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

Brook trout (Salvelinus fontinalis) are widely distributed through stream systems Canada’s Boreal shield region where they occupy a range of habitat types but have a common requirement for cold, clean water making them a useful indicator of stream ecosystem health. In many boreal streams, however, summer water temperature may reach or exceed the tolerance limits of brook trout.  We examined the distribution of brook trout in small streams during the summer to evaluate how spatial and temporal variability in stream temperature influenced distribution.  Stream bed temperatures vary as much as 10^oC with colder areas being associated with shallow groundwater inputs from the adjacent riparian areas.  Using underwater video monitoring we determined that stream fish abundance was 1.4 times higher in cool areas relative to warm but that brook trout were found almost exclusively in the cool areas. Using a tank experiment, we confirmed that in the absence of any other habitat characteristic brook trout responded to simulated groundwater inputs and remained within these colder areas when they were available.  Our results illustrate the importance of thermal variability in small stream systems and highlight the need to protect terrestrial areas that contribute shallow groundwater to these streams.