Evaluation of two monitoring approaches to assess effects of waste water disposal on histological alterations in fish

An active monitoring (caging experiment) and a passive monitoring (sampling of wild fish) were performed to investigate the effects of effluent from a sewage treatment works (STW) on brown trout (Salmo trutta) by histopathological examinations of the skin, gill, liver and kidney. Histopathological lesions were evaluated according to a standardised assessment tool, which allows calculation of indices for every organ. According to the results of both monitorings, trout exposed to river water supplemented with treated waste water from the STW Lyss showed higher histopathological indices than trout caught upstream of the discharge point of the STW or kept in river water only. These results indicate a negative effect of treated waste water from the STW on the histopathological status of the examined organs of brown trout. Both monitoring approaches revealed the liver to be the most affected organ compared with reference fish. However, data from the two monitoring approaches were not completely consistent: histologically the gills were the most sensitive organ to the effects of treated waste water in the active monitoring, but were not affected in the passive monitoring. The data provide relevant information about both the comparability and the pros and cons of the two monitoring approaches to assess effects of pollution on histopathological alterations in fish.