The European eel's singular spawning migration from European waters towards the Sargasso Sea remains elusive, including the early phase of migration at sea. During spawning migration, the movement of freshwater resident eels from river to sea has been thought to be irreversible. We report the first recorded incidents of eels returning to the river of origin after spending up to a year in the marine environment. After migrating to the Baltic Sea, 21% of the silver eels, tagged with acoustic transmitters, returned to the Narva River. Half returned 11–12 months after moving to the sea, with 15 km being the longest upstream movement. The returned eels spent up to 33 days in the river and migrated to the sea again. The fastest specimen migrated to the outlet of the Baltic Sea in 68 days after the second start—roughly 1300 km. The surprising occurrence of returning migrants has implications for sustainable management and protection of this critically endangered species.

欧洲鳗鱼从欧洲水域向马尾藻海的单一产卵迁徙仍然难以捉摸，包括海上迁徙的早期阶段。在产卵迁徙过程中，淡水鳗鱼从河流到海洋的移动被认为是不可逆转的。我们报告了首次有记录的鳗鱼在海洋环境中度过一年后返回原产地的事件。洄游到波罗的海后，21%的银鳗带着声学发射器标记返回纳尔瓦河。一半在出海后 11 至 12 个月返回，其中最长的上游移动距离为 15 公里。返回的鳗鱼在河里度过了长达33天的时间，然后再次洄游到大海。最快的标本在第二次出发后的 68 天内迁移到了波罗的海出海口——大约 1300 公里。令人惊讶的回返移民现象对这种极度濒危物种的可持续管理和保护产生了影响。