Use of a Minimally Invasive Stable Isotope Technique to Characterize Diets of Coastal Shark Species

Estuaries and coastal waters of South Carolina are inhabited and used as nursery grounds by multiple species of coastal sharks, many of which show reduced populations and high fishing mortality. Obtaining dietary and trophic level information from these shark species is important in order to develop ecosystem-based management plans. Diet analyses using stomach contents in other regions have revealed an ontogenetic shift in the diet of sandbar sharks, Carcharhinus plumbeus, and a diet restricted to crustaceans in the bonnethead shark, Shyrna tiburo. Sharks were captured by longlining and muscle samples were obtained using minimally invasive muscular biopsies before sharks were released.  D¹³C and D¹⁵N stable isotope analysis of muscle samples allowed the dietary characteristics of both species to be described in a non-lethal and effective manner. Significant differences in D¹³C signatures were found between young-of-year and juveniles of sandbar sharks, suggesting that South Carolina populations have a similar ontogenetic shift as sandbar sharks in other regions. Differences in total occupied niche area and D¹⁵N range indicated that young-of-year sharks have more diverse diets than juveniles. Very low variance in D¹³C and D¹⁵N values confirmed that bonnethead sharks fed almost exclusively on blue crabs. The use of stable isotope analyses of muscular tissues is a valid method for doing non-lethal dietary studies in elasmobranch fishes.