Scientific Journal Article Summary

Natal Homing and Connectivity in Atlantic Bluefin Tuna Populations
Jay R. Rooker, David H. Secor, Gregorio DeMetrio, Ryan Schloesser, Barbara A. Block, John D. Neilson
Science (2008) 322:742-744

Chemical analyses of Atlantic bluefin tuna earbones, or otoliths, revealed distinctness of the eastern (Mediterranean Sea-spawning) and western (Gulf of Mexico-spawning) populations, significant trans-Atlantic movements of eastern adolescents to western foraging grounds, and high fidelity to eastern or western spawning areas.  Carbon and oxygen stable isotopic ratios (δ¹³C and δ¹⁸O) in otolith cores were used to assign fish to the Mediterranean Sea or Gulf of Mexico spawning populations based on well understood associations of isotopic ratios to the differing water mass properties.

Otolith δ¹⁸O and δ¹³C values were significantly different for yearlings from the two spawning regions and served as natal tags to assess natal homing (i.e., return of spawning adults to their region of origin) and population mixing. Analysis of otolith cores of adults on spawning grounds supported high rates of natal homing to both eastern and western spawning areas (95.7% and 99.1%, respectively), rivaling those of Pacific salmon (Figure 1). When statistical error is considered, 100% spawning site fidelity is possible.  This is not surprising, given previous genetic results that no more than 1 bluefin per generation (~5 years) can transfer to the other population and interbreed to maintain the observed level of genetic difference.

Adolescent and adult bluefin tuna collected from foraging areas in the U.S. Mid Atlantic were comprised of both populations, indicative of substantial mixing of the populations, particularly for young bluefin tuna. A large fraction of the school (<47”, 56.9%) and medium (47”-81”, 40.9%) category bluefin present in the Mid-Atlantic were assigned to the eastern population (Figure 1).  The eastern component of the population decreased in older age classes. The largest and oldest adult bluefin tuna collected from high latitudes in the western North Atlantic were comprised primarily of western-origin fish (97.9% in the Gulf of Maine and 100% in the Gulf of St. Lawrence, Figure 1).

These results suggest that past abundance estimates of western Atlantic bluefin tuna may have been over-estimated, particularly for the younger age classes, due to a large subsidy of fish from the eastern population in western fisheries.  Furthermore, fish from the smaller, less productive western population who migrate east across the 45ºW management boundary will be disproportionately affected by higher eastern fishing rates.  The high concentration of western fish in the Gulf of St. Lawrence and Gulf of Maine indicates that these regions may serve as important refugia of western bluefin tuna biodiversity and should be given high priority for conservation.  This study, coupled with continued population declines despite three decades of international management, highlight a clear need for more spatially realistic fishery management.

Figure 1: Estimates of natal origin for school (S), medium (M), and giant (G) category Atlantic bluefin tuna from spawning areas (Mediterranean Sea, Gulf of Mexico) and foraging areas (Gulf of St. Lawrence, Gulf of Maine , Mid-Atlantic Bight).

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