Depressed resilience of bluefin tuna in the western atlantic and age truncation

• Published in: Conservation Biology
• Main Authors: Secor, D. H., Rooker, J. R., Gahagan, B. I., Siskey, M. R., Wingate, R. W.
• Other Authors: National Oceanic and Atmospheric Administration, Pew Charitable Trusts
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2014
• Subjects: Northeast Atlantic
• Online Access: http://dx.doi.org/10.1111/cobi.12392; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1111%2Fcobi.12392; http://onlinelibrary.wiley.com/wol1/doi/10.1111/cobi.12392/fullpdf

Abstract Following intense overfishing in the 1970s, the western stock of Atlantic bluefin tuna (Thunnus thynnus) experienced a long period of depressed abundance, which has been attributed to failure of the population to periodically produce large numbers of juveniles, the western stock mixing with the more highly exploited eastern stock (fisheries in the Northeast Atlantic Ocean and Mediterranean Sea), and regime shift in the population's ecosystem resulting in lower replacement rates. To evaluate the presence of relatively strong years of juvenile production, we analyzed age structure from a recent sample of otoliths (ear stones) collected from the western stock (2011–2013, North Carolina, U.S.A., winter fishery). Mixing levels for the recent sample were analyzed using otolith stable isotopes to test whether age structure might be biased through immigration of eastern stock bluefin tuna. Age structure from historical samples collected from United States and Canadian fisheries (1975–1981) was compared with more recent samples (1996–2007) to examine whether demographic changes had occurred to the western stock that might have disrupted juvenile production. Relatively high juvenile production occurred in 2003, 2005, and 2006. Otolith stable isotope analysis showed that these recruitments were mostly of western stock origin. However, these high recruitments were >2‐fold less than historical recruitment. We found substantial age truncation in the sampled fisheries. Half the historical sample was >20 years old (mean age = 20.1 [SD 3.7]; skewness = −0.3), whereas <5% of the recent sample was >20 years old (mean age = 13.4 [SD 3.8]; skewness = 1.3). Loss of age structure is consistent with changes in fishing selectivity and trends in the stock assessment used for management. We propose that fishing, as a forcing variable, brought about a threshold shift in the western stock toward lower biomass and production, a shift that emulates the regime shift hypothesis. An abbreviated reproductive life span ...