SEASONAL-CHANGES IN HABITAT USE AND PREFERENCE BY JUVENILE BROWN TROUT, SALMO-TRUTTA, IN A NORTHERN BOREAL RIVER

By means of electrofishing, we examined seasonal and size-class variat ion in habitat preference by juvenile brown trout (Salmo trutta) in a third-order river in northern Finland. Larger trout preferred deeper s tream areas than young-of-the-year fish. At the onset of winter, all b out size-classes moved into shallower water, but this mainly reflected seasonal variation in habitat availability. In winter, trout preferre d slowly flowing stream areas, whereas in other seasons the mean water velocities used by trout parallelled habitat availability. In summer and autumn, age-0 fish favoured stream areas with large amounts of aqu atic vegetation to provide cover. The largest trout (16-22 cm) occupie d habitats with little cover throughout the year, and in winter, all t rout avoided areas with high instream cover. In summer, all size-class es preferred small substrates, whereas in winter, areas with cobble-bo ulder substrates were preferred, especially by trout larger than 10 cm . Wintering trout often shelter among the interstitial spaces of coars e substrates, and to facilitate the survival of juvenile trout through winter, stream management programmes need to ensure that such particl es are abundantly available in trout wintering areas.