Diet- and tissue-specific incorporation of isotopes in the shark Scyliorhinus stellaris, a North Sea mesopredator

Elucidating predator-prey relationships is an important part of understanding and assessing the structure and function of ecosystems. Sharks are believed to play a significant role in marine ecosystems, although their specific trophic ecology is largely unexplored. Stable isotopes of nitrogen (delta N-15) and carbon (delta C-13) are a widely applied tool in food-web studies, but there is a need to quantify stable isotope dynamics in animals, particularly sharks. In this study, diet-tissue discrimination factors (DTDF = stable isotope in consumer tissue - stable isotope in diet) and turnover rates (time for the isotope to be assimilated into the consumer's tissue) of stable isotopes were estimated in blood, fin, and muscle tissue for the shark species Scyliorhinus stellaris fed 2 diets with different isotope values. Subsequently, these diet-and tissue-specific DTDFs were used in isotopic mixing models to quantify the diet of Scyliorhinus canicula caught in the North Sea and were compared with stomach content data. DTDFs for delta N-15 (Delta N-15) and delta C-13 (Delta C-13) ranged from -1.95 to 3.49 parts per thousand and from 0.52 to 5.14 parts per thousand, respectively, and varied with tissue and diet type. Isotope turnover rates in plasma and red blood cells, expressed as half-lives, ranged from 39 to 135 d. Most of the variability in DTDFs reported in this and other studies with sharks can be explained by linear relationships between DTDF and dietary isotopic values. From these relationships, we propose a method for isotope mixing models using diet-specific DTDFs, which improves diet reconstruction estimates for animals, particularly mesopredator sharks that consume a large range of prey types.