Abstract
Laboratory experiments were undertaken to investigate the influences of exposure duration and frequency on the toxicity of shortβterm pulses of low dissolved oxygen (DO) to fish. For the investigation of exposure duration, rainbow trout (Oncorhynchus mykiss [Walbaum]) and roach (Rutilus rutilus L.) were exposed to a range of DO concentrations in single pulses of 1, 6, or 24 h. For the investigation of exposure frequency, brown trout (Salmo trutta L.) were exposed to 24βh pulses of DO concentrations of 4.0 and 5.5 mg/L at frequencies of once or twice weekly over a period of 75 d. The results suggest that, for a given duration, there is a narrow threshold concentration range above which mortality does not occur and below which mortality rapidly becomes high. This threshold concentration range increases as exposure duration increases. Roach were able to survive lower DO concentrations than trout. Observations on experimental animals following exposure indicated no significant postexposure effects, even at very low DO levels. For the exposure frequencies used here, DO concentration rather than frequency of exposure was the important factor in terms of effect on fish. No significant effects on growth rate were observed but there were significant differences in hemoglobin levels, hematocrit, and organ weights. These results have important implications for the derivation of environmental quality standards aimed at the control of episodic pollution in rivers.