Evidence of marine mammal predation of the European eel (Anguilla anguilla L.) on its marine migration

Temperature and depth logging tags were implanted into adult eels released on Atlantic west coasts of France and Ireland to study their oceanic migration behavior. For three of the tags, 25 to 256 days after release there was a dramatic rise in temperature from 10°C to 36°C and the dive profile changed from depths of 300–1000m to repeated ascents to the surface. This indicated that the eels carrying the tags had been eaten by a mammalian predator. Two of the tags had sufficient sampling rate to resolve the dives in detail. They recorded a total of 91 dives to maximum depths of 250–860m lasting 11–12min and with surface intervals of 5–7min. More than two thirds of the dives included a rapid descent from approximately 500m to 600–700m. From this we infer that the predator was most likely a deep-diving toothed whale. The dives logged while the tags were inside the predator revealed that the temperature usually decreased during dives, and increased again during surface periods. The temperature drops during dives were probably caused by the ingestion of prey or water. These observations provide insights into the behavior of toothed whales foraging in the mesopelagic zone. •European eels tagged with electronic tags were eaten by deep-diving marine mammals.•Predation occurred at depths >600m, and show that eels are at risk even in the mesopelagic.•The tags continued to collect temperature and depth data until they were voided up to 50h after ingestion.•The depth data from the ingested tags show frequent diving to depths of up to 800m, and with dive profiles similar to toothed whales.•The temperature data from the ingested tags can be used to infer foraging success and ingestion of water by the whale.