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

Abstract

Atlantic bluefin tuna (Thunnus thynnus) is considered to be overfished, but the status of its populations has been debated, partly because of uncertainties regarding the effects of mixing on fishing grounds. A better understanding of spatial structure and mixing may help fisheries managers to successfully rebuild populations to sustainable levels while maximizing catches. We formulate a new seasonally and spatially explicit fisheries model that is fitted to conventional and electronic tag data, historic catch-at-age reconstructions, and otolith microchemistry stock-composition data to improve the capacity to assess past, current, and future population sizes of Atlantic bluefin tuna. We apply the model to estimate spatial and temporal mixing of the eastern (Mediterranean) and western (Gulf of Mexico) populations, and to reconstruct abundances from 1950 to 2008. We show that western and eastern populations have been reduced to 17% and 33%, respectively, of 1950 spawning stock biomass levels. Overfishing to below the biomass that produces maximum sustainable yield occurred in the 1960s and the late 1990s for western and eastern populations, respectively. The model predicts that mixing depends on season, ontogeny, and location, and is highest in the western Atlantic. Assuming that future catches are zero, western and eastern populations are predicted to recover to levels at maximum sustainable yield by 2025 and 2015, respectively. However, the western population will not recover with catches of 1750 and 12,900 tonnes (the “rebuilding quotas”) in the western and eastern Atlantic, respectively, with or without closures in the Gulf of Mexico. If future catches are double the rebuilding quotas, then rebuilding of both populations will be compromised. If fishing were to continue in the eastern Atlantic at the unregulated levels of 2007, both stocks would continue to decline. Since populations mix on North Atlantic foraging grounds, successful rebuilding policies will benefit from trans-Atlantic cooperation.

Discovery of a spawning ground reveals diverse migration strategies in Atlantic bluefin tuna (Thunnus thynnus)

“…Fourth, estimates of the nature and extent of mixing from tagging data need to be reevaluated to account for Slope Sea spawning. Spatially explicit population models show that changes in the distribution of catch can help achieve management goals, assuming levels of mixing in different areas of the ocean are known (33).…”

Crossing the line: migratory and homing behaviors of Atlantic bluefin tuna

“…Movement of bluefin tuna across the 45°W management boundary has been observed with electronic tags (Block et al 2005), highlighting the potential for consequential population mixing in certain regions of the North Atlantic Ocean (NAO). The impact of trans-Atlantic movement and mixing has been recently explored with simulation models, and findings clearly indicate that population estimates, fishing mortality, and long-term rebuilding expectations are strongly dependent on the movement and connectivity of eastern and western bluefin tuna populations (Taylor et al 2011, Kerr et al 2012.…”

Otolith shape variation provides a marker of stock origin for north Atlantic bluefin tuna (Thunnus thynnus)

“…Consequently, commercial fisheries are likely to exploit a mixed assemblage of eastern and western origin fish, particularly in the central north Atlantic and on the east coast of the US (Rooker et al 2007). The relative contributions of the two stocks to mixed assemblages vary spatially, temporally and ontogenetically (Taylor et al 2011). Exploitation in mixed fisheries is likely to have a disproportionate effect on the western stock because its productivity is estimated to be one-tenth that of the eastern stock (Fromentin and Powers 2005).…”

“…Exploitation in mixed fisheries is likely to have a disproportionate effect on the western stock because its productivity is estimated to be one-tenth that of the eastern stock (Fromentin and Powers 2005). Stock simulation results indicate that the incorporation of mixing rates into stock assessment within a mixed-stock modelling framework is essential for sound evaluation of quotas and effective stock-rebuilding plans, particularly for west Atlantic bluefin tuna (Taylor et al 2011).…”