Photo: Anders Asp, University of Southern Denmark

Migrating can be risky business for many animals, which is why scientists thought European eels took the deep-water route as they traveled across the Atlantic Ocean. Turns out, deep water doesn’t seem to be all that safe. A group of researchers from Denmark have discovered that whales may be hunting the eels down as they swim from the freshwater rivers and streams in Europe where they live most of their lives to the Sargasso Sea where they mate.

Curious about how scientists discovered this? The tell tale signs washed up on the beach in whale poop. Magnus Wahlberg, a biologist from the University of Southern Denmark placed devices called data loggers on about 156 eels before they made their migration across the Atlantic. Just as the name suggests, these instruments record all kinds of data about depth, temperature and location of the eels.

Three of the eels never made it to the Sargasso Sea. Their data loggers were found in whale poop that washed ashore. There’s only one way for that to happen–the eels traveled through the whale’s digestive system. The data loggers tell scientists exactly where and when. Apparently, they were swimming along at a depth of about 200 feet when the temperature got really warm. It jumped from 50 degrees Fahrenheit to almost 97 degrees Fahrenheit, the temperature inside the belly of a warm-blooded marine mammal.

The European eel is an endangered species, so it’s important that as many eels as possible have the chance to reproduce. That’s why scientists have been monitoring their migration. What they found was surprising for two reasons; One, they didn’t realize whales were a predator for these eels; and two, they thought deep water offered the eels protection on their journey.

Learn more about the European Eel at the Zoological Society of London

Read about this project here