A Comparison of the Foraging Ecology and Bioenergetics of the Early Life-Stages of Two Sympatric Hammerhead Sharks

Abstract

Juvenile scalloped hammerhead sharks, Sphyrna lewini (Griffith and Smith, 1834), were collected in northwest Florida to examine foraging ecology, bioenergetics, and trophic level [30-60 cm fork length (FL); mean FL= 41.5 cm; N = 196]. Diet analysis was performed using single and compound measures of prey quantity, as well as seven broad diet categories. Diet composition and estimated daily ration were compared to previously published information on bonnethead sharks, Sphyrna tiburo (Linnaeus, 1758). 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 juvenile S. tiburo has been documented to feed mostly on crustaceans and plant material. Plant material contributed little to the diet of S. lewini. Estimated daily ration was significantly lower for S. lewini than for S. tiburo, regardless of whether plant material was included in the model. 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 (delta)15N values and significantly lower (delta)13C values than S. tiburo, consistent with the difference observed in calculated trophic level. These results provide evidence that juvenile hammerhead species coexist in coastal northwest Florida by feeding at separate trophic levels.