The Influence of Thermal Variability on Brook Trout Distribution in Boreal Streams

Summer surface water temperature in boreal streams can exceed the thermal tolerance for brook trout (Salvelinus fontinalis) indicating that areas of thermal refugia are required for these populations to persist. We examined the influence of spatial and temporal thermal variability in streambed temperature on the distribution of brook trout. In the summer streambed temperatures varied spatially as much as 12 °C with the cold temperatures, associated with lateral groundwater inputs from the riparian zone, driving the variability. Streambed temperature loggers showed that locations with groundwater inputs had significantly less diurnal temperature variation and were significantly cooler in the summer (Jun-Oct) and warmer in the winter (Nov-Apr) relative to other areas in the stream.  Electrofishing and underwater video surveys indicated that brook trout occupation of areas with cool water inputs was significantly greater than other locations when stream surface water exceeded the optimal thermal level.  Additional laboratory experiments showed that randomly-distributed brook trout in sub-optimal water temperature quickly congregated at a cold water input as it became available.  Thermal variability provided by cool groundwater inputs is an important factor influencing the distribution of brook trout in stream systems and needs to be considered when managing the landscape.