Effluent from a sewage treatment works causes changes in serum chemistry of brown trout (Salmo trutta L.)

To evaluate the impact of effluent from a sewage treatment works on fish health, serum chemistry variables were investigated in brown trout (Salmo trutta L.) held in cages (active monitoring) and wild brown trout (passive monitoring). Means of the measured serum parameters of the different treatment groups were close or within normal ranges. However, the results of the active monitoring demonstrated that the serum variables of reference trout held in tap water were clearly different from those of the river treatment groups. In the active monitoring, fish exposed to effluent from the sewage treatment works had significantly different blood urea nitrogen and bilirubin values than fish kept in river water. In the passive monitoring, total protein, blood urea nitrogen, and alkaline phosphatase were significantly different between the two groups. Of the numerous correlations between serum chemistry parameters and histological lesions, blood urea nitrogen and alkaline phosphatase were found to most strongly indicate gill and liver lesions, respectively. In the passive monitoring correlations between serum chemistry variables and histopathological lesions were restricted to bilirubin and liver lesions. This indicates that the application of serum chemistry variables as indicators of histological lesions in case of chronic exposure is questionable. A multivariate discriminant analysis was used to consider relationships between the single serum variables concurrently.