LETHAL EFFECTS OF DRAINING ON BROWN TROUT - A PREDICTIVE MODEL BASED ON FIELD AND LABORATORY STUDIES

During the draining of dams, realized to remove the sediment from the reservoir, drastic fish mortalities have been observed down-stream in the river. These mortalities are correlated with an increase of the suspended solids and ammonia concentrations in the water and also related with a fall of the dissolved oxygen concentration. The purpose of this paper is to describe the methodology used and the results obtained in a field and laboratory survey of ecotoxicological hazards linked with dam draining. The field study was carried out in the River Isere in France. Different chemical and physical parameters of water quality were recorded with simultaneous toxicity testing using water from the river during the draining process. The results obtained are considered as reference data allowing the laboratory experimental conditions to be drawn up. This methodology enables the predictive model derived from laboratory surveys to be consistent with the observed environmental phenomenon. Finally we have found that the only way to predict the lethal impact of dam draining on fish (brown trout) was to take into account the strong synergistic effect between suspended sediment and the drop in dissolved oxygen. © 1990.