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1. Understanding of the winter ecology of stream salmonids is biased by research conducted in northern temperate and boreal regions dominated by hard rock geology. Such systems are driven by highly dynamic surface-flow regimes and tend to be physically diverse, nutrient poor and influenced by ice. This study investigated how the behaviour of brown trout, Salmo trutta, inhabiting a stable groundwater-fed, productive and comparatively warm southern English chalk stream differs from that described for other systems, and how this is translated to performance, measured as growth. 2. Physical characteristics were mapped, and high-resolution temperature data collected using a spatial array of data loggers installed throughout the study reach during the winter. A combination of passive integrated transponder and radio telemetry was used to monitor distribution, density, and movement of trout. Micro-archival data storage tags inserted in some individuals provided information on temperature regimes experienced. Growth performance was calculated for recaptured fish. 3. Trout density was positively related to depth and there was no evidence that temperature influenced microhabitat selection. Three patterns of movement were observed. Over three-quarters of tracked fish exhibited high site fidelity and tended to remain in a single focal position throughout the study. Fourteen per cent of trout exploited more than one distinct location, while the remainder were detected at multiple locations and showed no preference for any one. 4. Trout exhibited regular daily activity patterns and highly periodic local movements at dusk and dawn and tended to experience positive growth performance during periods that included winter. 5. This study challenges the conventional view of salmonid winter ecology, which is biased towards populations that inhabit hard rock surface-flow dominated rivers that experience the influence of ice. Despite inhabiting a distinctly different winter habitat templat

1. Understanding of the winter ecology of stream salmonids is biased by research conducted in northern temperate and boreal regions dominated by hard rock geology. Such systems are driven by highly dynamic surface-flow regimes and tend to be physically diverse, nutrient poor and influenced by ice. This study investigated how the behaviour of brown trout, Salmo trutta, inhabiting a stable groundwater-fed, productive and comparatively warm southern English chalk stream differs from that described for other systems, and how this is translated to performance, measured as growth. 2. Physical characteristics were mapped, and high-resolution temperature data collected using a spatial array of data loggers installed throughout the study reach during the winter. A combination of passive integrated transponder and radio telemetry was used to monitor distribution, density, and movement of trout. Micro-archival data storage tags inserted in some individuals provided information on temperature regimes experienced. Growth performance was calculated for recaptured fish. 3. Trout density was positively related to depth and there was no evidence that temperature influenced microhabitat selection. Three patterns of movement were observed. Over three-quarters of tracked fish exhibited high site fidelity and tended to remain in a single focal position throughout the study. Fourteen per cent of trout exploited more than one distinct location, while the remainder were detected at multiple locations and showed no preference for any one. 4. Trout exhibited regular daily activity patterns and highly periodic local movements at dusk and dawn and tended to experience positive growth performance during periods that included winter. 5. This study challenges the conventional view of salmonid winter ecology, which is biased towards populations that inhabit hard rock surface-flow dominated rivers that experience the influence of ice. Despite inhabiting a distinctly different winter habitat templat