (Figures 2.9-2.13). These simulations were designed to illustrate how stock responses to management measures could be affected under different stock-recruitment scenarios. The committee tested four models of stock-recruitment relationships and the effects of these relationships on stock projections:

  1. a Beverton-Holt spawner-recruit model used in NMFS assessments (Figure 2.9);

  2. a spawner-recruit model in which recruitment increases in proportion to spawning biomass (Figure 2.10);

  3. a spawner-recruit model in which recruitment is constant at the mean historical value, independent of spawning biomass (Figure 2.11); and

  4. a depensatory spawner-recruit model in which the ratio R/S increases at low spawning biomass (Figure 2.12). As a result, a stock at low spawning biomass will continue to experience low recruitment on average until spawning biomass increases beyond the depensation threshold.

These figures show that the trend and amount of uncertainty in future projections depends strongly on which spawner-recruit model is used; implications of these results were previously discussed in the section "Evaluating Consequences of Alternative Management Actions" (pp. 31-34). In Figure 2.13, an additional projection is shown for which the constraints on recruitment used in the NMFS analysis are removed (see Appendix F for details). The amount of variability increases by removing the constraints, thereby showing increased uncertainty in future projections.

Suggested Citation:"General Review of Northeast Groundfish Stock Assessments." National Research Council. 1998. Review of Northeast Fishery Stock Assessments. Washington, DC: The National Academies Press. doi: 10.17226/6067.
×

FIGURE 2.9 Results of stochastic projection runs for Gulf of Maine cod using a Beverton-Holt stock-recruitment model and three target fishing mortalities (F0.1 = 0.16, Fmax = 0.29, and a target fishing mortality F = 1.04 equal to the 10-year mean).

Suggested Citation:"General Review of Northeast Groundfish Stock Assessments." National Research Council. 1998. Review of Northeast Fishery Stock Assessments. Washington, DC: The National Academies Press. doi: 10.17226/6067.
×

FIGURE 2.10 Results of stochastic projection runs for Gulf of Maine cod using a stock-recruitment model in which R is proportional to S (R = 0.2825 SSB) and three target fishing mortalities (F0.1 = 0.16, Fmax = 0.29, and a target fishing mortality F = 1.04 equal to the 10-year mean).

Suggested Citation:"General Review of Northeast Groundfish Stock Assessments." National Research Council. 1998. Review of Northeast Fishery Stock Assessments. Washington, DC: The National Academies Press. doi: 10.17226/6067.
×

FIGURE 2.11 Results of stochastic projections for Gulf of Maine cod using a constant recruitment stock-recruitment model and three target fishing mortalities (F0.1 = 0.16, Fmax = 0.29, and a target fishing mortality F = 1.04 equal to the 10-year mean).

Suggested Citation:"General Review of Northeast Groundfish Stock Assessments." National Research Council. 1998. Review of Northeast Fishery Stock Assessments. Washington, DC: The National Academies Press. doi: 10.17226/6067.
×

FIGURE 2.12 Results of stochastic projection runs for Gulf of Maine cod using a depensatory S-R relationship recruitment model and three target fishing mortalities (F0.1 = 0.16, Fmax = 0.29, and a target fishing mortality F = 1.04 equal to the 10 year mean).

Suggested Citation:"General Review of Northeast Groundfish Stock Assessments." National Research Council. 1998. Review of Northeast Fishery Stock Assessments. Washington, DC: The National Academies Press. doi: 10.17226/6067.
×

FIGURE 2.13 Results of a stochastic projection for Gulf of Maine cod spawning stock biomass (SSB) using a spawner-recruit model with recruitment proportional to spawning biomass and a high target fishing mortality F=1.04 equal to the 10 year mean and removing the constraints on recruitment used in the NMFS analysis.

Georges Bank Cod (NEFSC, 1997a; pp. 108-170)

Stock Size and Condition

CPUE and survey trends from NMFS show that the spawning stock biomass of Georges Bank cod, in 1996, was roughly one-fifth the level of the late 1970s (NEFSC, 1997a, Figures B2, B4). The ADAPT estimate of spawning stock at its lowest, in 1994, was roughly one-third of the 1980 estimates, the highest in the historical record (Figure 1.2). Analysis of the long-term catch data (see Appendix F) suggests the stock in 1980 amounted to perhaps one-half the potential unfished spawning stock biomass (SSB). This indicates that the 1994 spawning stock biomass was likely in the range of 10-20% of unfished stock size. The latest ADAPT runs indicate some rebuilding of spawning stock (Figure 1.2). Canadian data from the most recent stock assessment in the 5Zj,m areas show a similar trend in the SSB of Georges Bank cod (Hunt and Buzeta, 1997).

