Modelling fish mortality due to urban storm run-off: Interacting effects of hypoxia and un-ionized ammonia

Storm run-off is a source of acute toxicity leading to fish mortality. The lethal main factors involved are hypoxia and the presence of ammonia. A series of experiments was carried out on juvenile rainbow trout exposed to ammonia and low concentrations of dissolved oxygen. Survival data analysis and generalized linear models were used to model the laboratory data obtained. A quantitative model of the instantaneous death probability as a function of time, concentrations of un-ionized ammonia and dissolved oxygen was obtained. The model was fitted with data obtained at a temperature of 17°C with concentrations of un-ionized ammonia between 0 and 0.5 mg/l, concentrations of dissolved oxygen between 1.7 and 2.9 mg/l and time less than 24 hours. The survival probability of rainbow trout exposed simultaneously to un-ionized ammonia and low dissolved oxygen was found to be lower than the predicted survival probability derived from the addition of individual effects of both ammonia and hypoxia. This kind of model can be used to understand the toxic impact of run-off and to draw up guidelines for the management of urban storm water.