Foraging ecology of the early life stages of four shark species (Rhizoprionodon terraenovae, Carcharhinus limbatus, Carcharhinus isodon, and Carcharhinus brevipinna) in Apalachicola Bay, Florida.

Abstract

As top predators, sharks have an important role in marine ecosystems in relation to populations of fish and invertebrates at lower trophic levels. Fishery management plans stress the need for an ecosystem approach, but few quantitative data on the foraging ecology of sharks have been published. Results from a literature review found that shark species close in taxonomic relation have high diet overlap. Stomach contents and catch data of early life stages of Atlantic sharpnose (<i>Rhizoprionodon terraenovae</i>), blacktip (<i>Carcharhinus limbatus</i>), finetooth (<i>Carcharhinus isodon</i>), and spinner sharks (<i>Carcharhinus brevipinna</i>) taken from fishery independent surveys in Apalachicola Bay, Florida, April-October 1999-2002 were examined to test for resource competition. All species are capable of taking teleost prey from birth, though Atlantic sharpnose and blacktip sharks show an ontogenetic shift in diet. Young-of-the-year Atlantic Sharpnose sharks feed mainly on shrimp, juveniles on sciaenids, and adults on clupeids. Young-of-the-year blacktip sharks feed mainly on sciaenids, whereas juveniles feed on clupeids. The primary prey of young-of-the-year and juvenile finetooth and spinner sharks is menhaden. Seven of ten size-selectivity tests showed neutral selection. Atlantic sharpnose and finetooth sharks consume relatively small-sized prey (over 60% <20% of their length) compared to teleost piscivores while blacktip sharks consume relatively larger prey (58% >20% of their length). Regardless of maturity state and species, diet overlap is high for species-life stage combinations that are similar in size; however, species-life stages did not show significant overlap in habitat use. One possible interpretation is that prey categories shared by similar-sized species are not limiting, but competition may exist for available habitat resources. More intensive monitoring is needed to fully understand temporal and spatial habitat use patterns among these early life stages. Quantifying the links among these sharks and the links between these sharks and resource species are critical for ecosystem modeling and a key step to a broader approach in fisheries management.