Recent Exploitation Rates

The age distribution and effort suggest a high fishing mortality rate until 1995, which is consistent with the ADAPT outputs (Figure 2.14). The current F is estimated to be 0.18 (NEFSC, 1997a), and estimates of F in 1995 and 1996 are much lower than in previous years (Figure 2.14).

Have Current Regulations Reduced Fishing Mortality (F)?

Reductions in effort and landings (Figure 2.14) are consistent with a significant drop in F in the last two years. Again, this drop is also indicated by the ADAPT output. The model-independent EFI shows a strong drop in the last two years (Figure 2.7), suggesting that fishing mortality has been reduced.

The Canadian assessments show a similar decrease in F, corroborating the U.S. assessment results, although there may have been a slight increase in F in the most recent year. Nevertheless, the exploitation rate in the last two years is dramatically lower than the 1978-1994 exploitation rates (Hunt and Buzeta, 1997).

Suggested Citation:"General Review of Northeast Groundfish Stock Assessments." National Research Council. 1998. Review of Northeast Fishery Stock Assessments. Washington, DC: The National Academies Press. doi: 10.17226/6067.
×

FIGURE 2.14 Commercial landings (metric tons, live) and fishing mortality of Georges Bank (GB) cod (ages 4-8). Based on ADAPT-tuned VPA.

SOURCE: Serchuk et al., 1994; NEFSC, 1997a.

Will Yield-Per-Recruit Change with Lower Fishing Mortality?

The yield-per-recruit analysis indicates minor gains in yield-per-recruit by reducing F from high values before 1995 to Fmax. An expected reduction of less than 10% of the maximum is predicted when F is reduced to F0.1 (see Appendix E for definition). Spawning biomass per recruit is only about 15% of the unfished level at the high values of F before 1995 and increases to about 25% at Fmax and 40% at F0.1 (which is near F40%).

Will Recruitment Increase with Increasing Spawning Biomass or Decline with Current Fishing Mortality (F)?

The spawner-recruit analysis for Georges Bank cod suggests a near linear spawner-recruit relationship (Figure 2.2), and as in the case of Gulf of Maine cod, U.S. data show that recruitments in the most recent years are particularly weak (Figure 2.8). Canadian data also show similarly poor recruitment in the most recent years (Hunt and Buzeta, 1997). This analysis suggests the possibility of depensation at low spawning stock sizes; it also provides support for the hypothesis that larger spawning stocks will result in larger recruitments. The committee suggests that increases in recruitment are not likely at spawning stock levels higher than those seen in the early 1980s. The possibility of depensation raises the concern of stock collapse if spawning biomasses were to decline below the levels of 1994.

Suggested Citation:"General Review of Northeast Groundfish Stock Assessments." National Research Council. 1998. Review of Northeast Fishery Stock Assessments. Washington, DC: The National Academies Press. doi: 10.17226/6067.
×

Georges Bank Haddock (NEFSC, 1997a; pp. 171-223)

Stock Size and Condition

The haddock stock is much less abundant now than it was before 1960. Although survey and CPUE data do not extend back before 1960, catch and survey data from the early 1960s provide evidence of higher historical levels of abundance (Figure 1.2, Appendix F; NEFSC, 1997a, Figures C2, C5). According to a long-term VPA (1931-1986), the spawning stock was between 100,000 and 300,000 metric tons prior to 1960 and was as low as 11,000 metric tons in 1993. The most recent NMFS assessment shows an increase from this historical low in abundance to 32,400 metric tons in 1996 (NEFSC, 1997a). Canadian results from the smaller 5Zj,m area assessment show a similar increase in spawning biomass since 1993 (Gavaris and VanEeckhaute, 1997). It appears that the recent small increases in spawning biomass (Figure 1.2) are due to lower fishing mortality on the existing biomass; recent recruitments for this stock are low (Figures 2.3, 2.17). The committee concludes that the Georges Bank haddock stock has collapsed.

Recent Exploitation Rates

The age structure and total effort suggest that F was very high prior to 1995. Current regulations have reduced F in 1995 and 1996 (Figure 2.15). The current F is estimated to be 0.18.

FIGURE 2.15 Commercial landings (metric tons, live) and fishing mortality of Georges Bank (GB) haddock (ages 4-7). Based on ADAPT-tuned VPA. SOURCE: O'Brien and Brown, 1997; NEFSC, 1997a.

