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

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 (<i>Galeocerdo cuvier</i>), 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 <i>vs</i> 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.

人类向野生动物投喂食物的行为是一项关键的资源管理议题，其不仅可能引发动物行为与生态系统结构的改变，还会对人类安全造成负面影响。在海洋领域，人类与鲨鱼的互动（通常由直接或间接投喂行为引发）需要整合社会、文化与环境视角的多维度管理方案。澳大利亚诺福克岛的情况凸显了管控人鲨互动的复杂性。由于小型偏远岛屿普遍存在废物处理能力不足的问题，历史上屠宰场产生的有机废物（如动物尸体、内脏、骨骼、兽皮）均被直接排入海洋。虎鲨（Tiger shark，*Galeocerdo cuvier*）作为大型广食性捕食者，常被观测到取食此类海洋有机废物。地区与联邦政府已开始逐步取消这类废物处置方式，当地居民因此产生安全顾虑，担忧投喂行为的终止可能改变鲨鱼的行为模式。据此，本研究旨在通过声学遥测（acoustic telemetry）获取的时空数据，以及脂肪酸、稳定同位素（stable isotopes）分析得到的膳食数据，探究47尾虎鲨在诺福克岛与有机废物处置活动的时空与营养关联，以评估虎鲨的行为是否受投喂行为调控。虎鲨对处置场地墓石湾（Headstone Bay）具有极高的栖息偏好（日栖息指数约为0.42），且该偏好与废物处置的频率（每日开展）及类型（骨骼/兽皮相较内脏/整只动物）均无关联。尽管墓石湾内的虎鲨存在较高的共现率，但网络分析结果显示其不存在固有的社会结构。牛源性食物在虎鲨膳食中的占比极低（稳定同位素混合模型（stable isotope mixing models）估算为6%~8%）；而地理距离较远的猎物来源（如新喀里多尼亚的鱼类与爬行动物，占比45%~46%）与本地猎物来源（如海鸟，占比33%~35%）在短期（血浆样本）与长期（肌肉样本）时间尺度上均占据更重要的地位。总体而言，本研究结果表明，虎鲨对墓石湾的栖息利用或许并非仅由废物处置活动导致；尽管如此，虎鲨仍可能将该区域作为机会主义觅食策略的一部分而频繁造访。本项多维度研究不仅对诺福克岛本地的资源管理具有重要参考价值，还为其他亟需开展应用生态学研究以理解并解决人兽互动问题的地区提供了借鉴。