Differences in habitat utilization among native, native stocked, and non-native stocked brown trout (Salmo trutta) in a hydroelectric reservoir

Native and native-stocked brown trout (Salmo trutta) in Lake Tesse, a regulated hydroelectric reservoir (southern Norway), were spatially segregated according to size: small individuals occurred mainly in the epibenthic habitat and larger individuals mainly in the pelagic habitat. In contrast, all size groups of non-native stocked brown trout were mostly restricted to the epibenthic habitat. Age-specific lengths were generally larger for non-native than for native stocked trout, which were larger than native fish. However, growth rate between age 3 and 4 was significantly lower for non-native stocked fish than for native and native stocked fish. Differences in body length were mainly due to strain but also to some extent to habitat. Native fish had significantly fuller stomachs in the pelagic than in the epibenthic habitat in summer. Epibenthic non-native fish had significantly fuller stomachs than native and native stocked fish in August but not in July. Native and native stocked fish fed mainly on surface insects and planktonic crustaceans in both habitats. We hypothesize that the non-native brown trout stocked in Lake Tesse do not use the pelagic habitat in the home lake and are therefore less adapted to utilize such habitat than populations originating from lakes where pelagic habitat is available.