Movements of adult sea trout Salmo trutta L. in the tailrace of a low-head dam at Włocławek hydroelectric station on the Vistula River, Poland

Abstract

Ultrasonic telemetry was used to study the movements and behaviour of 27 adult migratory sea trout, 60–90 cm in fork length. Fish were caught near the tailrace of a hydroelectric dam, tagged externally and released in October 1993, 1994 and 1996. Distribution of tagged sea trout depended on flow patterns in the tailrace. At high water discharge (maximum water current greater than 1 m s^-1) during the day, fish were observed about 250–500 m from turbines in the main stream or on the right of it. At lower water discharges, tagged fish were observed in the main stream both during the day and at night. Fish mainly moved upstream during the day and downstream at night. Fish were closer (about 50–250 m) to the turbines during the day than at night (about 300–500 m). They swam downstream to the right of main stream at night. The mean ground swimming speed of tagged sea trout was 1.98 cm s^-1 during the day and 8.33 cm s^-1 at night. Estimated swimming speeds correlated with time of day and fish length. Fish rarely entered the fishladder when water level near the entrance was low. Influence of water flow patterns on distribution of migratory sea trout is discussed. Fish transferred above the dam successfully migrated upstream through the reservoir under conditions of high water discharge.