Tissue-specific stable isotope ratios of shortfin mako (Isurus oxyrinchus) and white (Carcharodon carcharias) sharks as indicators of size-based differences in foraging habitat and trophic level

We evaluated whether the trophic level and feeding grounds (coastal versus offshore) of white and shortfin mako sharks captured in the northern Mexican Pacific varied as a function of size using carbon (C-13 values) and nitrogen (N-15 values) stable isotope analysis of soft tissues. A step increase in N-15 values was observed at approximately 85cm TL and 150-170cm TL for shortfin mako and white sharks, respectively, consistent with the dilution of the maternal isotopic signal following the incorporation of exogenous food. Shortfin mako and white shark muscle tissue and whole blood were enriched in N-15 as a function of size, which is consistent with increasing trophic level, but this pattern may be confounded by differences in habitat. To infer differences in foraging habitat throughout life, we compared isotope ratios of liver and blood fractions with those of white muscle tissue; these tissues have relatively faster and slower isotope turnover rates, respectively. Whole blood and muscle C-13 values of small white sharks (<200cm total length) caught near the coast suggested benthic foraging. Very limited differences between C-13 values of muscle tissue and whole blood from small and medium mako (<196cm TL) and small white sharks indicated equilibrium to local feeding grounds. In contrast, differences in the C-13 values of liver tissue from small and medium mako and small white sharks indicated changes in diet or feeding grounds. The C-13 values of plasma and muscle tissue of large white sharks (>350cm total length) caught offshore indicate feeding in coastal and oceanic areas and are consistent with recent migration to pelagic feeding grounds. Our study provides insight into size-based habitat use and migration patterns of two species of migratory sharks for which there is limited information, particularly in their nursery grounds in Mexican waters.