Golet et al.: Decline in condition of Thunnus thynnus in the Gulf of Maine 



393 



Grade B 



Grade B or better 



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'3-j 



„Ci^ ^ A' 



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l-D- liinc -fJ-JuK —'—Aug  



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Figure 2 



The predicted probabilities of a northern bluefin tuna iThiinnus thynnus) being in grade 

 j in month m and year _v, iAm.y) for shape grade B- and grade B or better. Similar to fat 

 and oil content, there is a consistent increase in poor fish shape and a decline in the good 

 shape grades for all months, except June, throughout the 14-year period. 



Seasonal migrations of adult northern bluefin tuna 

 are believed to be bound by reproduction and feed- 

 ing constraints: spawning in warm (>24°C) regions 

 in spring and early summer followed by dispersal to 

 continental shelves at higher latitudes for intensive for- 

 aging through late fall (Rivas, 1955; Clay, 1991). Their 

 body condition also varies through this cycle: feeding 

 periods presumably put individuals into positive lipid 

 balance, creating energy stores for gonadal develop- 

 ment and metabolism (Medina et al., 2002; Carruthers 

 et al., 2005). Since the mid 1990s, mixed size classes 

 of northern bluefin tuna appeared in North Carolina 

 coastal waters from December to February (Block et 

 al., 2001), extending the range of their inshore foraging 

 on Atlantic menhaden (Brevoortia tyrannus) and pos- 

 sibly other species. If the temporal or spatial aspects 

 of migration, reproduction, and feeding patterns have 

 changed over the past decade, alterations in the somatic 

 and bioenergetic condition of fish feeding in the Gulf of 

 Maine could be expected. Other possible explanations 

 for changes in somatic and bioenergetic condition of fish 

 include increased growth rates due to selective fishing 

 pressure (Polacheck et al., 2004), or skipped spawning 

 to increase growth (Jorgensen et al., 2006), but the 

 general declines in condition in fish of such large body 

 size are difficult to explain based on intrinsic changes 

 in growth. This is particularly true in this study where 

 every fish was larger than 110 cm, the size at which the 

 seasonal length-weight relationship begins to decrease 

 (Mather et al., 1995) and northern bluefin tuna gain 

 more in mass than length. 



In pelagic fishes, migrations require a substantial 

 energetic cost (Harden Jones, 1984), and migration 

 distance has been linked to body size and available 

 fat stores (N0ttestad et al., 1999). The longstanding 

 migration paradigm is that western Atlantic northern 



bluefin tuna spawn in the Gulf of Mexico and Straits of 

 Florida from April to June and then migrate northward 

 along the continental shelf to New England and Cana- 

 dian waters (Rivas, 1955; Mather et al., 1995; Block et 

 al,. 2005). A substantial increase in migrants from the 

 Eastern Atlantic may explain why the condition of fish 

 in this region has declined. Northern bluefin tuna mi- 

 grating to the Gulf of Maine from the eastern Atlantic 

 would have to swim a greater distance, travel against 

 major currents and through unproductive waters using 

 more stored energy than individuals coming from west- 

 ern Atlantic spawning grounds. To date, there is insuf- 

 ficient data to confirm that such a shift has occurred 

 (Fromentin and Powers, 2005). Another possibility is 

 that the timing and location of spawning has shifted. 

 The long held assumption that the New England as- 

 semblage spawns exclusively in the Gulf of Mexico and 

 adjacent regions (Rivas, 1955; Block et al., 2005) may 

 be incorrect (Lutcavage et al., 1999; Fromentin and 

 Powers, 2005; Goldstein et al., in press). If fish are 

 spawning outside of these traditional spawning grounds 

 during May and June (Mather et al., 1995; Lutcavage 

 et al., 1999; Wilson et al., 2005), or at different times, 

 their somatic condition and lipid allocations would be 

 expected to shift from historical patterns (Rajasilta, 

 1992). 



Given that northern bluefin tuna spend up to five 

 months on the feeding grounds, a decline in somatic 

 condition intuitively points to changes in the forage 

 base and energy transfer within the Gulf of Maine. 

 Northern bluefin tuna exploit several trophic levels, 

 including krill, before arrival in the Gulf of Maine (Es- 

 trada et al., 2005), but while there, they forage exten- 

 sively on herring (Chase, 2002), which has the highest 

 energy density of prey in the region (Lawson et al., 

 1998). The observed decline in condition could result 



