Diet of scalloped hammerhead shark in eastern Gulf of Mexico

Juvenile scalloped hammerhead sharks, Sphyrna lewini, were collected in northwest Florida to examine foraging ecology, bioenergetics, and trophic level (30-60 cm FL mean FL 41.5 cm n 196). Diet analysis was performed using single and compound measures of prey quantity. Diet was also analyzed using seven broad diet categories (DC). Diet composition and estimated daily ration were compared to previously published information on bonnethead sharks, S. tiburo. Diet overlap was low between species. Juvenile S. lewini feed on relatively small (85 of prey items 5 shark length) teleosts (mostly bothids and sciaenids) and shrimps, whereas S. tiburo have been documented to feed mostly on crustaceans and plant material in northwest Florida. Plant material contributed little to the diet of S. lewini. Estimated daily ration was significantly lower for S. lewini (4.6 BW d-1) than for S. tiburo in northwest Florida, regardless if plant material was included in the model (p0.02 including and p0.00001 excluding plant material). Trophic level was calculated at 4.0 for S. lewini and 2.6 for S. tiburo. Stable isotope analysis showed S. lewini had significantly higher 15N values and significantly lower 13C values than S. tiburo, supporting the difference observed in calculated trophic level. These results provide evidence that small juvenile hammerhead species co-exist in coastal northwest Florida by feeding at separate trophic levels.