Trophic overlap in mobulid rays: insights from stable isotope analysis

Mobulid rays, a group of closely related filter-feeders, are threatened globally by bycatch and targeted fisheries. Their habitat use and feeding ecology are not well studied, and most efforts have focused on temporally limited stomach content analysis or inferences from tagging data. Previous studies demonstrate a variety of different diving behaviors across species, which researchers have interpreted as evidence of disparate foraging strategies. However, few studies have examined feeding habitats and diets of multiple mobulid species from a single location, and it is unclear if the proposed differences in diving and inferred foraging behavior are examples of variability between species or regional adaptations to food availability. Here, we use stable isotope data from mobulids landed in fisheries to examine the feeding ecology of 5 species at 3 sites in the Indo-Pacific. We use Bayesian mixing models and analyses of isotopic niche areas to demonstrate dietary overlap between sympatric mobulid species at all of our study sites. We show the degree of overlap may be inversely related to productivity, which is contrary to prevailing theories of niche overlap. We use isotope data from 2 tissues to examine diet stability of Manta birostris and Mobula tarapacana in the Philippines. Finally, we observe a significant but weak relationship between body size and isotope values across species. Our findings highlight challenges to bycatch mitigation measures for mobulid species and may explain the multi-species mobulid bycatch that occurs in a variety of fisheries around the world.