Catalogus

Depending on the cell density, exposed fish showed behavioural changes, including increased ventilation rates possibly representing a physiological stress response. Susceptibility to ectoparasitic infestation and increased mortality in exposed fish suggested P. rubescens associated effects on fish fitness. Histopathological alterations in liver, gastrointestinal tract and kidney, which were also immunopositive for microcystin suggested causality of tissue damage and the presence of microcystins. In contrast, observed gill pathology appeared to result primarily from mechanical abrasion and irritation due to ectoparasitic infestation. The current exposure experiment confirmed the hypothesis that subchronic and chronic exposure to low cyanobacterial cell densities and hence microcystins can exacerbate physiological stress and sustained pathological alterations in exposed coregonids. The study therefore supports the theory that P. rubescens blooms may be causal to the observed weight reduction and hence fitness of coregonids in pre-alpine lakes such as Lake Ammersee (Germany).