A new element in the migration cycle of the European river lamprey Lampetra fluviatilis: downstream migration from a lake

In 2019, during spring flood, we collected unusual Lampetra fluviatilis smolts migrating from a small lake, as part of a multi-year survey in the Chernaya River (Gulf of Finland, Baltic Sea). They differed from the ordinary Lampetra fluviatilis individuals in some of the features, mainly size. The emigration occurred at the darkest period of night and during extreme flooding, which significantly worsened the conditions in the Lake Gladyshevskoe and Chernaya River. In these individuals, lacus smolts, emigration occurred in the spring season with the migration of ordinary flumen specimens. Several hypotheses are discussed to explain the observation of large lacus (148–165 mm) individuals: they are seaward migrants, within expected body sizes of European river lamprey species but unusual for Gulf of Finland tributaries; the lake is a transitional habitat, and the postmetamorphic lacus juveniles migrating through it from a tributary; postmetamorphic lacus juveniles spend some time feeding in a lake but emigrated due to environmental conditions; larvae of European river lamprey, developing in the eutrophic lake, reach greater sizes than riverine larvae before transformation and subsequently produce larger juveniles.