MEPS 636:107-121 (2020)  -  DOI: https://doi.org/10.3354/meps13221

ABSTRACT: 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 (δ¹³C and δ¹⁵N) 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 δ¹³C and δ¹⁵N values of all 4 species ranged from -18.6 to -16.2‰ and 8.3 to 13.8‰, respectively. The small δ¹³C 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. walkeri exhibited significant increases in δ¹⁵N (~3‰) and δ¹³C (~2‰) with size. Latitude, longitude, and depth had statistically significant but comparatively minor effects on isotope values (≤1‰) 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.

KEY WORDS: Elasmobranchs · Deepwater · Stable isotope analysis · Great Barrier Reef