The Expression of Bluegill Behavioral Types in Chronically Heated Environments

Global climate change is expected to exert selective pressures on behavioral phenotypes within freshwater ecosystems through environmental changes associated with chronic warming of water temperatures. We compared the behavioral profiles of bluegill sunfish (Lepomis macrochirus) from three power-plant cooling reservoirs to the behavioral tendencies of bluegill from three ambient reservoirs to investigate whether long-term exposure to increased water temperatures influences the expression of behavioral phenotypes. Power-plant cooling reservoirs were considered as model systems for global warming due to their year-round elevated water temperatures (~ 5°C) when compared to ambient reservoirs. We quantified activity, boldness, and exploration through 30-minute assays in a common laboratory setting that tested the spatial usage and response of individual fish to a suite of situations involving novel items and a predator, largemouth bass (Micropterus salmoides). For each assay, multiple measurements were recorded for each behavior, leading to the development of a principal component score (PCA) for activity, boldness, and exploration for each individual. PCA scores for each behavior were compared between groups (heated or ambient) and then used to determine how well behaviors correlated to one another within groups. Distinct behaviors did not differ between bluegill from heated and ambient lakes. However, we found significant directional changes between groups for the correlations of activity and exploration as well as for boldness and exploration. These results suggests that chronic exposure to warming can influence the expression of behaviors, providing insight for how the behavioral composition of bluegill populations may be modified in chronically warmed systems.