Eels on the move

Study tracks the fish for the first 1,300 kilometers of their migration

European eels really take the plunge when they embark on their 5,000-kilometer migration to the Sargasso Sea. Data from eels outfitted with pop-up satellite tags suggest that the slender fish spend the daytime hours of the journey well below the surface, diving to cool depths between 200 and 1,000 meters, researchers report in the Sept. 25 Science.

EEL ABOARD Data from the first leg of European eels’ 5,000-kilometer migration suggest daily dives to cooler waters and a leisurely pace, scientists report. Courtesy of Eva Thorstad

The tags allowed researchers to follow 22 eels for the first 1,300 kilometers of their trek from the coast of Ireland to mating grounds near the Bahamas. Understanding the details of the eels’ journey may help to protect this critically endangered species, a favorite of sushi eaters.

Some creatures are known to swim in cool waters during the day and warmer surface waters at night, an up and down migration pattern that can help the animals avoid predators or find food. For the eels, vertical movement might help control development. Spending time in cold water may prevent the eels’ reproductive organs from becoming fully mature before reaching mating grounds, the researchers speculate.

Migration speeds for this first leg were surprisingly slow — a mere 5 to 25 kilometers per day, much less than the 35 kilometers required per day to reach the destination in time for spring spawning. This finding suggests the eels may gain speed and efficiency when they hit currents closer to the Sargasso Sea, an idea the scientists hope to investigate with better tracking technologies.

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