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

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 (<i>Carcharhinus obscurus</i>; n = 18) and Galapagos (<i>C. galapagensis</i>; 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.

鲨鱼常通过改变行为来从人类处获取食物。人为投食（food provisioning）可导致行为条件反射（behavioural conditioning），进而改变物种内部及物种间的资源利用模式与相互作用。在太平洋偏远小岛诺福克岛（Norfolk Island），鲨鱼会聚集于两处码头——此处人们清洗鱼类并将废弃物抛入水中。我们采用声学遥测（acoustic telemetry）与饮食生物标志物（dietary biomarkers，包括脂肪酸（fatty acids）和稳定同位素（stable isotopes）），来量化灰真鲨（<i>Carcharhinus obscurus</i>；n=18）与加拉帕戈斯真鲨（<i>C. galapagensis</i>；n=16）与码头的关联程度。具体而言，我们评估了鲨鱼在昼夜周期内造访码头的频率与时间、共现模式，以及废弃物对两种鲨鱼饮食的贡献。个体平均每四天造访码头一次（每日约3小时），其中灰真鲨通常午后早些时候抵达并停留至黄昏，而加拉帕戈斯真鲨则于黄昏抵达并在清晨离开。尽管存在这种时间分隔，两种鲨鱼的饮食并无显著差异，且废弃物鱼类的贡献有限；不过，脂肪酸分析显示，在码头停留时间较长的鲨鱼饮食中，鱼类残渣更为常见。本研究结果表明，码头的人为投食并未显著改变鲨鱼的觅食行为，其定期造访码头仅为补充饮食。了解鲨鱼如何响应人类活动利用码头，将有助于评估诺福克岛当前的废弃物处理方式及持续监测需求。