RELATIVE SENSITIVITY OF BROWN AND RAINBOW-TROUT TO PULSED EXPOSURES OF AN ACUTELY LETHAL MIXTURE OF METALS TYPICAL OF THE CLARK-FORK RIVER,MONTANA

Brown trout (Salma trutta) and rainbow trout (Oncorhynchus mykiss) fry and juveniles were episodically or continuously exposed to a metals m ixture (Zn, Cu, Pb, Cd): the concentrations and ratios of the metals, and variations in water quality (pH, hardness), were selected to repre sent conditions measured during episodic storm events in the Clark For k River, Montana. Brown trout fry were more sensitive (lower LC(50)) t han rainbow trout fry to the metals in 8-h exposures with constant har dness and pH, but less sensitive to elevated metal concentrations in c onjunction with depressed hardness and pH. Fry were more sensitive tha n juveniles when exposure was continuous, but neither life stage was c learly more sensitive when exposure was pulsed. Whole-body concentrati ons of K+ and Ca2+ but not Na+ were significantly depressed in fry exp osed to metals. Results support the hypotheses that changes in water q uality during thunderstorms are lethal to fry and juvenile life stages of brown and rainbow trouts and that the relative sensitivity of the species to the metals mixture may explain their distributions in the C lark Fork River. Low-frequency extreme conditions may effectively act as a bottleneck on the viability of populations whose relative sensiti vities to such extremes may control distributions of species in a syst em.