Effects of short-term pulsed ammonia exposure on fish

Two series of laboratory experiments were undertaken to investigate the inf luences of exposure duration and frequency on the toxicity of short-term pu lses of ammonia to rainbow trout (Oncorhynchus mykiss Walbaum) and brown tr out (Salmo trutta L.). For the investigation of exposure duration, rainbow trout were exposed to a range of unionized ammonia concentrations in single pulses of 1, 6, or 24 h, and mortality was recorded as the endpoint. For t he investigation of exposure frequency, brown trout were exposed to repeate d sublethal ammonia pulses of different concentrations either once or three times weekly over a period of 53 d to simulate intermittent pollution epis odes. A range of endpoints were measured, including growth, gill condition, blood indices, and organ-to-body weight ratios. The results suggest that e xposure duration and frequency are both important factors influencing the s everity of effect in fish exposed to short-term ammonia peaks. Growth, gill condition, organ weights, and hematocrit were all significantly affected b y repeated exposures, particularly at the higher exposure frequency. The re sults of this study confirm that the duration and return period of transien t pollution events are critical dimensions when considering the development of environmental quality standards for intermittent pollution.