340 



Fishery Bulletin 104(3) 



terraenovae) (Hoffmayer, 2003); if such changes occur 

 in sandbar sharks, these fluctuations could also affect 

 the model's consumption estimates. 



Ecosystem interactions 



Our results downplay the top-down role of sandbar 

 sharks in the trophic economy of the lower Chesapeake 

 Bay. The model results presented above predict that 

 juvenile sandbar sharks consume -120,000 kg of prey 

 in an average summer in the nursery. In comparison, 

 the estimated annual prey consumption rates of the 

 dominant teleost piscivores (bluefish, P. saltatrix; striped 

 bass, M. saxatilis; and weakfish, Cynoscion regalis) in 

 Chesapeake Bay were 27,000,000 kg, 10,000,000 kg, and 

 5,000,000 kg, respectively (Hartman and Brandt, 1995a). 

 Moreover, the seasonal consumption of prey species by 

 juvenile sandbar sharks is insignificant compared to 

 fisheries landings. The total predicted consumption of 

 Crustacea and Teleostei by juvenile sandbar sharks 

 equals only 0.57% and 0.01% of the annual commer- 

 cial landings of blue crabs (C. sapidus) and Atlantic 

 menhaden (B. tyrannus) in Virginia, respectively (U.S. 

 Department of Commerce"*). 



Bottom-up effects on sharks as apex predators are 

 possible if lower trophic levels are overfished, but the 

 apparent opportunistic foraging strategy of sandbar 

 sharks (Medved and Marshall, 1981; Medved et al., 

 1985; Stillwell and Kohler, 1993; Ellis, 2003) prob- 

 ably reduces their vulnerability to declines of specific 

 prey species (Stevens et al., 2000). However, if current 

 fishery landings in Chesapeake Bay are not sustain- 

 able, the dietary overlap between the dominant piscivo- 

 rous teleost species (Hartman and Brandt, 1995b) and 

 sandbar sharks could lead to competition among these 

 predators for limited prey. 



Conclusions 



An updated sandbar shark bioenergetics model predicts 

 higher consumption rates than earlier bioenergetics esti- 

 mates, but the daily ration estimates generally agree with 

 reconstructed meal sizes from stomach contents data. Our 

 results will be useful for ongoing efforts to build ecosys- 

 tem-wide trophic models for the lower Chesapeake Bay. 

 As the sandbar shark population slowly recovers from 

 overfishing, the contributions of the summer nursery 

 grounds of the lower Chesapeake Bay to juvenile growth 

 and survival will remain critical. Meanwhile, the slow 

 growth rate and low consumption rate of these long- 

 lived elasmobranchs in a complex trophic system may 

 indicate a limited top-down ecosystem role for sandbar 

 sharks in Chesapeake Bay. Our results support the 



** United States Department of Commerce, National Oceanic 

 and Atmospheric Administration, National Marine Fisheries 

 Service. Commercial Fishery Landings Database. Website: 

 http://ww W.St. nmfs.gov/stl/commercial/inde.\. html (accessed 

 May 2004. J 



conclusion that the effects of anthropogenic activities — 

 fisheries and other activities — on shark populations 

 often greatly outweigh the effects of these populations 

 on their ecosystems (Stevens et al., 2000; Bush and 

 Holland, 2002; Kitchell et al., 2002; Baum et al., 2003; 

 Bascompte et al., 2005). 



Acknowledgments 



This work was supported by the U.S. National Shark 

 Research Consortium (NOAA/NMFS Grant no. 

 NA17FL2813 to J. A.M.) and an Indiana University 

 South Bend Faculty Research Award to P.G.B. 



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