336 



Fishery Bulletin 104(3) 



less bias than other diet indices (Cortes, 1997). For 

 the present study, prey species were grouped into four 

 categories for each age class of shark: teleost fishes, 

 mollusks, crustaceans, and elasmobranchs (Table 2). 

 The proportion of each prey type in the diet and the 

 mean energy content values for each category (calculated 

 from data in Thayer et al., 1973) were used to convert 

 daily energy ration (kj/d) to daily ration (percent body 

 mass per day, '7(BM/d). Diet composition was assumed 

 to remain constant during the simulation period. The 

 average daily ration and total seasonal prey consumption 

 were calculated for individuals of each age class. 



Population estimates The relative abundance and size- 

 class composition of the seasonal nursery population 

 were estimated from catch per unit of effort ( CPUE ) data 

 (Musick et al., 1993; VIMSM. Sminkey (1994) used vir- 

 tual population analysis to estimate the sandbar shark 

 cohort sizes in the Chesapeake Bay nursery from the 

 VIMS Shark Longline Survey data, using the standard 

 Mustad"' 9/0 J hooks between 1989 and 1993 (Table 3). 



However, the standard hooks select for larger animals, 

 yielding underestimates of abundance for ages 0-2 years. 

 Therefore, we indexed the VIMS CPUE data for ages 0-2, 

 using smaller Mustad"' 12/0 circle hooks against the 

 CPUE for larger hooks for 25 longline sets between 1997 

 and 2002 when both gears were fished simultaneously at 

 the two lower Chesapeake Bay survey stations. We then 

 used this index to produce a more realistic population age 

 structure (Table 3). The mean adjusted nursery popula- 

 tion size was 11,627 ±2483 individuals. 



For simplicity, we assumed negligible mortality and 

 zero emigration of juvenile sharks during the simula- 

 tion period. Consequently, the revised cohort sizes were 

 held constant throughout the simulation period. Low 

 natural mortality rates would be expected for these 

 sharks, particularly in light of the near absence of large 

 coastal shark predators in the nursery (Musick et al., 

 1993). Tracking, tagging, and survey data all indicate 

 that juvenile sandbar sharks remain within the nursery 

 throughout the summer (Grubbs et al., in press; Merson 

 and Pratt, 2001). 



