Many dangerous substances like pesticides, drugs, plasticizers, polycyclic aromatic hydrocarbons (PAHs) can be supplied via the stream water to fish farms as dissolved or bound particles.

The substances derive from sewage treatment plans, agriculture and atmospheric deposition and may affect the health of the fish. Drugs and disinfectants that are used by fish farmers and discharged into the streams are also harmful to stream organisms. The effect depends on the dose to which the organisms are exposed.

Concentrations of substances in different parts of a traditional fish farm with excavated earth ponds and a canalised fish farm with and without partial re-circulation will be presented. The obtained concentrations in the stream will be related to the water quality criteria. Means of reducing the discharge concentrations will be discussed.

Ole Sortkjær Ph.D: Ecotoxicological and ecological studies in streams, lakes and fjords. Studies of chemicals and medicine in aquaculture. Involved in the Danish Aquatic Monitoring and Assessment Programme, EEA National Focal Point and Topic Centre activities within Land Cover and EuroWaternet and riverine inputs of hazardous substances.

Contact Details: Senior Adviser of the Department of Freshwater Ecology, National Environmen­tal Rese­arch Institute, DK-8600 Silkeborg, Vejlsøvej 25 (Denmark). Email: os@dmu.dk