Stable isotope assessment of whale sharks across two ocean basins: Gulf of California and the Mexican Caribbean

The use of stable isotope analysis has proven to be a reliable approach to investigate the diet of whale sharks in their natural habitat. A total of 63 whale sharks (50 male, 13 female) were sampled from the two localities (La Paz Bay n = 32, Yucatan Peninsula n = 31) in Mexico. Isotopic signatures differed greatly between both localities with mean values for delta N-15 in La Paz Bay (13.5 +/- 0.1 parts per thousand) being considerably higher than the Yucatan Peninsula (8.3 +/- 0.1 parts per thousand). As for delta C-13 both localities represented similar means (La Paz Bay - 15.4 +/- 0.5%o, Yucatan Peninsula - 14.2 +/- 0.1 parts per thousand). A Mann-Whitney U test revealed no significant differences between sexes at localities for delta N-15 (La Paz Bay, p > .05, Yucatan Peninsula, P > .05) and also for delta C-13 (La Paz Bay, p > .05, Yucatan Peninsula, P > .05). Between size groups of individuals ( < 6 m - > 6 m), significant differences were observed in the delta N-15 for whale sharks solely from La Paz Bay (p < .05), while no differences were found for delta C-13 at both localities. A significant relationship was observed between animal size and delta N-15 values, only for La Paz Bay (r(2) = 0.27, p <= .05). Analysis focused on the isotopic niches within each locality, revealed that individuals from La Paz Bay occupied the largest isotopic niche area compared with the Yucatan Peninsula, suggesting that animals from this population are using a different isoscape outside of the Gulf of California compared to a more uniform use of the habitat in the Gulf of Mexico. Ultimately, this study provided the first isotopic assessment of whale sharks at two unique geographical sites, providing key information that reveals differences in the base of the food web can affect the overall isotopic values.