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1—
Introduction

Catches from fish populations managed within U.S. waters have fluctuated substantially over the last century and in many cases have declined precipitously, creating serious economic, social, and ecological problems. Accurate assessments of population characteristics (such as mean and variance of estimates of annual abundance) are crucial for sustaining* fisheries while allowing adequate levels of catches. Two primary roles of stock assessment are (1) to monitor the abundance and productivity of exploited fish populations and (2) to provide fishery managers a quantitative evaluation of the potential consequences of alternative actions, to help achieve management goals.

Overview of U.S. Fisheries

The United States has the largest exclusive economic zone (EEZ) of any nation, covering about 11 million square kilometers. The United States ranked fifth in the world in fish harvests in 1993, following China, Japan, Peru, and Chile (FAO, 1995b) and accounts for approximately 7% of the global wild catch of marine fish. The first-sale value of U.S. commercial landings (5.1 million metric tons [mmt]) in 1994 was estimated at $3.8 billion (NMFS, 1995), with a contribution to the U.S. gross national product of $20.2 billion when the extended (multiplier) effect on related industries and the national economy is included. The U.S. catch is dominated by a small number of species, with almost 50% of the catch (by weight) composed of walleye pollock (Theragra chalcogramma) from Alaskan waters and menhaden (Brevoortia tyrannus) from the Gulf of Mexico and the Atlantic Ocean. In terms of harvest value, sockeye salmon (Oncorhynchus nerka), walleye pollock, brown shrimp (Penaeus aztecus), and American lobster (Homarus americanus ) accounted for about one-third of the catch in 1995.

Recreational fishing is also important in the United States. Although the recreational catch is only about 2%

*  

''Sustainable development has been defined as the management and conservation of the natural resource base, and the orientation of technological and institutional change in such a manner as to ensure the attainment and continued satisfaction of human needs for present and future generations. Such development conserves land, water, plant genetic resources, is environmentally non-degrading, technologically appreciated, economically viable and socially acceptable …" (FAO, 1995a, p. 1). Sustainability does not necessarily imply biologic or economic optimality.



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--> 1— Introduction Catches from fish populations managed within U.S. waters have fluctuated substantially over the last century and in many cases have declined precipitously, creating serious economic, social, and ecological problems. Accurate assessments of population characteristics (such as mean and variance of estimates of annual abundance) are crucial for sustaining* fisheries while allowing adequate levels of catches. Two primary roles of stock assessment are (1) to monitor the abundance and productivity of exploited fish populations and (2) to provide fishery managers a quantitative evaluation of the potential consequences of alternative actions, to help achieve management goals. Overview of U.S. Fisheries The United States has the largest exclusive economic zone (EEZ) of any nation, covering about 11 million square kilometers. The United States ranked fifth in the world in fish harvests in 1993, following China, Japan, Peru, and Chile (FAO, 1995b) and accounts for approximately 7% of the global wild catch of marine fish. The first-sale value of U.S. commercial landings (5.1 million metric tons [mmt]) in 1994 was estimated at $3.8 billion (NMFS, 1995), with a contribution to the U.S. gross national product of $20.2 billion when the extended (multiplier) effect on related industries and the national economy is included. The U.S. catch is dominated by a small number of species, with almost 50% of the catch (by weight) composed of walleye pollock (Theragra chalcogramma) from Alaskan waters and menhaden (Brevoortia tyrannus) from the Gulf of Mexico and the Atlantic Ocean. In terms of harvest value, sockeye salmon (Oncorhynchus nerka), walleye pollock, brown shrimp (Penaeus aztecus), and American lobster (Homarus americanus ) accounted for about one-third of the catch in 1995. Recreational fishing is also important in the United States. Although the recreational catch is only about 2% *   ''Sustainable development has been defined as the management and conservation of the natural resource base, and the orientation of technological and institutional change in such a manner as to ensure the attainment and continued satisfaction of human needs for present and future generations. Such development conserves land, water, plant genetic resources, is environmentally non-degrading, technologically appreciated, economically viable and socially acceptable …" (FAO, 1995a, p. 1). Sustainability does not necessarily imply biologic or economic optimality.

