Shark Predation on Migrating Adult American Eels (Anguilla rostrata) in the Gulf of St. Lawrence

In an attempt to document the migratory pathways and the environmental conditions encountered by American eels during their oceanic migration to the Sargasso Sea, we tagged eight silver eels with miniature satellite pop-up tags during their migration from the St. Lawrence River in Québec, Canada. Surprisingly, of the seven tags that successfully transmitted archived data, six were ingested by warm-gutted predators, as observed by a sudden increase in water temperature. Gut temperatures were in the range of 20 to 25°C—too cold for marine mammals but within the range of endothermic fish. In order to identify the eel predators, we compared their vertical migratory behavior with those of satellite-tagged porbeagle shark and bluefin tuna, the only endothermic fishes occurring non-marginally in the Gulf of St. Lawrence. We accurately distinguished between tuna and shark by using the behavioral criteria generated by comparing the diving behavior of these two species with those of our unknown predators. Depth profile characteristics of most eel predators more closely resembled those of sharks than those of tuna. During the first days following tagging, all eels remained in surface waters and did not exhibit diel vertical migrations. Three eels were eaten at this time. Two eels exhibited inverse diel vertical migrations (at surface during the day) during several days prior to predation. Four eels were eaten during daytime, whereas the two night-predation events occurred at full moon. Although tagging itself may contribute to increasing the eel's susceptibility to predation, we discuss evidence suggesting that predation of silver-stage American eels by porbeagle sharks may represent a significant source of mortality inside the Gulf of St. Lawrence and raises the possibility that eels may represent a reliable, predictable food resource for porbeagle sharks.

为记录美洲鳗鲡向马尾藻海（Sargasso Sea）远洋洄游的路径及途中遭遇的环境条件，我们于加拿大魁北克省圣劳伦斯河（St. Lawrence River）的洄游途中，为8条银阶段美洲鳗鲡（silver eels）佩戴了微型卫星弹出式标签（satellite pop-up tags）。令人意外的是，在成功传回归档数据的7枚标签中，有6枚被温腹捕食者摄食，该现象可通过水温骤升的特征观测得到。其腹腔温度介于20℃至25℃之间：该温度对海洋哺乳动物而言过低，但恰好处于恒温鱼类（endothermic fish）的体温区间内。为鉴定该鳗鲡的捕食者种类，我们将其垂直洄游行为与经卫星标签标记的鼠鲨（porbeagle shark）及蓝鳍金枪鱼（bluefin tuna）的行为进行比对——这两种恒温鱼类是圣劳伦斯湾（Gulf of St. Lawrence）内非边缘分布的唯一恒温鱼类。通过比对这两个物种与未知捕食者的潜水行为所构建的行为判据，我们精准区分了蓝鳍金枪鱼与鼠鲨。多数鳗鲡捕食者的深度剖面特征与鼠鲨的相似度远高于蓝鳍金枪鱼。在标记后的初始阶段，所有鳗鲡均停留在表层水域，未表现出昼夜垂直迁移（diel vertical migrations）行为，其中3条于该阶段被捕食。另有2条鳗鲡在被捕食前数日呈现出反向昼夜垂直迁移——即白天活动于表层水域。4条鳗鲡于白天被捕食，剩余2起夜间捕食事件均发生在满月时段。尽管标记操作本身可能会提升鳗鲡被捕食的风险，但我们的讨论表明，鼠鲨对银阶段美洲鳗鲡的捕食可能是圣劳伦斯湾内鳗鲡死亡的重要诱因，同时也提示鳗鲡或许是鼠鲨可稳定获取且可预测的食物资源。