On Epic Spawning Migration, Eels May Travel at Their Own Pace
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Version 0 of 1. Every year, when the autumn chill sets in, eels start to transform. Their skins thicken, donning a coat of mucus and changing from a muddied yellow to a gunmetal silver. Their stomachs shrivel and turn white. Their eyes expand, so they can see better in the dark. On moonless, stormy nights, when the rivers flow fast, they travel downstream until they reach the sea. Then, they disappear. For a century, researchers have been trying to untangle what happens to European and American eels on their migration from rivers to their spawning grounds in the Sargasso Sea, thousands of miles away in the North Atlantic. Scientists have long assumed that migrating eels arrive to the Sargasso by spring to spawn together, as one reproductive cohort. A new study of European eels, published Wednesday in Science Advances, challenges that belief. The results suggest that this population of eels employs a mixed migration strategy, with some eels arriving at the Sargasso in time for spawning season and others arriving much later, to spawn the following season. These findings could help conservationists better understand patterns of population loss for the critically endangered fish, said David Righton, a behavioral ecologist at the Center for Environment, Fisheries and Aquaculture Science in Britain and one of the authors of the study. In the five-year study, Dr. Righton and his colleagues attached electronic tags to female European eels captured in the wild and released them from the coasts of France, Ireland, Sweden and Germany. From the trackers they recovered, the scientists reconstructed detailed routes of the first half of the Sargasso migration for more than 80 eels. They pieced together travel speeds from their tracking data, and checked the speeds against periods of peak departure and spawning estimated from historical surveys and other studies. From there, the scientists deduced that few eels departing Europe in the fall actually reach the Sargasso by the peak spawning season, when most larvae hatch. Many, in fact, wait for the next season to spawn. How quickly an eel makes the journey seems to come down to individual variation, Dr. Righton added. “It’s just like people. Some go fast, some go slow and most are somewhere in the middle.” Predators like whales probably pose a serious threat along the migration, as suggested by the extreme depths of well over half a mile that the eels keep to during the day. A mixed migratory strategy probably benefits the species by spreading the risk to reproduction posed by traveling in groups, Dr. Righton said. Many questions about the European eel’s legendary migration remain. Scientists still do not know how they navigate to the Sargasso, for instance, which they share with their American eel cousins as spawning grounds. For now, that many eels take their time when making the journey speaks to its importance. The fish make the trip only once in their life — it’s their only chance to reproduce. By the time females make it to the Sargasso, they have undergone puberty and have endured months of rough conditions, all under starvation. With the little energy they have left, they release millions of eggs into the sea. With their mission finally complete, they are free to die. |