Friday, August 6, 2010

PS 98-73: Stable isotope analysis of the sandbar shark, Carcharinus plumbeus: A minimally invasive method for comparison of diet and trophic relationships between genders, locations, and age classes

David Shiffman1, Bryan Frazier2, Gorka Sancho1, John Kucklick3, Daniel Abel4, Tracey Sutton5, and Kristene Parsons5. (1) College of Charleston, (2) South Carolina Department of Natural Resources, (3) Holling Marine Laboratory, (4) Coastal Carolina University, (5) Virginia Instittue of Marine Science


The 2006 National Marine Fisheries Service SEDAR (Southeast Data Assessment and Review) for large coastal sharks recommended the gathering of additional diet and trophic relationship data for the sandbar shark, Carcharinus plumbeus. No diet studies of any kind have been performed on South Carolina subpopulations of C. plumbeus, and δ13C and δ 15N stable isotope analysis has never been performed on this species. Muscle samples were taken from C. plumbeus caught by the South Carolina Department of Natural Resources and the Virginia Institute of Marine Science shark surveys in several estuaries along the coast of the Southeastern United States. Samples were analyzed using δ13C and δ 15N stable isotope analysis to elucidate diet and trophic level. Isotope values were compared between age classes, genders, and subpopulations (different estuaries) to determine if there are any ontogenetic, sex-based, or location-based differences in diet and trophic level.


Preliminary results show that in most studied subpopulations, δ 15N increases with increasing age class, indicating that larger, older animals feed at a higher trophic level than smaller, younger animals. Overall, the highest δ 15N level documented came from an adult, and five of the seven lowest δ 15N levels documented came from young-of-the-year animals. δ13C levels also vary with age class, indicating that adults have a different diet than young-of-the-year. 

Major differences in δ13C and δ 15N levels between male and female young-of-the-year sharks were detected in one estuary, suggesting that males may have different diets than females. In most studied subpopulations, there is very little difference in δ13C and δ 15N levels between genders at any age class.

Though most juveniles had similar δ13C and δ 15N levels, there is some evidence of different diet between juveniles from different estuaries within South Carolina.

The analysis is ongoing, and these isotope values will be compared with those of suspected prey species to attempt to elucidate preferred prey of sandbar sharks.