European eels are critically endangered. To better understand their lives, researchers tracked them during a migration. Suddenly, three trackers returned very strange data. What, oh what, could have happened?

European eels are reverse salmon. They live most of their adult lives in rivers, but will eventually migrate out into the ocean and journey to the Sargasso Sea. There they spawn and die, starting the cycle over.

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Once at sea the eels stay in deep water, so little is known about their behavior. Researchers decided to implant temperature and depth logging tags into a group of eels. Most of the eels made it to the Sargasso Sea, but three eels weren’t so lucky. The report states:

“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 300–1000 m to repeated ascents to the surface. . . . 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 250–860 m lasting 11–12 min and with surface intervals of 5–7 min. More than two thirds of the dives included a rapid descent from approximately 500 m to 600–700 m.”

Deep dives were accompanied by short drops in temperature. It wasn’t a mystery what had happened. The eels had been eaten by whales. That was what caused the quick rise in temperature—and let the researchers know they were dealing with a warm-blooded animal. The whale went on their way, the implants still transmitting data inside them. The depth of the dives indicated they were toothed whales, probably pilot whales, and the quick drops in temperature during those dives happened when either water or some other unfortunate prey had joined the eel in the whale’s stomach.

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The report concludes by saying, “The tags continued to collect temperature and depth data until they were voided up to 50 h after ingestion.” The process of getting out of the whale was apparently more traumatic than going in, because it broke the tags, although the remains of the tags did eventually wash up on a beach and were turned in to the scientific organization by people who found the debris.

[Source: Evidence of mammalian predation of the European eel during its marine migration, Whales Devour European Eels]

Image: Barney Moss