Sub-ice thermal gradients strongly influence winter depth use of an iconic cold-water fish

In a thermally stratified lake, freshwater fish select habitats that align with their thermal preferences. While most studies of dimictic lakes have focused on habitat selection during summer stratification, less attention has been given to winter when inverse thermal stratification occurs with water temperatures between 0 to 4 °C. Our results show that winter thermal stratifications vary considerably among lakes. Much of the variation in winter depth use by the cold-water predator, Lake Trout ( Salvelinus namaycush ), can be explained by the depth of the 2 °C isotherm. Selection ratios further support preferential use of this thermal boundary after accounting for habitat availability. Larger, shallower, and windier lakes tend to exhibit colder mean temperatures near 0 °C and deeper isotherms, typically within the 1 to 2 °C range. In contrast, deeper or more sheltered lakes maintained warmer temperatures close to 4 °C and a shallower 2 °C isotherm. Using multiyear acoustic telemetry of Lake Trout from 11 different lakes, we found that in warmer lakes with depth-averaged temperatures above 2 °C, Lake Trout used thermal habitat averaging 2.6 ± 0.6 °C, which occurred in relatively shallow depths (0.1 to 11.3 m). In contrast, in the one large, shallow lake studied, where maximum water temperatures were below 2 °C, Lake Trout occupied the deepest waters during winter (16.8 to 26.4 m) and experienced average temperatures of 1.5 ± 0.4 °C. Applying our model to 877,927 Canadian and Alaskan lakes predicted a roughly even split between warm and cold winter conditions; however, 73% of the 66,500 known Lake Trout lakes exhibited warmer winter conditions, where Lake Trout would occupy the upper water column during winter.