Age and Sex Segregation in Habitat Utilization by Brown Trout in a Norwegian Lake

Abstract

The resident brown trout Salmo trutta in Lake Myrkdalsvatnet utilized benthic and pelagic areas down to a depth of 40 m. Most 1- and 2-year-old fish were littoral; with increasing age, pelagic and deeper benthic habitats were utilized. It is hypothesized that the significant habitat segregation is caused by differential foraging profitabilities and social interactions among age groups. The proportion of females was significantly higher in pelagic than in benthic areas. Among mature fish, the males, but not the females, congregated in onshore areas in September before entering the tributaries to spawn in October-November. Age groups 1–2 fed mainly on littoral zoobenthos. In August, when zooplankton and surface insects were abundant, age-2 fish also were numerous offshore. Older benthic fish fed largely on chironomid larvae and pupae. Surface insects and zooplankton were important food for pelagic brown trout. Adult chironomids mainly were eaten during early summer, swarming ants during late summer. Holopedium gibberum and Bythotrephes longimanus were the selected zooplankton; smaller cladocerans were not eaten.