This study presents an experimental analysis of the effects of midwinter flow reduction (50–75%, reduction in discharge in 4 h daily pulses) on the physical habitat and on behaviour and physiology of overwintering brook trout Salvelinus fontinalis in a small mountain stream.
Flow reduction did not result in significant lowering of temperature or formation of surface or subsurface ice. The main findings were (1) daily movement by S. fontinalis increased (c. 2·5‐fold) during flow reduction, but was limited to small‐scale relocations
(<10 m). (2) Undercut banks were the preferred habitat and availability of these habitats was reduced during flow reduction. (3) Although both experimental and reference fish did lose mass and condition during the experiment, no effects of flow reduction on stress indicators (blood cortisol
or glucose) or bioenergetics (total body fat, water content or mass loss) were detected, probably because access to the preferred type of cover remained available. Like other salmonids, S. fontinalis moves little and seeks physical cover during winter. Unlike many of the more studied
salmonids, however, this species overwinters successfully in small groundwater‐rich streams that often remain ice‐free, and this study identifies undercut banks as the critical winter habitat rather than substratum cover.