Dowd et al,: Consumption rates of Carcharchinus plumbeus in Chesapeake Bay 



333 



a number of commercially important species such as 

 menhaden (Brevoortia tyranniis), blue crabs (Callinectes 

 sapidus), striped bass (Moroi^e saxatilis), and bluefish 

 (Pomatomus saltatrix) (Medved and Marshall, 1981; 

 Medved et al., 1985; Stillwell and Kohler, 1993; El- 

 lis, 2003). Interestingly, previous ecosystem models 

 have predicted both significant (Stevens et al., 2000) 

 and negligible (Kitchell et al., 2002) top-down effects 

 of changes in shark biomass on ecosystem structure, 

 depending primarily on the trophic complexity of the 

 system and the incidence of omnivory (Bascompte et 

 al., 2005). 



Because the sandbar shark is one of the few species 

 for which many of the necessary modeling parameters 

 have been measured, it serves as an excellent system 

 for assessing the bioenergetics and ecosystem role of 

 large coastal elasmobranchs. This article has the fol- 

 lowing objectives: 



1 to construct a realistic bioenergetics model for juve- 

 nile sandbar sharks in the Chesapeake Bay summer 

 nursery grounds. Because previous sandbar shark 

 models have suffered from a lack of species-spe- 

 cific data (Medved et al., 1988; Stillwell and Kohler, 

 1993), we have incorporated updated species-specific 

 and habitat-specific data. 



2 to use the model to assess the role of juvenile sand- 

 bar sharks as predators in the Chesapeake Bay to 

 aid ecosystem modelers and fishery management 

 efforts. 



3 to test the sensitivity of the model to uncertainty in 

 parameter estimates using error analysis to identify 

 future research priorities (Kitchell et al., 1977). 



Materials and methods 



Study area and nursery habitat 



The core sandbar shark nursery area (-500-1000 km^; 

 Grubbs and Musick, in press) in the lower, eastern Ches- 

 apeake Bay supports a seasonal population of -10,000 

 individuals (Sminkey, 1994), composed almost entirely of 

 sandbar sharks <90 cm precaudal length (PCL) (Musick 

 et al., 1993; VIMS'). Juvenile sandbar sharks move 

 actively throughout the nursery area, covering large 

 activity spaces (>110 km-) and the entire water column, 

 as shown in telemetry studies (Medved and Marshall, 

 1983; Grubbs, 2001). 



Sandbar sharks in the nursery area are exposed to 

 both long-term and short-term changes in water tem- 

 peratures. Juvenile sandbar sharks inhabit Chesapeake 

 Bay at seasonal temperatures ranging from 15 to 29°C 

 (VIMSM. During the months of July and August, a 

 seasonal thermocline also develops in the lower Chesa- 

 peake Bay, which sandbar sharks will cross repeatedly 

 throughout the day (Grubbs, 2001), The magnitude of 

 the temperature gradient from top to bottom is typically 

 5-6°C (VIMS', Chesapeake Bay Program^). 



VIMS (Virginia Institute of Marine Science) Shark Ecology 

 Program Longline Survey. 1973-2003. Unpubl. data (as 

 a Microsoft Excel file). [Available from J. A. Musick. 1208 

 Greate Road, Gloucester Point, VA 23062-1346.] 

 ' Chesapeake Bay Program Water Quality Database. Website: 

 http://www.chesapeakebay.net/data/index.htni (accessed on 

 March 2003.] 



