Estimating the Food Requirements and Prey Size Spectra of Larval American Shad

Widespread declines in American shad Alosa sapidissima along the Atlantic coast have been attributed to overfishing, a decrease in water quality, and loss of habitat. Recent surveys along the Roanoke River and Albemarle Sound, North Carolina, suggest that stocks are continuing to decline despite extensive management and stock enhancement efforts. Laboratory experiments were conducted to evaluate the effect of prey density on the growth and survival of American shad and to determine whether larvae can survive and grow in a riverine environment with a limited forage base. Larvae were reared from 11 to 20 d posthatch in one of five treatments: (1) no food; (2) low food (1 prey/L), which simulated the prey densities in the Roanoke River; (3) medium food (50 prey/L), which simulated the prey densities typical of coastal watersheds; (4) high food (500 prey/L); and (5) Artemia spp. (500/L). Larval survival was 35 +/- 7% (mean +/- SE) and was not significantly different among treatments. Treatments with starved fish had the lowest survival (22 +/- 12%), while the highest survival was observed in treatments with high densities of wild zooplankton (46 +/- 18%) and Artemia (40 +/- 16%). Length-specific growth rates were 0.017 mm/d for the starved treatments and 0.024, 0.029, 0.034, and 0.039 mm/d for the low-prey, medium-prey, high-prey, and Artemia treatments, respectively. Larval growth as a function of length was not significantly different between the Artemia and high-prey treatments; however, growth in these treatments was significantly higher than in those with lower prey densities. Weight-specific growth rates (G(w)) were significantly higher for the Artemia treatment (G(w) = 0.129) than for all the other treatments (G(w) = 0.081). Analysis of stomach contents indicated that American shad were selectively feeding on the smallest zooplankton (80-250 mu m) and that larvae exhibited a strong preference for copepod nauplii and rotifers. These results suggest that spatial and temporal overlap between larvae and zooplankton is important for larval growth and survival.