Summary

Atlantic bluefin tuna (Thunnus thynnus) are remarkable fish of considerable value, bringing a high selling price on the raw seafood market in Japan. Bluefin tuna are among the largest bony fishes in the ocean reaching lengths of over 10 feet (3.05 meters), weights of over 1,200 pounds (544 kilograms), and ages of over 30 years. These fish are prized catches of both commercial and recreational fishermen worldwide. The historic popularity of fishing for bluefin tuna and their increasing market value have contributed to the calamitous exploitation of this species, especially in the North Atlantic Ocean.

In response to concerns about the declining abundance of bluefin tuna in the North Atlantic Ocean during the mid-1960s, and in recognition of the need for coordinated international management of highly migratory fish species in the Atlantic Ocean, the International Convention for the Conservation of Atlantic Tunas was signed in 1966. The convention is implemented by an international body called the International Commission for the Conservation of Atlantic Tunas (ICCAT), presently consisting of 22 member nations, including the United States, Canada, Japan, Spain, and France. ICCAT is responsible for providing internationally coordinated research on the condition of Atlantic tunas and other large, highly migratory species (e.g., swordfish) and their environment, as well as for developing regulatory harvest proposals for consideration by member nations. The two most contentious issues concerning the management of Atlantic bluefin tuna are the definitions and sizes of management units and the indices of abundances now used in stock assessments.

In 1981, ICCAT adopted the premise of a two-stock structure for Atlantic bluefin tuna, one in the eastern and the other in the western Atlantic Ocean.



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An Assessment of Atlantic Bluefin Tuna Summary Atlantic bluefin tuna (Thunnus thynnus) are remarkable fish of considerable value, bringing a high selling price on the raw seafood market in Japan. Bluefin tuna are among the largest bony fishes in the ocean reaching lengths of over 10 feet (3.05 meters), weights of over 1,200 pounds (544 kilograms), and ages of over 30 years. These fish are prized catches of both commercial and recreational fishermen worldwide. The historic popularity of fishing for bluefin tuna and their increasing market value have contributed to the calamitous exploitation of this species, especially in the North Atlantic Ocean. In response to concerns about the declining abundance of bluefin tuna in the North Atlantic Ocean during the mid-1960s, and in recognition of the need for coordinated international management of highly migratory fish species in the Atlantic Ocean, the International Convention for the Conservation of Atlantic Tunas was signed in 1966. The convention is implemented by an international body called the International Commission for the Conservation of Atlantic Tunas (ICCAT), presently consisting of 22 member nations, including the United States, Canada, Japan, Spain, and France. ICCAT is responsible for providing internationally coordinated research on the condition of Atlantic tunas and other large, highly migratory species (e.g., swordfish) and their environment, as well as for developing regulatory harvest proposals for consideration by member nations. The two most contentious issues concerning the management of Atlantic bluefin tuna are the definitions and sizes of management units and the indices of abundances now used in stock assessments. In 1981, ICCAT adopted the premise of a two-stock structure for Atlantic bluefin tuna, one in the eastern and the other in the western Atlantic Ocean.

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An Assessment of Atlantic Bluefin Tuna Implementation of this assumption began in 1982 for member nations. Since 1991, international management of the two stocks has been separated to the degree that each stock is alternately reviewed every second year by the commission. Because of the perceived decline in abundance of western Atlantic bluefin tuna, the two stocks have been subject to different regulations, the most striking difference being the absence of a quota for the eastern fishery and the imposition of a strict harvest quota, allowing catches only for scientific monitoring, for the western fishery. The United States is represented in ICCAT by three commissioners. An official from the U.S. Department of Commerce's National Oceanic and Atmospheric Administration (NOAA) serves as one of the U.S. commissioners; of the other two, one is required to have experience with commercial fishing and the second with recreational fishing. In preparation for the November 1994 ICCAT meeting, Douglas K. Hall, Assistant Secretary for Oceans and Atmosphere in the Department of Commerce, asked the National Research Council's Ocean Studies Board to conduct a peer reviewed study within six months to enable NOAA to use the results for the ICCAT meeting. Accordingly, the Ocean Studies Board established the Committee to Review Atlantic Bluefin Tuna to review and evaluate the scientific basis of U.S. management of fisheries for Atlantic bluefin tuna and to recommend research to resolve remaining stock structure issues. The report focuses primarily on the scientific basis for assumptions about stock structure and for indices of abundances used in the stock assessments for western Atlantic bluefin tuna. The issue of stock structure of Atlantic bluefin tuna is discussed in Chapter 2, and movement of Atlantic bluefin tuna is discussed in Chapter 3. Information on the indices of abundances and the results of sensitivity analyses conducted by the committee using data sets obtained from NOAA's National Marine Fisheries Service (NMFS) and the industry are presented and discussed in Chapter 4. In addition, the committee makes specific recommendations to address the detailed problems that are identified and discussed in each section of Chapters 2 through 4. GENERAL RECOMMENDATIONS In response to its charge and based on the analyses presented in Chapters 1 through 4, the committee presents in Chapter 5 the following major conclusions and recommendations for improving the scientific basis of management of Atlantic bluefin tuna: Available biological evidence of stock structure although sparse is consistent with a single stock hypothesis for bluefin tuna in the North Atlantic Ocean, with at least two spawning areas. Furthermore, the committee's reevaluation of tagging results confirms that movement of bluefin tuna between the western and eastern Atlantic Ocean is sufficient to alter the previous ICCAT

