AbstractToxicity studies were performed on two natural populations of fathead minnows(Pimephales promelas).One group of organisms was taken from a metal‐contaminated flyash pond associated with a coal‐fired power plant and the other population was collected from Newtown hatchery ponds. As determined in acute toxicity tests, ash pond minnows were significantly more tolerant of cadmium and copper than were hatchery minnows. At an exposure concentration of 6.0 mg Cd/L in moderately hard water, the median period of survival (LT50) for flyash pond minnows was 50.0 h, compared with 6.8 h for hatchery minnows. The LT50 values for ash pond and Newtown animals exposed to 0.50 mg Cu/L in moderately hard water were 17.0 and 4.5 h, respectively. Both populations were about equally tolerant of zinc. After ash pond minnows were transferred to reconstituted water in the laboratory, tolerance to cadmium and copper decreased significantly after 7 d. Conversely, tolerance was increased in hatchery minnows following acclimation in the laboratory to sublethal concentrations of cadmium. For example, after exposing hatchery minnows for 35 d to 10 μg Cd/L, an acute toxicity test gave LC50 values of 2.88 and 1.71 mg Cd/L for acclimated and control organisms, respectively. This induced tolerance in cadmium‐acclimated hatchery minnows was lost when organisms were transferred to cadmium‐free water for 7 d. The results justify the conclusion that fathead minnows develop increased tolerance to cadmium and copper following prolonged sublethal exposure to these metals, and that this metal‐induced tolerance is not sustained once organisms are removed from toxicant stress. As determined in biochemical studies, gill metallothionein concentrations followed the fluctuations in tolerance exhibited during cadmium acclimation and deacclimation. Thus, tolerance induction was, in part, attributed to increased production of metallothionein, a protein which selectively binds and deactivates cadmium, copper and certain ot
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