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--> as large as commercial landings for all species combined (90,000 metric tons in 1994), there are more than 17 million marine recreational fishers (anglers) who make more than 66 million fishing trips per year, catch about 360 million fish, and spend $25.3 billion per year on fishing-related activities (NMFS, 1995a); thus, recreational and commercial fishing activities contribute roughly equally to the U.S. economy. For some fisheries in which both commercial and recreational fishers participate (e.g., summer flounder [Paralichthys dentatus] and bluefish [Pomatomus saltatrix]), the recreational catch is a significant portion of the total. The allocation of available marine fish resources between commercial and recreational sectors is a major issue for fishery management councils and for state, regional, and federal fisheries agencies. In its most recent assessment of the condition of U.S. fisheries, the National Marine Fisheries Service (NMFS) evaluated 275 stocks caught by fishers in nearshore coastal waters, the EEZ, and the high seas beyond the EEZ (NMFS, 1996). Of these stocks, there was not enough information available to evaluate the status of 31% of the stocks in 1995; 23% of the stocks were overutilized, 34% were fully utilized, and 12% were underutilized. In 1993, 29% of the stocks were of unknown status, 28% were overutilized, 31% fully utilized, and 12% underutilized (NMFS, 1993). Nearly half the stocks are below the level of abundance that will produce the greatest long-term potential yield (LTPY).* The LTPY of the U.S. fisheries within the U.S. EEZ is estimated to be 8.1 mmt per year, which is more than 60% greater than the recent yield of 5.1 mmt (NMFS, 1996). For the United States to achieve its potential increase in LTPY, currently underutilized fisheries will have to be developed further, but more importantly, fishing on overutilized stocks will have to be reduced so that stocks can rebuild. The estimated LTPY and maximum sustainable yield (MSY) levels can be used as targets to regulate fishing activities to levels that will sustain or rebuild marine fish stocks. Setting of appropriate LTPY and MSY levels depends on accurate stock assessments. Overview of the Use of Assessments in Management Fishery managers are responsible for sustaining fish stocks, and stock assessment scientists are responsible for providing analyses and abundance indices that can make such management possible. There are five steps in a stock assessment (see Appendix D): stock definition, choice of data collection procedures and collection of data, choice of an assessment model and its parameters and conduct of assessments, evaluation of alternative actions and specification of performance indicators, and presentation of results. Stock Definition A fish stock can be defined as all fish belonging to a given species that live in a particular geographic area at a particular time, that is, all individuals actually capable of interbreeding. For practical management purposes, a stock is often further defined by political boundaries. That is, the management unit, often still called a stock, includes those members of a biological stock that are under management by a single governmental agency. Units so defined, however, do not necessarily reflect meaningful biological entities or the spatial heterogeneity of fish distributions. When biological stock boundaries extend beyond national jurisdictions, only international cooperation can permit accurate assessments and wise management of the joint resources. The National Research Council *   "Long-term potential yield (LTPY) is the maximum long-term average yield that can be achieved through conscientious stewardship, by controlling the fishing mortality rate through regulating fishing effort or total catch. LTPY is a reference point for judging the potential of a resource … A fishery resource is considered overutilized when more fishing effort is employed than is necessary to achieve LTPY … A fishery resource is classified as underutilized when more fishing effort is required to achieve LTPY" (NMFS, 1996, pp. 151-152).

