Brown trout (Salmo trutta) growth and condition along a winter thermal gradient in temperate streams

Winter harshness and the degree to which it can impact stream-dwelling salmonid populations has received considerable attention from fisheries biologists, although some debate exists regarding the importance of winter severity for local populations. Groundwater input may buffer stream water temperature and benefit fish in buffered versus unbuffered streams. Overwinter growth and condition of individual brown trout were measured in 24 groundwater-dominated streams, and the relations between winter growth and condition to stream thermal regime (quantified by regressions of air and water temperature) and diet quality (amount and caloric density of prey) were examined in a subset of 16 streams. Brown trout (Salmo trutta) growth rate (mg.g(-1).day(-1)) was positive overwinter in 18 of 24 streams, and there was no significant change in condition between early and late winter. Juvenile fish grew faster than adults, but there was no significant difference in condition between adults and juveniles. Thermal regime positively influenced winter growth for both adults and juveniles, likely mediated through moderation of water temperature by groundwater, whereas diet had no significant effect on growth.