Effects of siltation on resource utilization and dynamics of allopatric brown trout,Salmo trulta,in a reservoir

Synopsis The water level of the old hydroelectric Ringedal Reservoir in western Norway was unusually low during 1985, resulting in severe erosion and siltation. The secchi disc transparency was 18 m in July 1984, decreased to 0.3 m in July 1985, and increased to 13 m in July 1986 after a rise in the water level. The abiotic changes induced major effects in the zooplankton community, with a strong reduction in occurrence of Cladocera (Holopedium gibberum andBosmina longispina). The allopatric population of brown trout,Salmo trutta, which mainly fed on zooplankton before the siltation, fed predominantly on surface insects during the summer of high siltation. The food consumption was lower than in the pre- and post-siltation years, resulting in reduced k-values, reduced number of spawners, and increased mortality of mature fish.