We assessed thermal habitat use of lake-dwelling Brook Trout (Salvelinus fontinalis) populations in 39 lakes of varying size with short-set duration, stratified-random netting surveys in Algonquin Park, Ontario, Canada. Temperature at capture depth was determined from vertical temperature profiles and used as a proxy of Brook Trout temperature selection. Almost all Brook Trout observations fit within their aerobic scope, indicating that our approach adequately captured the distribution of body temperature for this species. However, we found a bimodal distribution of temperatures at capture depth. Most fish occupied a cold mode at capture, but a smaller subset of fish was captured in a warm-temperature mode that occurred in all lake-size categories, indicating Brook Trout that appeared to be actively moving into the warm-temperature mode. Adult Brook Trout occupied colder thermal habitats, on average, than published literature estimates and were found in significantly-colder temperatures than subadult Brook Trout within the same lake. Variation in thermal habitat use increased as lake size decreased. For lakes <100 ha, temperatures at capture depth were highly variable and ranged from 7.2 to 17.7°C. We report weighted means and distributions of thermal habitat boundaries because these boundaries may aid future efforts to project the influence of climate change on Brook Trout. Dissolved oxygen at capture depth suggested that fish occupy either an upper mode >5 ppm or a lower mode, including individuals captured at depths where dissolved oxygen is <2 ppm.

Depth stratified random netting surveys were conducted in 39 brook trout lakes in Algonquin Provincial Park, Ontario, Canada.  Set duration was for one hour providing as close to an instantaneous sample of fish location as could be done in a multi-lake survey.  Temperature and dissolved oxygen depth profiles were used to convert catches at depth to capture temperature or dissolved oxygen. Patterns in thermal habitat were compared to an aerobic scope curve to determine if field thermal habitat selection matches physiological performance. 

Data are partitioned by life stage (adult or sub-adul) and lake. All figures and tables are based on this data set.