Lake Stratification and the Impacts of an Extreme Summer on Brook Trout Reproduction

Changing climate conditions are expected to cause an increase in the frequency and severity of hot summers in northeastern North America.  The hypolimnion of stratified lake ecosystems provides thermal refuge for coldwater fish during warm summer conditions. By contrast, thermal refuges in non-stratified lakes are restricted in extent.  We quantified gonad development and spawning activity of a resident coldwater salmonid (brook trout, Salvelinus fontinalis) during two summers in four lakes with contrasting thermal regimes.  Summer lake temperatures were unusually warm in 2012 and at or below long-term averages in 2013.  As expected, gonad development was substantially reduced in the two unstratified lakes in 2012 relative to 2013, with much lower gonad development in female fish by comparison with males.  In stratified lakes with reduced or delayed female gonad development in 2012, the timing of fall spawning was also delayed. These results highlight the importance of thermal refuge for lake-dwelling brook trout in the face of increasingly frequent extreme summer conditions and the potential vulnerability of trout to sub-lethal effects of increased summer temperatures in unstratified or weakly stratified lakes.