Effects of Continuous Zinc Exposure on Sockeye Salmon during Adult-to-Smolt Freshwater Residency

This chronic zinc toxicity test with sockeye salmon (Oncorhynchus nerka) consisted of a 3-mo adult exposure followed by an 18-mo exposure of embryonic through smolt stages. The zinc concentrations utilized were 30 to 112 μg/liter during the adult-to-smolt exposure period, and an additional 242 μg/liter concentration was used for the embryo-to-smolt exposure period. These zinc concentrations produced no adverse effects on survival, fertility, fecundity, growth, or on the subsequent survival of smolts transferred to seawater. Exposure to 242 μg/liter produced sufficient acclimation to markedly decrease acute mortality at zinc levels lethal to unacclimated sockeye salmon juveniles. The 112- and 242-μg/liter concentrations were 0.15 and 0.32, respectively, of the 749-μg/liter, 96-h LC50 for the 8-mo-old sockeye salmon. An application factor relating 96-h LC50 and “safe” concentrations of zinc to anadromous sockeye salmon in soft water appears to be >0.15 and at least 15 times larger than the often recommended 0.01 application factor for zinc based on studies with other species.