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An Assessment of Atlantic Bluefin Tuna
in the calculation of the U.S. rod and reel index of abundance of bluefin tuna and (2) rejection of the two-stock hypothesis and subsequent reanalysis employing a two-area mixing model.
Given the results of reanalysis, further reductions in catch quotas in the western Atlantic Ocean from 1992-1993 levels cannot be based on a conclusion of a decline in western Atlantic stock abundance since 1988.
The mixing model points to the importance of bluefin tuna in the eastern Atlantic Ocean and Mediterranean Sea when evaluating the status and future of bluefin tuna in the western Atlantic Ocean. Even with relatively low mixing rates, the extremely large abundance of young tunas estimated recently in the eastern Atlantic Ocean causes the abundance of fishable age classes in the west to be strongly influenced by westward migrants.
Fishing mortality in the east is perhaps four times greater than fishing mortality in the west.
Considerable uncertainty exists about the relationship between fish on the fishing grounds and those on the spawning grounds.
The committee recommends that:
Given the revised view of stock structure, management strategies for the east and west should be revisited and should include minimum size limits, catch limits, and (spawning ground) area closures.
Constant fishing mortality (that is, fixed at a constant annual rate for several years) may offer greater opportunity for rebuilding western stocks or reaching high long-term yields than constant catch quotas over many years.
Future assessments of Atlantic bluefin tuna should be conducted with an assessment procedure that explicitly accounts for mixing. As a consequence, assessments of both eastern and western fisheries should be made together at each SCRS meeting. Given the sensitivity of results to the transfer rates, further inspection of the tagging data is warranted and further analysis to estimate rates should be undertaken.
Alternative models to VPA, such as migratory catch age analysis and migratory stock synthesis analysis, should be considered.