In boreal streams during winter, low temperatures reduce the ability of fish to avoid terrestrial endothermic predators. Juvenile salmonids react to this increased predation risk by minimizing exposure during the day. Surface ice offers overhead cover from aerial attacks by mammals and birds, and ice cover should thus reduce the need for vigilance and daytime concealment and increase food intake, affecting growth and diel patterns in habitat use. These predictions were tested in a series of field and laboratory experiments with brown trout as test species. In a natural stream, trout that spent the winter underneath ice cover (simulated by plastic sheeting) had higher growth rate and broader habitat use than trout in control sections. In an artificial stream in the laboratory, the presence of ice cover increased swimming activity, aggression and food intake rates, reduced stress levels and affected social interactions. The magnitude of the effects differed to some extent depending on the resting metabolic rate of individual trout. The results from these studies show that reduction in the extent and duration of ice cover due to global warming will likely alter wintertime habitat use and time and energy budgets of juvenile salmonids.