Forty years of fishing: changes in age structure and stock mixing in northwestern Atlantic bluefin tuna (Thunnus thynnus) associated with size-selective and long-term exploitation

Over the past 40 years, northwestern Atlantic bluefin tuna (Thunnus thynnus) have experienced high rates of exploitation, targeted fishing on the largest size classes of the population, and an unknown degree of Mediterranean-stock contribution. Lack of recovery despite targeted rebuilding efforts by management prompted an evaluation of the population for changes in age-structure, size-at-age, and stock mixing over three samples (1974-8, 1996-2002, and 2009-14) coinciding with a cycle of exploitation that initially targeted smaller fish, but then showed strong selectivity for the largest and oldest members of the population. Ages and mixing levels were estimated using port-and observer-sampled otoliths collected by the US National Marine Fisheries Service. A comparison of age structure between the 1970s sample and two later samples indicated strong age truncation had occurred, where both mean age (13, 4, and 7 years) and mean length (191, 166, and 159 cm curved fork length) declined among samples. In addition, minor changes in size-at-age were detected among the three samples. Otolith stable isotope analysis indicated fluctuating stock composition, with a substantially higher contribution of Mediterranean-origin fish in the 1990s (48% eastern stock contribution) than in the 1970s (0% contribution) and the most recent sample (4% contribution). Higher mixing and severe age truncation in the 1990s indicated that the northwestern Atlantic population was at a depressed state. Reduced mixing and a slightly expanded age structure in the most recent sample could suggest that recovery has begun. Still, to evaluate the hypothesized cycle of collapse and modest recovery more rigorously, an integrative assessment framework is needed to consider the dynamic nature of stock productivity, trans-oceanic migrations, and fishing selectivity.