Resource use of two sympatric sharks in response to fishing activities at a remote island - fatty acid and stable isotope data

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Sharks often modify their behaviour to obtain food from humans. Food provisioning can lead to behavioural conditioning, which can alter resource use patterns and interactions within and between species. At Norfolk Island, a remote small island in the Pacific Ocean, sharks aggregate at the two piers where fish are cleaned and discards are thrown into the water. We used acoustic telemetry and dietary biomarkers (fatty acids and stable isotopes) to quantify dusky (Carcharhinus obscurus; n = 18) and Galapagos (C. galapagensis; n = 16) shark associations with the piers. Specifically, we evaluated the frequency and timing of visitations to piers across diel periods, patterns of co-occurrence, and contribution of discards to the diet of both species. Individuals visited the piers, on average, once every four days (~3 hours daily), with dusky sharks usually arriving in the early afternoon and staying until dusk while Galapagos sharks arrived at dusk and left in the early morning. Despite this temporal partitioning, the diets of both species were indistinguishable from each other and showed only limited contribution of discarded fish; however, fatty acids indicated that fish scraps were more common in the diet of sharks with high residency to the piers. The findings of this study indicate that food provisioning at the piers has not markedly altered foraging behaviour and that regular visits to the piers only serve to supplement diet. Understanding how sharks use the piers in response to human activities will help assess current waste disposal practices at Norfolk Island and ongoing monitoring needs.