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An Assessment of Atlantic Bluefin Tuna Standing Committee on Research and Statistics (SCRS) stock assessments. The committee recommends that NOAA/NMFS conduct new scientific assessments explicitly to include mixing of Atlantic bluefin tuna between eastern and western fishing grounds. In response to the first question posed to the committee by NOAA, the committee concludes that recent ICCAT SCRS assessments of abundance of eastern and western Atlantic bluefin tuna do not provide the most defensible interpretations of available scientific data. The committee's reanalyses show that there is no evidence that abundance of western Atlantic bluefin tuna has changed significantly between 1988 and 1992. The committee recommends that NOAA/NMFS use alternative methods of data management, data analyses, and peer review for estimating abundance indices, movement rates, and mixed population assessments (as discussed in Chapters 3 and 4 of this report). The committee notes that the ICCAT SCRS uses a variety of uncertainty analyses. The committee recommends that NOAA/NMFS and ICCAT SCRS act to include transatlantic movement of fish and adaptive management techniques in future uncertainty analyses. The committee cannot determine the maximum sustainable yield (MSY) for Atlantic bluefin tuna under a one-stock hypothesis with two spawning grounds. Available biological information on stock structure, mixing on the spawning and fishing grounds, spawning site fidelity, and spawner/recruit relationships is too sparse. We do know that the present abundance of bluefin tuna in the western Atlantic Ocean is lower than that in the early 1970s, although the committee did not analyze similar data for the bluefin tuna in the eastern Atlantic Ocean. We also know that the present abundance and fishing mortality are much higher in the eastern Atlantic Ocean than in the west, and that some physical mixing occurs between the fishing grounds in the eastern and western Atlantic Ocean. The committee recommends that NOAA/NMFS reevaluate MSY for Atlantic bluefin tuna. RESEARCH RECOMMENDATIONS The committee notes that research on the biology of Atlantic bluefin tuna is not continuing at an intensity necessary to answer major biological questions pertaining to the management of the fisheries. Therefore, the committee recommends that NOAA/NMFS carry out the research described below using the best available science and techniques within and outside NOAA. For example, research supported by other U.S. government agencies, including the National Science Foundation, the Department of Energy, and the Office of Naval Research, could contribute to the goals of the studies funded by NOAA. Finally, the committee urges NOAA/NMFS to work cooperatively with ICCAT to implement

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An Assessment of Atlantic Bluefin Tuna these research recommendations. The following recommendations are not listed in order of importance or priority. Tagging data show that there is movement of bluefin tuna between the eastern and western Atlantic fishing grounds, but the degree of gene flow between spawning areas is not known. Such knowledge is essential in defining population genetic structure and useful for refining stock assessments. The committee recommends that the one-stock hypothesis be tested rigorously, using the most appropriate technologies capable of detecting contemporary population genetic structure. Estimates of spawning fidelity to a particular area are essential for stock assessments. The committee recommends that microconstituent analysis and archival tags be used to provide information on spawning fidelity. Stock assessments can be refined by better estimates of life history characteristics such as spawning biomass, larval abundance, sex ratio, age at maturity, fecundity, and recruitment. The committee recommends that spawning biomass, sex ratio, age at maturity, and fecundity in the spawning grounds be estimated and that larval performance, as affected by environmental conditions, be studied. The committee recognizes that knowledge of movement patterns is essential for estimating abundance and distribution and that movement rates and patterns may change over time. The committee recommends that a tagging program be undertaken, with an appropriate combination of conventional, PIT, acoustic, and archival tags to provide improved estimates of the magnitude and patterns of movement. This program should be designed to answer scientific questions pertinent to stock assessment and should be coordinated among all nations involved in the bluefin tuna fishery. Estimates of abundance are confounded by the interaction between fishing and changes in distribution caused by interdecadal climatic and oceanic variability. The committee recommends a synthesizing analysis of existing data on distributions of bluefin tuna in relation to spatial and temporal dynamics of major oceanographic features. The committee notes that a greater use of peer review would have improved the quality of some of the research reviewed during the preparation of this report. The committee recommends that review of all research proposals and resulting manuscripts include a process of external peer review. The committee believes that the analyses and results in this report present a challenge to government and to conservation and industrial organizations for the conservation and management of bluefin tuna in the Atlantic Ocean. Responding to this challenge will require a new view, better science, and a commitment to international cooperation. The committee hopes that this report will serve as a catalyst for obtaining better scientific information to improve the status of Atlantic bluefin tuna.