Atlantic Bluefin Tuna: A Novel Multistock Spatial Model for Assessing Population Biomass

Atlantic bluefin tuna (Thunnus thynnus) is consideredto be overfished, but the status of its populations has been debated, partlybecause of uncertainties regarding the effects of mixing on fishing grounds.A better understanding of spatial structure and mixing may help fisheriesmanagers to successfully rebuild populations to sustainable levels while maximizingcatches. We formulate a new seasonally and spatially explicit fisheries modelthat is fitted to conventional and electronic tag data, historic catch-at-agereconstructions, and otolith microchemistry stock-composition data to improvethe capacity to assess past, current, and future population sizes of Atlanticbluefin tuna. We apply the model to estimate spatial and temporal mixing ofthe eastern (Mediterranean) and western (Gulf of Mexico) populations, andto reconstruct abundances from 1950 to 2008. We show that western and easternpopulations have been reduced to 17% and 33%, respectively,of 1950 spawning stock biomass levels. Overfishing to below the biomass thatproduces maximum sustainable yield occurred in the 1960s and the late 1990sfor western and eastern populations, respectively. The model predicts thatmixing depends on season, ontogeny, and location, and is highest in the westernAtlantic. Assuming that future catches are zero, western and eastern populationsare predicted to recover to levels at maximum sustainable yield by 2025 and2015, respectively. However, the western population will not recover withcatches of 1750 and 12,900 tonnes (the “rebuilding quotas”) inthe western and eastern Atlantic, respectively, with or without closures inthe Gulf of Mexico. If future catches are double the rebuilding quotas, thenrebuilding of both populations will be compromised. If fishing were to continuein the eastern Atlantic at the unregulated levels of 2007, both stocks wouldcontinue to decline. Since populations mix on North Atlantic foraging grounds,successful rebuilding policies will benefit from trans-Atlantic cooperation.