Whole Effluent Toxicity Testing with Oncorhynchus mykiss (Walbaum 1792): Survival and Behavioral Responses to a Dilution Series of a Mining Effluent in South Africa

Abstract.

Survival, behavioral early warning responses to, and behavioral effects of a complex effluent from Richards Bay Minerals in Natal, South Africa, were studied using rainbow trout Oncorhynchus mykiss (Walbaum 1792) as a biosensor. Three replicates of eight juvenile fish were exposed to a dilution series of the effluent (0, 5, 10, 20, 50, 75, 100%) and the behavioral pattern (ventilation, locomotion) was measured online with quadropole impedance conversion. Survival decreased with increasing effluent concentration. Before death, O. mykiss coloration darkened, demonstrating skewed bodies subsequently after death. Decreased activity and increased ventilation combined with increased ventilation frequency were found within the first 2 h of exposure at ≥96-h LC₅₀ value of 10% effluent concentration indicating early warning responses. During 4 days of exposure, activity decreased and ventilation increased further at concentrations around the 96-h LC₅₀ value indicating toxic effects.