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--> (NRC, 1994b) provides an example (for Atlantic bluefin tuna) of the implications of stock definitions and discusses a variety of means to delineate stocks. Even when political considerations do not compromise the definition of a stock, its biological (genetic) boundaries are not always known. Knowledge of such boundaries may not always be vital to proper management of a stock because every breeding individual does not need to be included in a specific stock for management purposes, if the vast majority are included. To the fishery manager, a stock is a collection of individual fish, of similar morphology and habitat use, that occurs in a certain locality at one time. The collection may or may not have genetic integrity. That is, (1) the managed stock may be a Mendelian population reflecting the genetic concept of a stock; (2) it may contain more than one genetically isolated population; or (3) it may contain only part of a population. The important point is that the stock defined on a management basis is managed as a unit whether or not it is identical to a genetic stock. This introduces a level of uncertainty to demographically based assessment models. Choice of Data Collection Procedures and Collection of Data Data regarding the numbers of fish caught, effort expended in catching the fish and other economic data, and biological information about the captured fish are gathered from commercial and recreational fishers (Chapter 2). Catch and yield can be determined from fish tickets, creel surveys, and fishery logbooks. Sampling of landings and/or catch (including discards) provides information regarding number of fish and their length, age, gender, maturity, and fecundity. To verify information obtained from fishers, independent observers can be placed on fishing vessels to record catch, bycatch, discards, and other biological characteristics. Economic performance and efficiency of fisheries can be estimated from effort and catch per unit effort (CPUE); price, cost, and auxiliary variables; and logbooks or interviews. To verify and complement information obtained from fishers, independent surveys are used to estimate biomass, abundance, and other biological characteristics. Survey methods include trawl, longline, pot or trap, mark-recapture, and aerial techniques (Doubleday and Rivard, 1981; Gunderson, 1993). Fishery-independent research can yield other information to aid stock assessments and to improve models. For example, the movement and distribution of fish (determined through tagging and relative CPUE) and the genetic and morphometric features of populations are necessary for determining movement among stocks such as Atlantic bluefin tuna (Gunderson, 1993; NRC, 1994b). Choice of an Assessment Model and Parameters and Conduct of Stock Assessments Any stock assessment model involves choices at two levels: the structural model and the parameter values and data to be used (Chapter 3). The purpose of stock assessments is to provide information to fishery managers that will allow them to control the catch of each species or species complex (or the effort directed to it) so that, ideally, populations can be maintained to produce the MSY* or LTPY for each species. Generally, stock assessments estimate the current abundance of a stock, its rate of removal due to fishing, and/or the abundance needed to sustain the stock in the future. Most assessments of U.S. fisheries are conducted by scientists employed by NMFS, although assessments are also conducted by some states, interstate marine fisheries commissions, and in the case of international stocks, international assessment groups. Stock assessments can be conducted by directly analyzing data characteristics and/or by using models to integrate data. As described below, informal estimates and more formal methods based on abundance are used to manage about half of U.S. marine species (by number of species). More sophisticated modeling approaches are used for the other species, to synthesize data of various types. Different processes affecting changes in stock abundance can be modeled, including recruitment, natural and fishing mortality, gear selectivity and catchability, *   Maximum sustainable yield (MSY) is the largest average catch that can be captured from a stock under existing environmental conditions on a sustainable basis.

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--> FIGURE 1.1 Stock assessment methods used for U.S. fisheries. The data for this figure were complied in 1995 and were provided to the committee by the National Marine Fisheries Service. stock-recruitment relationships, and movement rates. Different assumptions about process errors* and measurement errors can be used to account for variability in parameters arising from biological or environmental factors and to estimate the extent of such effects. Stock Assessment Methods Used by NMFS NMFS surveyed the stock assessment methods used for marine fisheries of the United States and provided summary information to the committee for 212 of the 275 species or complexes it manages. For some fish stocks, no stock assessment is conducted. For those stocks that are assessed, the following methods are used: (1) informal estimates based on professional judgment, (2) abundance analyses, (3) production models, (4) stock reduction models, and (5) age-structured models (Figure 1.1; see Chapter 3 for descriptions of models). For some species, more than one method is used. There is a cost associated with assessments, and there may be some relation of the value of a fishery to the cost expended in conducting stock assessments. Tradition and experience within a region may also influence the method selected. No Stock Assessment (16%)—No stock assessment is performed for some species, presumably because managers have not requested assessments, the value of the catch does not merit the expense of assessments, and/or the stock is distributed across wide geographic areas. Most of these species are large pelagic fish (e.g., dolphinfish, minor tuna species, some billfish, some sharks) or reef-dwelling fish (e.g., grouper, sea bass, and many forage fish). Informal Estimates (2%)—Informal estimates based on the professional judgment of assessment scientists are used for a small percentage of managed species, including the Nassau grouper, Atlantic croaker, Pacific jack mackerel, and Caribbean corals. Analysis of Abundance Trends (28%)—This category of methods includes the use of trends in CPUE and relative abundances over time. CPUE is obtained from fishers. Relative abundances are determined through fishery-independent surveys. For example, swept-area surveys can be used to estimate fish density from the area *   Process errors refer to variability in the population dynamics that cannot be accounted for using deterministic population models, but can be modeled as random processes. An example is the relationship between the number of spawning adults and the resulting juveniles entering the population as catchable fish.

