Vertical distribution of brown trout (Salmo Trutta) and perch (Perca Fluviatilis) in an acidified lake

Abstract

Brown trout (Salmo trutta) and perch (Perca fluviatilis) had different vertical distributions throughout the year in the acidified lake Gjerstadvann. During summer, the brown trout lived in the 0 to 16 m depth interval, whereas the perch lived in the 0 to 8 m interval. In Gjerstadvann, the thermocline lies at 8 to 10 m depth. The epilimnion pH was usually > 5.6, and the thermocline pH was about 5.2. Brown trout and perch in Gjerstadvann, thus experienced different chemical environment during summer-stratification. Two rivers, one of them acidified, the other circum-neutral, were the most important spawning areas for the Gjerstadvann brown trout. Brown trout parr in the acidified river migrated to the lake and matured at younger ages than the brown trout spawning in the non-acidic river. The brown trout stocks were juvenilized because of low survival rate of adult fish (S = 0.15 to 0.19). The short life span was probably dus to low pH and elevated Al. The perch in Gjerstadvann showed fluctuating year class strength but the survival rate of adult perch (S = 0.57) did not seem to be affected by acidification during this investigation. This may due to its vertical distribution during summer.