Suggested Citation:"General Review of Northeast Groundfish Stock Assessments." National Research Council. 1998. Review of Northeast Fishery Stock Assessments. Washington, DC: The National Academies Press. doi: 10.17226/6067.
×
Have Current Regulations Reduced Fishing Mortality (F)?

Reductions in effort and implementation of closed areas are consistent with the drastic declines in fishing mortality that emerge from ADAPT runs. The model-independent EFI shows a strong drop in the last two years (Figure 2.16), supporting a decrease in fishing mortality.

Will Yield-Per-Recruit Improve with Lower Fishing Mortality (F)?

The yield-per-recruit analysis shows that the expected yield continues to increase slightly when fishing mortality increases to more than one. An expected reduction of about 20% of the maximum yield is predicted when F is reduced to F0.1. Spawning biomass per recruit is less than 10% of the unfished level at the highest values of F shown on the yield-per-recruit graph. Spawning biomass per recruit increases to about 40% at F0.1 (which is near F40%).

Will Recruitment Increase with Increasing Spawning Biomass or Decline with Recent High Fishing Mortality (F)?

The spawner-recruit analysis for haddock is particularly complex. The data show that recruitment has varied since 1968 (Figures 2.3, 2.17, Appendix F). It has fluctuated without a trend about an average of 13.5 million recruits, with two large year classes in 1975 and 1978. Thus, at first sight, if the data prior to 1968 are ignored, there are no indications that higher spawning stock biomass produced larger recruitment from 1968 to 1996. However, since 1968, the spawning stock biomass has never been higher than 80,000 tons, whereas it had never been below that value from 1931 to 1967, the period during which substantially higher recruitment was observed. Potentially, there would be significant losses in not rebuilding the stock if 80,000 tons of spawning stock biomass were in fact a real biological threshold below which the average productivity is substantially lower. Of the two strong year classes produced since 1968, the 1975 year class was apparently a result of the particularly good survival of the spawning products from a small spawning stock biomass, whereas the 1978 spawning stock biomass was one of the highest during 1968-1996. As a result of the strong 1975 and 1978 year classes, the SSB remained higher than 40,000 tons from 1977 to 1982, but no other strong year classes were produced during that period. Recent studies (Marshall and Frank, 1994; Chambers and Trippel, 1997) strongly suggest that reproductive success may be a function of the quality of spawners, not just their quantity. Therefore, it is possible to imagine a scenario in which the spawning stock biomass during 1979-1982 consisted mostly of first-time spawners whose spawning products have a low probability of survival.

There are indications that the higher recruitments recorded prior to 1968 may have been produced from two major spawning aggregations, one on the northeast peak of Georges Bank and the other in Nantucket Shoals-West Gulf of Maine (McCracken, 1960; Grosslein, 1961; Clark et al., 1982). One hypothesis is that the Nantucket Shoals/West Gulf of Maine spawning unit may have been severely overexploited and that a larger proportion of the recruits are now produced from the northeast peak spawning unit. Recent work suggests that elimination of local stocks is a major problem for Gulf of Maine cod (Ames, 1997), and that serious attention needs to be given to this situation. Hydroclimatic changes have occurred in this area, but their magnitude has been substantially lower than in the northern areas off Newfoundland and Labrador, where they have been invoked as one of the causative factors in stock collapses (Myers et al., 1996). However, haddock in this area are at the southern limit of their distributional range, and small hydroclimatic changes may have a proportionately greater effect on stock dynamics.

Suggested Citation:"General Review of Northeast Groundfish Stock Assessments." National Research Council. 1998. Review of Northeast Fishery Stock Assessments. Washington, DC: The National Academies Press. doi: 10.17226/6067.
×

FIGURE 2.16 The ratio of commercial landings to spring survey index spawning stock biomass (SSB) for Georges Bank (GB) haddock. SOURCE: NEFSC, 1997a.

FIGURE 2.17 Recruitment of Georges Bank (GB) haddock in millions of fish at age 1.

SOURCE: NEFSC, 1997a.

Suggested Citation:"General Review of Northeast Groundfish Stock Assessments." National Research Council. 1998. Review of Northeast Fishery Stock Assessments. Washington, DC: The National Academies Press. doi: 10.17226/6067.
×

Although it is possible that the low recruitments since 1968 have been caused primarily by hydroclimatic or environmental changes, it would seem to be extremely important to rebuild the spawning stock biomass to more than 80,000 to 100,000 metric tons. It should be noted that the historically smaller haddock stock on Browns Bank and in the Bay of Fundy (Northwest Atlantic Fisheries Organization Division 4X) has consistently produced higher year classes since 1978, perhaps because spawning stock biomass there has been maintained closer to an optimal value.