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--> sampled by the fishing gear. Some representative species assessed using abundance trends include spiny dogfish, long-finned squid, striped bass, Atlantic shark, and most crustacean species. Production Models (8%)—These methods include equilibrium, nonequilibrium, and delay-difference models. Representative species include highly migratory species in the Atlantic and Pacific Oceans (for which age-structured sequential population analyses are also conducted), Spanish mackerel, and northern anchovy in the Pacific Ocean. Stock Reduction or DeLury-Type Models (6%)—Examples of species managed using these methods include lobsters and bivalve molluscs in the Atlantic, mackerel in the Gulf of Mexico and Atlantic Ocean, and sablefish in the Pacific Ocean. Age-Structured Models (39%)—These methods are used for 8 of the top 10 (by weight) U.S. commercial fish species. The models include ADAPT (24% of category), Cansar (1%), CAGEAN (13%), Stock Synthesis (28%), and virtual population analysis (33%). Stock Assessment Methods Used by States and Interstate Commissions Interstate fishery management commissions were created to manage stocks shared among states within their coastal waters (including 3 miles from shore on open coasts, bays, and estuaries). Stock assessments are conducted for species in Atlantic state waters by the Atlantic States Marine Fisheries Commission (ASMFC). ASMFC manages 18 Atlantic coast species, with cooperation among states, NMFS, the U.S. Fish and Wildlife Service, the District of Columbia, and the Potomac River Fisheries Commission. ASMFC supports stock assessment committees for individual stocks, whose work is reviewed by a technical committee for each managed species. The technical committee uses information from the stock assessment to develop options for management measures. Species management boards select and implement a specific management strategy for the species. ASMFC has provided a list of its concerns about stock assessment procedures, which appears in Appendix H. The Pacific States Marine Fisheries Commission (PSMFC) does not conduct assessments; instead, assessments are conducted by the individual states or by NMFS. PSMFC's primary functions are to maintain fishery databases that contribute to Pacific Coast fishery assessments and to coordinate the fishery management activities of California, Oregon, Washington, Idaho, and Alaska, including interjurisdictional planning among its member states for nonfederal fisheries. Oregon conducts assessments for pink shrimp, red sea urchins, Dungeness crabs, and Pacific herring (Table H.2). Alaska supports a large stock assessment effort for marine organisms harvested from state waters. Tables H.3, H.4, and H.5 show the methods used for assessment of Alaskan shellfish, Pacific salmon, and Pacific herring. Concerns of the PSMFC and of Alaskan stock assessment scientists are given in Appendix H. Finally, the Gulf States Marine Fisheries Commission (GSMFC) operates similarly to the PSMFC; states in this region (e.g., Florida) conduct their own stock assessments and manage a number of species in their own waters. Concerns of the GSMFC also can be found in Appendix H. Evaluation of Alternative Actions and Specification of Performance Indicators After data have been collected and the size and population structure of stocks have been estimated, harvest and management strategies must be developed (Chapter 4). The fishery management task is complicated by the fact that many fish stocks overlap the state-federal jurisdictional boundary at 3 miles from shore and state-to-state boundaries. For stocks that are shared by states and the federal government and feature a combination of commercial and recreational fishers, there are sometimes joint management activities and data sharing to allow management of fish populations as complete units. Tools used by managers of commercial activity vary from fishery to fishery, ranging from no control to specifying total allowable catch that can be removed from a fish population each year. Management measures can also include the amount of fishing and times of the year that fishing can take place; the gear that can be used;

