1. Behaviour and microhabitat selection by brown trout in two streams presenting severe winter conditions, were investigated in south-central Norway. Fish were studied by direct underwater observation in different habitat types, during day and night, and with simultaneous sampling for dietary analysis and invertebrate drift. 2. During the day in winter, brown trout exhibited two different behavioural strategies, depending on fish size. Smaller fish (<25 cm) sheltered passively in the substrate or in submerged vegetation, and were almost never active during daylight hours. This behaviour was consistent throughout the winter until water temperatures rose in spring. Larger trout (25-60 cm) were actively aggregating in deep-slow stream areas during the day. 3. During the night the trout exhibited a third behavioural strategy. Fish were exposed and holding positions above or on the substrate. The trout were active throughout the night from dusk to dawn, and this behaviour was consistent throughout the winter. 4. Measured microgradients in temperature did not affect spatial niche selection. Spatial niches at night were selected principally on the basis of focal microwater velocities. The fish minimized energy expenditure by selecting habitats with low water velocites (pools, back eddies, stream bank areas). When passively sheltering during the day, interstices in the substrate or macrophytes providing shelter were the more important microhabitat characteristic. 5. Feeding continued at night during winter. However, the nocturnal activity pattern was not principally determined by food availability, since invertebrate drift did not show such periodicity. 6. The behaviour and spatial niche selection by brown trout is explained as an ecologically adaptive homeostatic behavioural response to buffer the energy deficit and adverse environmental conditions experienced by the wintering fish. The nocturnal activity strategy reduces the risk of habitat exclusion and microhabitat inclusion by ice formation, and sheltering in the day reduces the energy expenditure and the risk of predation. Thus, the behaviour in winter is best described as a cost-minimizing shelter-and-move strategy. 7. Little aggression in trout during winter suggests that social interactions may be less important as a factor regulating populations in winter than in summer. Instead, they may be more regulated by abiotic factors such as the physical structures of the stream.
