European eels grow up in rivers, lakes and canals all over Europe, the Scheldt included. After ten years or more they turn silver, swim out to sea in autumn and set off for the Sargasso Sea, up to 10,000 km away, to spawn once and die. For a century this was known only from the larvae. In the early 1900s the Danish scientist Johannes Schmidt caught ever smaller eel larvae the further he sailed south-west, until the smallest of all turned up in the Sargasso Sea. No adult eel had ever been followed there. In 2018 and 2019 a team led by the Environment Agency tagged 26 large female eels in the Azores and released them into the Atlantic. The tags rode along for 1 to 12 months, then let go, floated to the surface and sent their position to a satellite. All the eels headed south-west, slowly and deep, at 3 to 12 km a day. Five reached the Sargasso Sea, and one was inside the spawning area itself: the first direct evidence that adult European eels make it there. The eels spawn in late winter and spring, though nobody has yet seen it happen. Their larvae, flat and transparent like a willow leaf, are carried back by the Gulf Stream and the North Atlantic Current. The crossing takes at least ten months, and probably about two years. On the edge of the continental shelf they turn into glass eels and swim up the rivers. Since the 1980s the number of glass eels reaching Europe has fallen by about 95%. The European eel is now listed as critically endangered. How this is drawn: each tagged eel is a straight line from its release point to the point where its tag surfaced, at constant speed, which is how the study itself treats the tracks. The dashed line is the average heading, which the study projected forward to the spawning season. The larvae are drawn along the route known from larval surveys, as an illustration, and are not a simulation.
Source: Adult tracks: Wright et al. (2022), First direct evidence of adult European eels migrating to their breeding place in the Sargasso Sea, Scientific Reports 12, 15362. Larval drift duration: Bonhommeau et al. (2009), Fisheries Oceanography. Spawning area: Miller et al. (2019). Coastline: GLOBE land mask. Visualisation by The Moving Sea
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