A COMMENTARY ON WESTERN ATLANTIC BLUEFIN TUNA ASSESSMENTS

Intense fishing for bluefin tuna Thunnus thynnus in the western Atlant ic Ocean began in the 1960s, when landings peaked at nearly 20,000 met ric tons (mt). During the 1970s, landings averaged about 5,000 mt. The International Commission for the Conservation of Atlantic Tunas (ICCA T) established a total allowable catch (TAC) of 1,160 mt in 1982 and h as set limits ranging from 1,995 to 2,660 mt annually since. The Commi ssion's assessments indicate that unrestricted fishing through the 196 0s and 1970s resulted in a sharp decline in abundance, primarily becau se heavy fishing on young fish wasted potentially good recruitment. Si nce the late 1980s, ICCAT management has stabilized the western Atlant ic population, and recently there are signs of improvement. Resource a ssessments and management of western Atlantic bluefin tuna are subject s of severe controversy. Two of the most controversial issues are the stock assessment implications of fish migrations between the western a nd eastern Atlantic management units and the strategies for rebuilding abundance in the western Atlantic. In 1994, the U.S. National Researc h Council (NRC) was commissioned to review bluefin tuna stock assessme nts with particular emphasis on the issue of population mixing. The NR C report was widely misinterpreted as being more optimistic than it re ally was for the western population. Analyses by the NRC committee ind icated that the abundance of spawning age fish in the west was higher than the value estimated in the 1993 ICCAT assessment but also that re cruitment in the western Atlantic had failed so badly that some year-c lasses were estimated to have zero fish. Projections of future populat ion size based on the NRC analyses indicated that recent levels of cat ch could not have been sustained. The critical issue now facing fisher y managers is how to rebuild the population to a size, estimated to be about eight times the current size, that can produce maximum sustaina ble yield. One strategy (referred to as ''active'') is to reduce the f ishing mortality, which would permit some immediate rebuilding and enh ance the likelihood of better recruitment in the future. Another strat egy (referred to as ''passive'') is to wait for natural variability in recruitment to bring a fortuitously strong year-class that would be i nvested in rebuilding, rather than in harvest. The most recent ICCAT a ssessment evaluated rebuilding strategies, but the method used has lim itations that should be understood before decisions about rebuilding a re made.