Abstract
Large areas of the Łeba River catchment are inaccessible for fish because of barriers, and this reduces significantly their spawning possibilities. The sea trout Salmo trutta L. population in the Łeba River is sustained through compensatory stocking with smolts. In the autumn of 2007 and 2008, seventy two sea trout were caught in a lake, through which the river flows, several kilometers upstream from its mouth. These individuals were tagged with radio transmitters that were recorded by two automatic stations in the river. They were also monitored with active telemetry. Only 26 of these fish moved upstream (41% in 2007 and 33% in 2008), and five reached the spawning grounds. Neither the release location in the lake nor the size of the fish influenced the timing of the fish entering the river or the extent of their migration. Among the fish originated from smolt stocked in the river segment below the lake, 24% continued to migrate and entered the river after approximately 5.7 days, as compared to 45% of the wild fish entering the river after approximately 2.3 days. Most of the stocked fish remained in the lower reaches of the river, upstream from the lake.