Effects of short-term oxygen depletion on fish

Laboratory experiments were undertaken to investigate the influences of exp osure duration and frequency on the toxicity of short-term pulses of low di ssolved oxygen (DO) to fish. For the investigation of exposure duration, ra inbow trout (Oncorhynchus mykiss [Walbaum]) and roach (Rutilus rutilus L.) were exposed to a range of DO concentrations in single pulses of 1, 6, or 2 4 h. For the investigation of exposure frequency, brown trout (Salmo trutta L.) were exposed to 24-h pulses of DO concentrations of 4.0 and 5.5 mg/L a t frequencies of once or twice weekly over a period of 75 d. The results su ggest that, for a given duration, there is a narrow threshold concentration range above which mortality does not occur and below which mortality rapid ly becomes high. This threshold concentration range increases as exposure d uration 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 expo sure frequencies used here, DO concentration rather than frequency of expos ure was the important factor in terms of effect on fish. No significant eff ects on growth rate were observed but there were significant differences in hemoglobin levels, hematocrit, and organ weights. These results have impor tant implications for the derivation of environmental quality standards aim ed at the control of episodic pollution in rivers.