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--> minimum size limits; and in some cases, the amount of fish that any single fisher, community, company, or other entity can catch. Recreational fisheries more often impose minimum size limits, daily catch limits, seasons, and sometimes gear restrictions and requirements to release fish that are caught. Presentation of Results A variety of approaches are used to present stock assessment results to managers. For fisheries managed by the regional fishery management councils, NMFS is responsible for preparing Stock Assessment and Fishery Evaluation (SAFE) documents. These documents receive scientific user-group review of varying intensity in different parts of the country. Results are presented in annual or periodic stock assessments. Stock assessment results are summarized in various NMFS publications, such as Our Living Oceans (NMFS, 1996). In many cases, there are also public presentations of stock assessment results. Ultimately, stock assessment results are presented to the regional fishery management councils or the boards that govern other fishery management organizations. Stock assessment documents generally contain information about a fishery, its catch and CPUE, survey biomass estimates, and estimates of other important population parameters. When sufficient information is available, population models (see Chapter 3) are constructed to make better use of the information. Frequently, recommendations about acceptable harvest levels are deduced from the population models or "yield or spawning biomass per recruit" studies. Although not yet common, risk assessment and explicit treatment of uncertainty is emerging as an important component of the reports. Approach of the Committee In 1994, the National Oceanic and Atmospheric Administration (NOAA) asked the NRC to evaluate the scientific basis of U.S. management of Atlantic bluefin tuna and to recommend research to resolve remaining stock structure issues. The NRC completed its work in time for the November 1994 meeting of the International Commission for the Conservation of Atlantic Tunas (ICCAT) and published a report entitled An Assessment of Atlantic Bluefin Tuna (NRC, 1994b). The central conclusion of the report was that there was no basis for the use of a stock assessment model that assumed relatively unmixed eastern and western Atlantic stocks of tuna, which was then the practice. The report suggested that NMFS test different models for analyzing tuna data (e.g., Stock Synthesis) and described several technical issues involved in the proper conduct of bluefin tuna assessments, including choice of transformation constants, error structures, weighting, and techniques to standardize catch rates. As a result of the impact of the 1994 report, the NRC began to receive requests to review stock assessments and models used in other U.S. fisheries. In discussions with NOAA, it was determined that the NRC could play a more strategic role by conducting a scientific review of all stock assessment methods and models used for U.S. marine fishery management, rather than examining stock assessments one by one (see Appendix A for letter of request). The NRC Committee on Fish Stock Assessment Methods was formed in early 1996 to conduct the review (see Appendix B for committee biographies). The committee chose to evaluate existing approaches by asking stock assessment analysts to conduct blind runs of simulated data sets through existing models, an approach similar to that used by the International Council for Exploration of the Sea (ICES) to evaluate its stock assessment models (ICES, 1993). The features of the simulated data sets were known only partially to the analysts and were designed to include characteristics that would test the sensitivity and robustness of the models to realistic permutations of data quality and type. The remainder of this report elaborates how data are obtained for stock assessments (Chapter 2); how assessments are conducted, including various assessment issues (Chapter 3); and how harvest strategies are developed based on assessments (Chapter 4). The procedure for the committee's simulation is discussed in detail in Chapter 5 and Appendix E. Results of the simulations are also presented in Chapter 5. The committee used the results of the simulations and its knowledge of existing U.S. stock assessment practices to identify new approaches to stock assessments and to recommend other ways in which the assessment of fish stocks could be improved in the United States (Chapter 6). Stock assessment methods are relatively uniform worldwide, so the information in this report may be useful for stock assessment scientists in other nations.