Staying cool: behavioral thermoregulation during summer by young-of-year brook trout in a lake

Thermal habitat selection and behavior by young-of-year brook trout Salvelinus fontinalis was studied in a lake in central Ontario, Canada. In May, trout foraged actively within 2 m of shore in the warmest water available (∼15°C). In early June, trout foraged near the bottom within 4 m of shore, where bottom water temperatures were near, or at, the upper thermal tolerance, for trout, of 20°C. In July, when ambient water temperatures ranged from 23°C to 27°C, trout lay on the bottom in the coldest water available (18–20°C) in discrete areas 3–8 m from shore. Flow rate of cold groundwater accounted for 87% of the variance in trout density in these areas, and the data suggest that a minimum flow rate of 125 mL·m−2·min−1 is required for trout to take up station. When trout were displaced from holding positions, sites with greater groundwater flow were more quickly reoccupied by trout than sites with lower flow. Experimentally created trough-like depressions at these sites attracted higher densities of trout than the same sites with their natural topography and restricted trout distribution to within each depression. Preliminary behavioral observations suggest that trout lie on the bottom and defend cool microhabitats at the expense of daytime feeding. These results suggest that areas with cold groundwater may be a limiting resource for young-of-year brook trout in the littoral zone during summer, and resource managers should consider protecting such areas from lakeshore development and logging.