Fin‐icky samples: an assessment of shark fin as a source material for stable isotope analysis

Abstract

Analyzing stable isotopes (SI: δ15N and δ13C) in a new tissue requires rigorous testing before its general application in examining aspects of animal ecology. Shark fin provides a novel, minor invasive source material, which is important considering the conservation status of many large sharks. Fin, however, is not a single tissue but composed of multiple tissues, primarily skin and cartilage. This may complicate the interpretation of SI, as fin can be sampled from multiple fins and different regions of a fin from an individual. Here, we examined the variation in δ15N and δ13C with sample location on the anal fin of Caribbean reef sharks (Carcharhinus perezi). Values of δ15N and δ13C were highly correlated across sampling locations indicating that mean population or size class fin SI data would be reliable. At the individual level, large variation in δ15N and δ13C between anal fin sampling locations indicates that the varying proportional contributions of tissues would complicate individual level analyses. For three pelagic shark species, dorsal fin δ13C values were consistently higher than δ13C muscle tissue values, identifying tissue‐specific diet discrimination factors. This would confound multiple tissue studies that assume that SI values across tissues will be equal if the animal is in equilibrium with its diet. Proposed sampling protocols for fin material will negate many of these issues, but caution is warranted for comparisons of SI data between shark fin and other tissues or across species until the isotope dynamics of fin have been experimentally validated.