Assessment of the developmental success of brown trout (Salmo trutta f. fario L.) embryos in two differently polluted streams in Germany

In order to assess the effects of pollution on recruitment of brown trout (Salmo trutta f. fario L.) exposure experiments with brown trout early life stages were performed in two differently contaminated small streams near Stuttgart, southwest Germany, and in a control situation in the laboratory. Pollution in the more polluted stream is mainly due to effluents of municipal sewage treatment plants and to the influx of surface waters from areas of intense agriculture activity. Additionally, the water carries high particle loads, especially after rainfall events. Water quality of the less polluted stream is occasionally influenced by agriculture activity in the vicinity. For the exposure of trout early life stages incubators were used, which allowed frequent examinations without serious disturbance of the exposed organisms and thus the development could be monitored over time. Compared to the less polluted stream and the control, in the more polluted stream high mortality rates, the lowest hatching success, and lowest values for growth occurred. In the less polluted stream, mortality rates were comparably low in prehatching stages, but high in hatchlings and juveniles due to heavy infestations with protozoic ectoparasites Chilodonella cyprini (Moroff, 1902) and Ichthyobodo necator (Henneguy, 1883). However, pollution-related effects were not detected in this stream. Differences in developmental rates between the different treatments were correlated with different water temperatures in the two streams and the control. For the more polluted stream, previous studies have indicated embryotoxic potentials mediated by pollutants. The present study indicates that in this steam, additionally, fine solids which infiltrate stream gravels may seriously affect development of brown trout early life stages.