Brook Trout Thermal Habitat Selection in Stratified Lakes: Diel Patterns, Body Size, and the Importance of Thermal Time

Thermal habitat selection is a key consideration when assessing the capacity of ectotherms to tolerate warming conditions during summer. We used high dimension acoustic arrays in three lakes to determine diel thermal habitat selection by free-ranging adult brook trout (Salvelinus fontinalis (Mitchill, 1814)), with surgically implanted acoustic tags (fork length range, 26.2–54.5 cm). Brook trout occupied a range of temperatures during lake stratification when warm surface waters and cold hypolimnetic waters were present. The overall mean occupied temperature was close to literature estimates of a single preferred temperature (15°C), but few individuals spent most of their time at this temperature. The preferred temperature range was 13–17°C in each lake while < 10°C and > 20°C were avoided. Individual variation was extensive and closely associated with body size. Despite wide diel variation in patterns of thermal habitat selection, thermal time (degree-hours; °C-hrs) was consistent and inversely related to body size over a month. Large adult brook trout accumulated fewer degree-hours than smaller brook trout. The intercept of this relationship was close to their sublethal threshold as defined by physiological stress. We hypothesize that adult brook trout actively manage metabolic rate as a function of body size over periods greater than the diel cycle.