Behavioral thermoregulation of brook and rainbow trout: Comparison of summer habitat use in an Adirondack River, New York

The body temperatures of 15 hatchery-origin and two wild brook trout Salvelinus fontinalis and I I hatchery-origin rainbow trout Oncorhynchus mykiss were monitored with internally implanted, temperature-sensitive radio transmitters from June through September 1997 in a fifth-order Adirondack river (New York). The fish were released into a 12-km reach that during summer had maximum temperatures near those that are lethal for salmonids. Body temperatures were compared between species and with river temperatures. In 1997, the maximum river temperature was 26.4degreesC, and the highest average daily temperature was 25.0degreesC. The brook trout were usually cooler than the main river flow because they used two of five tributary confluences or groundwater discharge areas in two pools within the main river. The temperatures of the brook trout from June through September were an average of 2.3degreesC cooler than the main flow of the river and differed significantly (P = 0.002) from those of the rainbow trout, which were 1.5degreesC cooler than the river. When the river temperatures were 20degreesC or higher, the mean temperatures of the brook trout were 4.0degreesC cooler than the river and differed significantly (P = 0.002) from those of the rainbow trout, which were 2.3degreesC cooler than the river. Brook trout, and to a lesser extent rainbow trout, used localized coolwater areas to lower their body temperatures below that of the main river. Groundwater discharge areas within pools and some tributary confluences were critical habitats for behavioral thermoregulation because they provided refuge areas from warm river water during the summer.