Downstream migration of early larvae of the European river lamprey Lampetra fluviatilis.

Abstract

Migrations at early stages of development are char� acteristic of many animal species. Their adaptive sig� nificance is related to distribution of individuals and occupation of the trophic part of the area. As a rule, juveniles of aquatic animals are distributed through downstream migration due to the transporting force of water flow. Many studies deal with downstream migra� tions and consider in detail regularities and mecha� nisms of this process (1). Detailed investigations on Petromyzontiformes have been performed only on the Arctic lamprey ( Lethenteron camtschaticum) from riv� ers of the northwest coast of Kamchatka (2, 3). We are not aware of such investigations on the European river lamprey Lampetra fluviatilis. The present study is aimed at elucidation of presence of downstream migration and its special traits in larvae of the Euro� pean river lamprey in rivers of the Baltic Sea basin. The Gladyshevka River flows through Vyborgsky District of Leningrad oblast and Kurortnyi district of the city of St. Petersburg. It originates from Lake Gla� dyshevskoe and mingles with the Roshchinka River. Together, they form the Chernaya River flowing to the Gulf of Finland of the Baltic Sea. The lengths of the rivers Gladyshevka and Roshchinka from their origins to the confluence are 12 and 15 km, respectively. The coordinates of the confluence point are 60°13'15.74'' N, 29°30'56.26'' E. The length of the Chernaya River is 5 km. The total area of the drainage basin of these riv� ers is 668 km 2 . In the lowwater period, the depths on riffles is 1 m; on broads, 2.5 m. The bottom is sandy� gravelly, sandy along the banks, with a layer of silt up to 10 cm thick. The width of the Gladyshevka River and the Roshchinka River at their confluence is 15-20 m. Fishing gears were set in the Gladyshevka River and the Roshchinka River 100 m and 20 m upstream of the confluence, respectively; in the Chernaya River, 1 km from its inflow to the Gulf of Finland. At places of set� ting of the fishing gears, the current rates were the fol� lowing: in the Gladyshevka River, 0.76 m/s; in the Roshchinka River, 0.56 m/s; in the Chernaya River, about 1.5 m/s. The Kamenka River is a small stream 10 km long in Luzhski District of Leningrad oblast. It is a firstorder tributary in the middle stream of the Luga River flow� ing into the Gulf of Finland of the Baltic Sea. The coordinates of the mouth of the Kamenka River are 58°52'46.28'' N, 29°49'12.32'' E. On the investigated stretch, up to 1 km from the mouth, the river bottom is pebblegravelly, sandsludge deposits occur along the banks. The river widths does not exceed 4 m, the depth varies from 0.2 to 0.5 m. At the place of setting of the fishing gear 200 m upstream of the mouth, the current rate in the main channel was 0.45 m/s. Youngoftheyear (YOY) lamprey were caught using the standard methods (4) with a passive fishing gear, an ichthyoplankton conic net (ICN). The aper� ture area of the net (0.4 × 0.4 m) was 0.16 m 2 . The net