Lake-dwelling populations of brook trout (Salvelinus fontinalis) represent a coldwater fish species potentially susceptible to climatic warming trends. Global climate change and associated warming trends may eliminate or limit temperature sensitive coldwater fish species in lake ecosystems. Important insights into the influence of summer water temperature can be gained by investigating coldwater fishes inhabiting thermally marginal lakes. This thesis examines the influence of summer water temperature conditions in an unstratified Adirondack lake, upon brook trout catch, growth, consumption, energy density, reproductive activity and mortality. The cumulative number of degree-days exceeding 20?C in each year was used as a measure of annual thermal stress. Catch, growth, size selective mortality and consumption were evaluated using data collected in two contrasting thermal years (cool: 2000, warm: 2001). Brook trout energy density and growth were also evaluated in 2007, during which ambient water temperatures were even warmer than 2001. Summer thermal conditions had a negative impact on brook trout catch and growth (e.g. both length and weight) in 2001. Evidence of size selective mortality against larger individuals was observed at all ages in 2000 and 2001. Maximum stomach fullness was negatively related to water temperature (r2 = 0.78). Positive growth in 2007, a more severe thermal year than 2000 or 2001 but with a lower brook trout density, indicated that the negative impact of temperature on growth can be mediated by decreased competition for food resources. Brook trout energy density (i.e. percent water content) appeared to be negatively correlated with water temperature.

Eight continuous years of data (2000 ? 2007) were used to evaluate the effect of summer thermal conditions on reproductive activity and mortality. Spawning activity (i.e., redd construction) was negatively correlated with summer thermal conditions (r2 = 0.85) and was largely independent of mature female density. Summer thermal conditions caused the total mortality of all age two and older fish and all age one and older fish when the number of annual degree-days above 20?C exceeded 156 and 210, respectively. Thermal conditions below 115 degree-days above 20?C did not cause total mortality of any year class. This work highlights the importance of annual temperature monitoring, knowledge of thermal refuge habitat availability, and the use of degree-day temperature metrics when assessing the influence of temperature on brook trout populations.