Carbon and nitrogen discrimination factors for elasmobranch soft tissues based on a long-term controlled feeding study

The foraging ecology of elasmobranchs (sharks, skates, and rays) is difficult to study because species have spatially and temporally diverse diets. Many diet and habitat preference studies for mammals, birds, and teleosts use stable isotope analysis, but interpretations are limited for elasmobranch studies because taxon-specific isotope discrimination factors from a controlled experiment are unavailable. Trophic discrimination factors for plasma, red blood cells, and muscle were determined from an experiment with leopard sharks (Triakis semifasciata) fed a constant diet of squid over 1000 days. The δ ¹³C values for shark tissues at equilibrium with the squid diet did not vary significantly among individuals, but plasma and red blood cell δ ¹⁵N values differed significantly among individuals and sampling day. Individual variation of muscle δ ¹⁵N averages was observed and likely related to growth. Overall, carbon and nitrogen discrimination factors corresponded to previous studies featuring high-protein diets and carnivorous taxa. The muscle-to-diet discrimination factors from the controlled feeding study were applied to blue sharks (Prionace glauca) and smooth hammerhead sharks (Sphyrna zygaena) caught offshore from Baja California, Mexico. This case study demonstrates the potential of stable isotope analysis to illuminate differences in foraging patterns between elasmobranch species.