Resilience of Bluefin Tuna in the Western Atlantic Ocean: The Storage Effect

Atlantic bluefin tuna (Thunnus thynnus), like other pelagic fishes, are known to transition between stationary demographic states.  Long-term depressed abundance in the western stock of Atlantic bluefin tuna followed a period of intense overfishing in the 1970s and has been attributed to several causes: (1) during the past 30 years the ecosystem has not supported strong years of juvenile production, (2) the stock mixes with the more highly exploited eastern stock (fisheries in the Northeast Atlantic Ocean and Mediterranean Sea), and (3) the population underwent a regime shift.  Analysis of age and stable isotopes in otoliths from a North Carolina fishery confirmed recent years of relatively high juvenile production in 2003, 2005, and 2006 and low levels of mixing during the recent period.  In support of a historical transition in demographic states, substantial age truncation was observed for adults sampled in 1996-2007 (mean age=13.4±3.8 SD; skewness=1.3) in comparison to 1975-1981 (mean age=20.1±3.7 SD; skewness= -0.3).  An abbreviated reproductive life span has compromised the capacity of adults to sample conditions favorable for offspring survival over decadal time scales.  Because recruitment dynamics by the western stock exhibits threshold dynamics, returning it to a higher production state will entail greater fishing controls.