Distribution of Juvenile Bull Trout in a Thermal Gradient of a Plunge Pool in Granite Creek, Idaho

The distribution of juvenile bull trout Salvelinus confluentus was observed on three nights (0000–0200 hours) during July 24–28, 1992, in a large plunge pool in Granite Creek, a direct tributary to Lake Pend Oreille in northern Idaho. The pool contained a strong side-to-side thermal gradient (8–15°C) created by the confluence of Granite Creek (water temperature 15°C) with Sullivan Springs, a much colder (8°C) stream of comparable discharge. Juvenile bull trout chose the coldest water available (8–9°C). Distribution of bull trout within the plunge pool could not be attributed to differences in water depth, substrate, velocity, overhanging cover, or interactions with other fish.