Surf or turf? Behavioural associations between tiger sharks and butcher’s waste at Norfolk Island

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The activity of humans provisioning food to wild animals is a major resource management issue, not only because it may facilitate changes in animal behaviour and ecosystem structure, but also because it can impact human safety. In the marine realm, human interactions with sharks — often initiated through direct or indirect provisioning — require multifaceted management integrating social, cultural, and environmental perspectives. The complexity of managing human-shark interactions is highlighted at Norfolk Island, Australia. Due to the limited waste management capacity typical of small remote islands, organic waste from butchers (e.g., carcasses, offal, bones, hides) has historically been discarded directly into the ocean. Tiger sharks (Galeocerdo cuvier), a large predatory generalist, are often observed consuming this waste. Regional and federal governments have begun phasing out this waste disposal, raising safety concerns among locals that the cessation in provisioning may alter the behaviour of sharks. Accordingly, the purpose of this study was to measure spatiotemporal (acoustic telemetry) and dietary (fatty acids and stable isotopes) associations of tiger sharks (n = 47) with organic waste disposal activities at Norfolk Island to evaluate whether the behaviour of tiger sharks is regulated by provisioning. Tiger sharks were highly resident to the disposal site Headstone Bay (daily residency index ~ 0.42), independent of the frequency (daily) and type (i.e., bones/hides vs offal/whole animal) of disposal. Despite high levels of co-occurrence among sharks in Headstone Bay, network analysis showed no inherent social structure. There was minimal contribution of cow to the diet (6 – 8% contribution in stable isotope mixing models), with geographically distant prey sources (e.g., fish and reptiles from New Caledonia; 45 – 46%) and local sources (e.g., seabirds; 33 – 35%) being more important at both short- (plasma) and long-term (muscle) temporal scales. Overall, these findings highlight that the use of Headstone Bay may not be solely attributable to waste disposal activities; nevertheless, sharks may still frequent the area as part of an opportunistic foraging strategy. Our multidimensional research has important bearing on resource management locally at Norfolk Island but also extends beyond to other areas where applied ecological examinations are needed to understand and resolve human-animal interactions.