394 



Fishery Bulletin 105(3) 



from a decrease in the amount, quality, or availability 

 of herring (Marshall et al., 1999; Diamond and Dev- 

 lin, 2003), or an increase in the energy required for 

 northern bluefin tuna to acquire and process sufficient 

 amounts of prey (Marshall et al., 1999; Nottestad et al., 

 1999; Carruthers et al., 2005). 



Atlantic herring spawning stocks in the Gulf of Maine 

 and Georges Bank-Nantucket Shoals are at historically 

 high levels (Overholtz et al.^l; this fact argues against 

 a reduction in herring abundance as a causal factor 

 for the declining condition in northern bluefin tuna. A 

 reduction in the energy density of herring itself, as seen 

 in seabird-herring-copepod ecosystem studies (Diamond 

 and Devlin, 2003; Durant et al., 2003) are other pos- 

 sible reasons for the decline. A coincident decline in 

 northern bluefin tuna and Atlantic herring condition in 

 the Gulf of St. Lawrence indicate that similar changes 

 are occurring in other Northwest Atlantic shelf systems. 

 In view of changes also detected in other predators, 

 such as seabirds, and in the distribution and abundance 

 of baleen whales, there appears to be a major shift in 

 energy transfer and dynamics across the Gulf of Maine 

 ecosystem over a period when oceanographic changes 

 linked to the North Atlantic Oscillation were also evi- 

 dent (Greene and Pershing, 2003). 



If the abundance of forage fish has been reduced, 

 dispersed into smaller schools, or shifted, northern blue- 

 fin tuna would have to expend more energy in search 

 of prey, shift their diet to less energetic prey (i.e., the 

 junk food hypothesis [Piatt and Anderson, 1996; Golet 

 et al., 2002]), or move to regions with a greater biomass 

 of forage fish (Anderson and Piatt, 1999). Northern 

 bluefin tuna are arriving in poorer condition than they 

 were 10-14 years ago, and despite long residency in the 

 area they are no longer attaining the good condition of 

 previous decades. Of additional concern is that after 

 disappearances in the North Sea and in some historic 

 locations on the Canadian Atlantic shelf, northern blue- 

 fin tuna have not returned to these areas (Tiews, 1978; 

 Clay and Hurlbut, 1989). 



Condition data alone do not allow us to identify the 

 cause of observed declines in this top predator, but 

 decreased energy stores on feeding grounds could re- 

 duce energetic allocations to growth and reproduction, 

 as observed in gadids in the northwest Atlantic and 

 elsewhere (Marshall et al., 1999; Lambert and Du- 

 til, 2000), and may have unexpected and detrimental 

 consequences for rebuilding the northern bluefin tuna 

 population. Given that northern bluefin tuna are cur- 

 rently overexploited throughout most of their range, it 

 is essential to identify root causes for these declines 

 on one of their most important foraging grounds in the 

 western Atlantic. 



s Overholtz, W. J., L. D. Jacobson, G. D. Melvin, M. Cieri, M. 

 Power, D. Libby, and K. Clark. 2004. Stock assessment of 

 the Gulf of Maine-Georges Bank Atlantic herring complex, 

 2003. Northeast Fish. Sci. Cent. Ref. Doc. 04-06, 300 p. 



Acknowledgments 



This work was supported by a National Oceanic and 

 Atmospheric Administration Grant NA04NMF4550391 

 to M. Lutcavage. We thank M. Perkins for all his work 

 processing the bluefin tuna, and several anonymous 

 reviewers for their constructive comments that improved 

 our manuscript. 



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