Thermal Habitat Use by Lake Trout in Two Contrasting Yukon Territory Lakes

Thermal habitat use by lake trout Salvelinus namaycush in two northern lakes in the southern portion of the Yukon Territory that differ in morphometry and thermal regime was monitored using temperature‐sensitive acoustics and radiotelemetry. We then contrasted in situ temperature selection by lake trout in these lakes with previously published estimates of the species' optimal thermal range of 8–12°C. We found that thermal habitat use by lake trout in the two northern lakes is not consistent with these literature‐derived expectations. In Dezadeash Lake, which is isothermal in summer, temperatures typically exceeded the literature‐derived upper limit of 12°C. Throughout the summer lake trout sought the coldest water in the lake, which was in the form of shallow coldwater plumes derived from alpine ice‐pack meltwater streams. Once lake temperatures declined in the fall, lake trout were distributed throughout the lake. In Kathleen Lake, where water temperatures ranged from approximately 2°C to 12°C in the summer, the majority of lake trout selected habitats throughout the summer and fall that were colder than the 8°C lower limit of their literature‐derived optimal thermal range. Our results highlight the importance of summer thermal refugia for lake trout inhabiting marginal systems and the variation in thermal habitat use among populations inhabiting different thermal environments. Given the established importance of thermal habitat availability to lake trout production, our results suggest the need to better understand optimal thermal habitat characteristics in nature, particularly in light of factors such as climate warming.