Georges Bank Yellowtail Flounder (NEFSC, 1997a; pp. 224-259)

Stock Size and Condition

The four survey indices of stock size available for Georges Bank yellowtail flounder indicate that the lowest stock sizes were observed from 1987 to 1989 (Figure 2.18). The U.S. spring and fall surveys, which have been conducted since the 1960s, suggest that stock sizes during this period were considerably smaller than those of the late 1960s. The trends of the various indices are inconsistent for the recent period: the scallop and Canadian surveys suggest increases in stock biomass since at least 1993 (Neilson et al., 1997), the 1995 U.S. spring and fall surveys indicate that there has not been any increase in stock size in recent years. VPA results (NEFSC, 1997a, Figure D14) also suggest substantially lower biomass in the late 1980s (less than 3,000 tons in 1987-1988) than in the early 1970s (21,000 tons in 1973).

Recent Exploitation Rates

The high fishing mortality rates estimated by the VPA prior to 1995 are consistent with the almost total lack of individuals older than 5 years observed in the survey data. The current F is estimated to be 0.25, and estimates in 1995 and 1996 are much lower than in previous years (Figure 2.19).

FIGURE 2.18 Commercial survey indices for Georges Bank yellowtail flounder (mean catch [kg] per tow for all age classes).

Suggested Citation:"General Review of Northeast Groundfish Stock Assessments." National Research Council. 1998. Review of Northeast Fishery Stock Assessments. Washington, DC: The National Academies Press. doi: 10.17226/6067.
×

FIGURE 2.19 Commercial landings (metric tons, live) and fishing mortality of Georges Bank (GB: ages 3-7) and southern New England (SNE: ages 3-6) yellowtail flounder. Based on ADAPT-tuned VPA.

SOURCE: yellowtail flounder (GB): NEFSC, 1994b, 1997a; yellowtail flounder (SNE): NEFSC, 1994c; 1997a.

Have Current Regulations Reduced Fishing Mortality (F)?

Reductions in effort and implementation of closed areas are consistent with the drastic declines in fishing mortality that emerge from ADAPT runs. The model-independent EFI shows a strong decline in the last two years (Figure 2.20), suggesting that fishing mortality has been reduced. However, these data should be interpreted with care: there are indications, at least for the Canadian survey, that the catchability or availability may have gone up in 1996.

Will Yield-Per-Recruit Improve with Lower Fishing Mortality (F)?

The yield-per-recruit graph is shown in the SARC advisory report (NEFSC, 1997b). Cadrin et al. (1997) showed Fmax = 0.6 and a 12% reduction in predicted yield per recruit when F is reduced to F0.1 = 0.24. The reduction may be inconsequential if recruitment increases due to lower F (see next section). Spawning biomass per recruit is only about 10% of the unfished level at the high values of F before 1995 and increases to about 20% at Fmax and 40% at F0.1 (which is near F40%).

Will Recruitment Increase with Increasing Spawning Biomass or Decline with Current Fishing Mortality (F)?

On average, larger spawning stock sizes have produced more recruits in Georges Bank yellowtail flounder, and both Canadian and U.S. data indicate that increasing spawning stock sizes should provide a higher probability of producing good year classes (Neilson et al., 1997). All four strong year classes

Suggested Citation:"General Review of Northeast Groundfish Stock Assessments." National Research Council. 1998. Review of Northeast Fishery Stock Assessments. Washington, DC: The National Academies Press. doi: 10.17226/6067.
×

FIGURE 2.20 Ratio of commercial landings to spring survey index spawning stock biomass (SSB) for Georges Bank (GB) and southern New England (SNE) yellowtail flounder. SOURCE: NEFSC, 1997a.

since 1973 have been formed from spawning stocks in excess of 7,000 metric tons (Figures 2.4, 2.21, Appendix F). The 1995 year class is among the weakest in the series, and it was produced from a spawning stock close to 7,000 metric tons. If fishing mortality had not been reduced, spawning biomass would have been lower, and there is no indication whether this would have resulted in even poorer recruitment. Thus, the major choice is between a future similar to the recent past or larger recruitments based on improved spawning stocks. It would be particularly important, in this case, to extend the assessment periods back to the early 1960s, when survey catch rates indicate substantially higher adult biomass than estimated for 1973-1996, the period considered in the current assessment (see Appendix F).

