Redd (nest) surveys for resident brook trout (Salvelinus fontinalis) were conducted annually in a mountain lake in northern New York for 11 years with multiple surveys conducted during the spawning season in eight of those years. Repeated surveys throughout the spawning season allowed us to fit an individually based parametric model and estimate the day of year on which spawning was initiated, reached its midpoint, and ended during each year. Spawning phenology was then assessed relative to (1) mean of maximum daily air temperature and (2) mean of maximum daily water temperature at the lake bottom during summer in each year using a linear model. Elevated temperatures in summer were correlated with a delay in spawning and a reduction in the total number of redds constructed. Increasing the summer mean of maximum daily air temperatures by 1°C delayed spawning by approximately 1 week and decreased the total number of redds constructed by nearly 65. Lake spawning brook trout select redd sites based on the presence of discharging groundwater that is relatively constant in temperature within and across years, leading to relatively consistent egg incubation times. Therefore, delayed spawning is likely to delay fry emergence, which could influence emergence synchrony with prey items. This work highlights non-lethal and sub-lethal effects of elevated summer temperatures on native resident salmonids in aquatic environments with limited thermal refugia.

Large‐scale observational studies in eastern Canada and the northeastern USA have concluded that introduced littoral predators are responsible for reductions in native fish diversity and abundance. To determine whether nonnative predator removal could increase native littoral fish abundance, we removed 47,682 smallmouth bass Micropterus dolomieu from a 271‐ha Adirondack lake during a 6‐year period. Two years after removal began, habitat‐stratified snorkel surveys indicated a greater than 90% reduction in smallmouth bass abundance. The relative abundances of six native littoral species increased (4‐90 times preremoval abundances) within 2 years of smallmouth bass removal. Decreased relative predation risk during the experiment reflected the reduction in littoral predators and identified seasonal differences in nearshore predation risk. The smallmouth bass population was resilient to removal, producing strong year‐classes throughout the experiment. Mechanical removal was successful at decreasing smallmouth bass abundance and increasing native fish abundance, but removal must be conducted on a yearly basis to maintain low smallmouth bass population abundance. Our results provide experimental evidence regarding the need to prevent littoral predator introductions in Adirondack waters and offer support for nonnative control wherever native fish species conservation is a management priority.

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.