Influence of Variable Interannual Summer Water Temperatures on Brook Trout Growth, Consumption, Reproduction, and Mortality in an Unstratified Adirondack Lake

—Stressful water temperatures negatively affect physiological processes in fishes, yet evidence for how elevated temperatures influence population-level characteristics is rare. An 8-year field study of brook trout Salvelinus fontinalis in an unstratified Adirondack lake revealed that an aggregate measure of chronically stressful summer water temperatures strongly influenced brook trout population-level characteristics. We quantified chronic thermal stress using the cumulative degree-days over which bottom temperatures exceeded a range of thresholds (18–228C) and found that the strongest relationships were with cumulative degree-days over 208C (DD .20 ). Across years with similar brook trout densities, warmer summer water temperatures resulted in decreased brook trout growth, but growth was not reduced in a year with high water temperatures and low density. Maximum stomach fullness was negatively related to water temperature. Reproductive activity was negatively correlated with stressful summer thermal conditions and was less dependent on female brook trout density. Periods of chronically stressful summer water temperatures resulted in the apparent mortality of age-2 and older fish during a moderately stressful summer (156 DD .20 ) and apparent mortality of age-1 and older fish during the most stressful summer (210 DD .20 ). In contrast, extensive mortality was not evident in any year-classes when DD .20 was less than 115. Anthropogenic impacts on the thermal conditions of aquatic ecosystems occur at small (hydrology) and large (climate change) spatial extents, and in situ studies at the margin of an organism’s thermal range can provide important insights into changes that will occur if temperatures increase. Temperature plays a primary role in governing most life processes in fish (Brett 1971). Thermal preferences and temperature effects on fishes have been described for many taxa and are commonly determined in controlled laboratory settings (Baldwin 1956; Hokanson et al. 1973, 1977; Selong et al. 2001) or through bioenergetics simulations (Schofield et al. 1993; Reis and Perry 1995; Johnson et al. 2006). Less common are field studies that specifically document and quantify the influence of warm water temperatures on fish populations over multiple years (Drake and Taylor 1996; Baird et al. 2006; Biro et al. 2007). The effect of water temperature on fish varies with body size and life stage (Schmidt-Nielsen 1990; Lester et al. 2004). For a given species, larger fish are generally more sensitive to temperature increases than smaller fish due to greater metabolic demands and lower thermal preferences (Baldwin 1956; Coutant 1977; Hartman and Cox 2008). However, most studies examining the influence of temperature on fish have focused on juvenile life stages due to the ease of sampling and rearing under laboratory conditions (Elliott 1975; Dockray et al. 1996; Biro et al. 2007; Spina 2007). Long-term studies in intact lake ecosystems are needed to understand how variability in annual temperature conditions affect different life stages in fish populations. Studies at this spatial and temporal scale provide relevant insight into how potential changes in annual temperature regimes may impact population-level characteristics in the presence of environmental variation. Anthropogenic impacts on the thermal conditions of aquatic ecosystems occur at small (e.g., land management influences on groundwater seeps) and large (climate change impacts on lake stratification) spatial * Corresponding author: [email protected] 1 Present address: Chesapeake Biological Laboratory, University of Maryland Center for Environmental Science, Post Office Box 38 (1 Williams Street), Solomons, Maryland 20688, USA Received September 17, 2008; accepted October 22, 2009 Published online March 4, 2010 685 Transactions of the American Fisheries Society 139:685–699, 2010 Copyright by the American Fisheries Society 2010 DOI: 10.1577/T08-185.1 [Article]