Southern New England Yellowtail Flounder (NEFSC, 1997a; pp. 260-290)

Stock Size and Condition

Catch and survey data and ADAPT results all show a major decline in the abundance of southern New England yellowtail flounder (Figures 1.1, 1.2; NEFSC, 1997a, Tables E1, E8). Autumn survey abundances for the mid-1990s are less than 5% of the values observed in the late 1960s (Appendix F). Catch data show similar strong reductions (see Figure 1.1), and recruitment has been weak for a number of years (Figure 2.21). The committee considers the southern New England yellowtail flounder stock to have collapsed.

Mid-1990s Exploitation Rates

A truncated age distribution indicates high exploitation rates in the 1990s consistent with model outputs. The current F is estimated to be 0.12 (Figure 2.19).

Suggested Citation:"General Review of Northeast Groundfish Stock Assessments." National Research Council. 1998. Review of Northeast Fishery Stock Assessments. Washington, DC: The National Academies Press. doi: 10.17226/6067.
×

FIGURE 2.21 Recruitment of Georges Bank (GB) and southern New England (SNE) yellowtail flounder in millions of fish at age 1.

SOURCE: NEFSC, 1997a.

Have Current Regulations Reduced Fishing Mortality (F)?

Reductions in effort and implementation of closed areas are consistent with major declines in fishing mortality that emerge from ADAPT runs. The model-independent EFI shows a strong drop in the last two years (Figure 2.20).

Will Yield-Per-Recruit Improve with Lower Fishing Mortality (F)?

Yield-per-recruit analysis shows that expected yield continues to increase slightly when fishing mortality increases well beyond 1. An expected reduction of about 15% of the maximum is predicted when F is reduced to F0.1. The reduction may be inconsequential if recruitment increases due to lower F (see next section). Spawning biomass per recruit is about 20% of the unfished level at the highest values of F shown on the yield per recruit graph (which are near the levels of F before 1995). Spawning biomass per recruit increases to about 40% at F0.1 (which is near F40%).

Will Recruitment Increase with Increasing Spawning Biomass or Decline with Current Fishing Mortality (F)'s?

Spawner-recruit data show that recruitment has been fluctuating without a clear trend over a broad range of spawning stocks, with indications that the most recent years (at low spawning stock biomass) have produced poor recruitments (NEFSC, 1997a; Overholtz et al., 1997; see Appendix F). One of the largest year classes, 1987, was formed from a small spawning stock. Most of the largest year classes, 1976-1981, came in a sequence of years. Recruitments larger than the recent average may not occur from increased spawning stock sizes. Except for the 1987 year class, no strong year class has been produced at spawning stock biomass less than 5,000 metric tons. So there are indications that

Suggested Citation:"General Review of Northeast Groundfish Stock Assessments." National Research Council. 1998. Review of Northeast Fishery Stock Assessments. Washington, DC: The National Academies Press. doi: 10.17226/6067.
×

spawning stock biomass should be kept greater than 5,000 metric tons. In addition, most of the largest year classes (1976-1981) came in a sequence of years. Thus, if only the period covered by the assessment is examined, the two most likely recruitment hypotheses are: (1) although strong recruitment is not necessarily associated with the largest spawning stocks but rather with favorable environmental conditions, recruitment will decline if high F is maintained; and (2) recruitment would stay reasonably unchanged if F is maintained. However, earlier survey catch rates at age that extend back to 1963 (NEFSC, 1997a; see Appendix F) indicate that much larger year classes might have been recruited in the 1960s when biomass was substantially larger. The appearance of large year classes in years when biomass was greater would provide support for a third hypothesis, namely, that stronger recruitments may be possible under favorable environmental conditions when spawning biomass is higher.

Suggested Citation:"General Review of Northeast Groundfish Stock Assessments." National Research Council. 1998. Review of Northeast Fishery Stock Assessments. Washington, DC: The National Academies Press. doi: 10.17226/6067.
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The collapse of cod, flounder, and haddock fish stocks in the Northeast United States has caused widespread concern among managers and fishers in the United States and Canada. The diminishing stocks have forced managers to take strict regulatory measures. Numerous questions have been raised about the adequacy of stock assessment science used to evaluate the status of these stocks and the appropriateness of the management measures taken. Based on these concerns, Congress mandated that a scientific review of the methodology and data used to evaluate these stocks be conducted. In this volume, the committee concludes that although there are improvements to be made in data collection, modeling uncertainty, and communicating between fishers, managers, and scientists, the scientific methods used in the Northeast stock assessments are sound. Recommendations are made on how the stock assessment process can be improved.

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