A fish-kill in the river, Akerselva, Oslo, Norway: The use of benthos and fish to trace the source of pollution

Abstract

In connection with a fish-kill in Akerselva, Oslo, in October 1986, studies of fish and benthos were initiated to find the source of pollution and to evaluate the consequences for the pollution status of the river. Dead Atlantic salmon (Salmo salar), brown trout (S. trutta) and minnow (Phoxinus phoxinus) were reported from below the waterfall, Øvre Foss, down to the Oslofjord at Bjørvika. From Øvre Foss up to the water intake pool for the De-No-Fa/Lilleborg factory, no fish were found, while above this minnows and trout were recorded. By following the distribution of two indicator species, Baetis rhodani (Ephemeroptera) and Rhyacophila nubila (Trichoptera), a locality was found where they suddenly disappeared from benthic samples. At this point a surfacewater culvert enters the river. Benthos samples taken along the underground culvert demonstrated that the pollution had originated from the De-No-Fa/Lilleborg factory. Subsequently, the factory disclosed that strongly alkaline effluent had been discharged into the culvert. Pollution indices, incorporating the whole benthos, also showed that water quality had deteriorated downstream of the discharge compared to previous surveys of the river.