Evidence for inter- and intraspecific trophic niche separation among deepwater elasmobranchs on the southern Great Barrier Reef, Australia

Quantifying the trophic structure and interactions of deepwater (>200 m depth) elasmobranch assemblages is required to improve our understanding of deepwater ecosystems and the impacts of increased deepwater exploitation. To this end, we investigated the trophic ecology of deepwater elasmobranchs on the Great Barrier Reef (GBR) using a stable isotope (delta C-13 and delta N-15) approach. Our study included 4 species captured in the southern GBR deepwater eastern king prawn trawl fishery: the eastern spotted gummy shark Mustelus walkeri, the piked spurdog Squalus megalops, the pale spotted catshark Asymbolus pallidus, and the Argus skate Dentiraja polyommata. The delta C-13 and delta N-15 values of all 4 species ranged from -18.6 to -16.2 parts per thousand and 8.3 to 13.8 parts per thousand, respectively. The small C-13 range was likely due to the limited number of unique carbon baseline sources typically found in deepwater environments. Despite this, 3 of the 4 species exhibited relatively low core (40% SEA b ) isotopic niche overlap (<1 to 44%). Isotopic niche separation may be driven by multiple interacting factors including morphology, feeding strategies, or resource partitioning to reduce competition. Isotope analysis also provided evidence for intraspecific variation; S. megalops, D. polyommata and M. walked exhibited significant increases in delta N-15 (similar to 3 parts per thousand) and delta C-13 (similar to 2 parts per thousand) with size. Latitude, longitude, and depth had statistically significant but comparatively minor effects on isotope values (<= 1 parts per thousand) of the 4 species. Cumulatively, our results indicate that isotopic variation among deepwater elasmobranchs on the GBR is principally driven by size and species-level differences in resource use