Abstract
Sri Lankan fisheries have substantial elasmobranch catches, but the local ecology of individual species is not well characterized. We examine the tiger shark (Galeocerdo cuvier) and Bengal whipray (Brevitrygon imbricata), two elasmobranch species with variable life history and feeding ecology that represent differing trophic guilds. Tiger sharks have a global distribution and are well-studied in some regions, but there is a lack of ecological information specific to the Indian Ocean. In contrast, Bengal whiprays are often misidentified at the species level and are thought to largely feed on benthic flatworms. Here, we investigate the trophic ecology of these two species with stable isotope analysis, which tracks the nutrient flow through food webs. Morphometric measurements and samples were obtained from tiger sharks [muscle (n = 24), teeth (n = 17)] and Bengal whiprays [muscle (n = 44)] after boats were onshore; tissues were sampled and dried before transport for stable isotope preparation. Both tiger sharks and Bengal whiprays have a wide range of δ13C values spanning from -17.8 to -14.8‰, indicating diverse feeding habitats. In general, tiger sharks have higher δ15N values (13.3 ± 0.6‰) than Bengal whiprays (12.1 ± 0.7‰), although five Bengal whiprays had similar δ15N values to tiger sharks. There were also δ15N differences by sex among Bengal whiprays, which suggests some foraging or baseline differences within the population. The isotopic differences among market locations were subtle and difficult to discern given differences in sample size. These insights into the ecology of tiger sharks and Bengal whiprays in Sri Lanka, along with other studies, including tagging and stomach content analysis, are critical in developing ecosystem-based management strategies. For example, the identification of essential habitats for the Marine Protected Area designation would restrict fishing and help mitigate impacts on population structure and dynamics, two critical considerations for these two species, which are listed as Near Threatened and Vulnerable, respectively, on the International Union for Conservation of Nature Red List.