This study had two primary objectives. The first was to determine if the length of exposure necessary for acclimation to Cu to develop in larval fathead minnows (Pimephales promelas) is different than that for juveniles. The second objective was to determine whether the acclimatory response, as determined by organism survival, is consistent with acclimation as determined by whole-body Na + . Six experiments were conducted: four using larval (<20-d-old) and two using juvenile (<60-dold) fathead minnows. Within each experiment, fish were allocated to one of four groups: unexposed, continuously exposed, episodically exposed or naïvely exposed. The continuous group was exposed to a sublethal Cu exposure (125 µg/L) for 8, 12, 16 or 20 d and then subjected to a survival test at a lethal dose (375 µg/L). Fish in the episodic group were exposed to the sublethal dose for either 4 or 8 d, given a depuration period of varying lengths (4-16 d) then subjected to a survival test. Naïve minnows were maintained in clean water then given the survival challenge. Results from survival tests show that the larvae acclimate after only a 4-d sublethal exposure to Cu. In contrast, juveniles require a 16-d exposure to acclimate. Once acclimation had developed, there was a strong relationship between larval survival and whole-body Na + . Acclimated larvae maintained whole-body Na + relative to unexposed fish, while unacclimated larvae did not. Interestingly, this was not the case for juveniles, as acclimated and unacclimated groups did not differ with respect to whole-body Na + concentrations. The results of this study show that age influences the time course and possibly the mechanisms of acclimation in fathead minnows exposed to Cu.

Fathead minnows exposed to sublethal Cu concentrations may undergo branchial damage followed by repair, and may also develop enhanced Cu tolerance. The primary objective of this study was to determine whether the cycle of damage and repair was necessary for the development of enhanced Cu tolerance. Inferences regarding damage and repair were made from changes in the whole body Na + of juvenile (0.5 g) fathead minnows (Pimephales promelas). Two experiments were conducted in which juvenile minnows were exposed to various sublethal Cu concentrations for 16 d. In the first experiment, fish were exposed to one of four different Cu concentrations (0, 73, 118, 189 μg/L Cu) then challenged with an 8-d exposure at 302 μg Cu/L. Only the fish exposed to the highest Cu dose experienced enhanced Cu tolerance relative to the other three doses. In the second experiment, whole body Na + was monitored in fish exposed to one of five different Cu exposures (0, 70, 127, 202, 289 μg/L Cu). Fish exposed to 70 μg/L Cu did not experience a significant decline in whole body Na + at any point during the 16-d exposure period. The reduction in whole body Na + was short lived and moderate (17%) in the fish exposed to 127 μg/L Cu, but more severe (>30%) and longer lasting in the fish exposed to 202 μg/L Cu. At 289 μg/L Cu, the fish experienced irreversible reductions in whole body Na + and ultimately died. When taken together, results from these two experiments suggest that enhanced tolerance will only develop in fathead minnows that have experienced a pronounced, relatively long-term cycle of branchial damage and repair.