Stable isotope analysis reveals resource use by rays in an <scp>Australian</scp> temperate estuary

Estuarine ecosystems are increasingly threatened by anthropogenic activities that deplete or replace natural habitats. This may lead to cascading effects in estuarine food webs as the availability of primary producers changes. Rays are ecosystem engineers that feed in sedimentary environments, yet their main prey items and the primary producers supporting their nutrition are unknown. The aim of this study was to assess resource use of two ray species in Wallis Lake, New South Wales, Australia, the estuary stingray (Hemitrygon fluviorum) and the common stingaree (Trygonoptera testacea). Using stable isotope analysis and Bayesian mixing models, we evaluated the contribution of likely prey and primary producers that contribute to the nutrition of these rays. Our results from Bayesian mixing models revealed that filter-feeders, including oysters, contributed negligibly to the assimilated diet of both ray species (estuary stingray: 5 ± 4%, common stingaree: 8 ± 6%). Benthic fish and crustaceans contributed 37 ± 10% and 28 ± 11%, respectively, in average to the diet of the estuary stingray, whereas Nassarius gastropods contributed 27 ± 11% to the diet of the common stingaree. Both species relied primarily on particulate organic matter for nutrition (estuary stingray: 46 ± 7%, common stingaree: 49 ± 9%). We estimated the trophic positions of the estuary stingray (3.60 ± 0.51) and the common stingaree (3.76 ± 0.50). We also assessed the isotopic niche of the estuary stingray relative to a common estuarine mesopredator, yellowfin bream (Acanthopagrus australis). Bayesian ecological niche modelling showed disproportionate overlap between the two species, with 72.1% of the estuary stingray's niche overlapping with that of the yellowfin bream, whereas only 35.2% of the yellowfin bream's isotopic niche overlapping with that of the estuary stingray. The estuary stingray also occupied a smaller niche (147.44 ± 40.25) compared to the yellowfin bream (339.61 ± 107.50), indicating that the estuary stingray may be more vulnerable to environmental perturbations that impact its sources of prey. This study highlights the resource use of two estuarine ray species and provides valuable insights for estuary managers regarding the potential impacts of future estuary modifications and aquaculture development on nutrient sources, benthic communities and the implications for these mesopredators. Future work should apply additional complementary dietary study methods (e.g. stomach content analysis, prey availability) to confirm estuary stingray vulnerability.