Time-specific and population-level differences in physiological responses of fathead minnows (Pimephales promelas) and golden shiners (Notemigonus crysoleucas) exposed to copper

► Time-specific changes in gill Na+/K+ ATPase activity and metabolic rate was examined in fathead minnows and golden shiners exposed to Cu. ► Patterns of Na+/K+ ATPase activity and metabolic rate were similar and support predictions based on the concept of acclimation. ► Population-specific differences in metabolic rate were observed for both species of fish. The influence of exposure time on gill Na+/K+ ATPase activity and metabolic rate in populations of fathead minnows (Pimephales promelas) and golden shiners (Notemigonus crysoleucas) hatcheries in Ohio (OH) and Pennsylvania (PA) when exposed to sublethal concentrations of copper (Cu) was examined. The pattern of change in gill Na+/K+ ATPase activity was similar in all species/populations and results support expectations based on the concept of acclimation. In all populations, Na+/K+ ATPase activity declined significantly compared to reference values within 24h, recovered by 48h, and then continued to increase before exceeding reference values by 192h. With the exception of PA fathead minnows, Na+/K+ ATPase activities returned to reference levels by 384h. Although metabolic rates of individual fish were not strongly correlated with Na+/K+ ATPase activities, the pattern of change in mean values of these physiological parameters was very similar. However, OH populations of both fathead minnows and golden shiners demonstrated much more dramatic changes in metabolic rate compared to PA fish. At 24h, metabolic rate of PA fathead minnows had decreased by 16% compared to the reference value whereas the OH population had decreased by 31%; metabolic rate of PA golden shiners declined by 23% compared to 59% in OH shiners at 24h. Similar differences were observed in the maximum metabolic rates achieved at 192h. While the increased sensitivity of OH fish to Cu is not readily explainable by genetic or environmental factors, results suggest the need for considering population level differences when evaluating the physiological effects of toxicants.