Active biomonitoring with brown trout and rainbow trout in diluted sewage plant effluents

Brown trout Salmo trutta populations of numerous Swiss rivers are declining . Sewage plant effluents are discussed as a possible cause. To investigate the influence of sewage plant effluents, brown trout as well as rainbow tro ut Oncorhynchus mykiss were exposed to 10% diluted waste water over a perio d of 12 months. The effects were compared to those on trout kept in commerc ial tap water. The mortality rate was low and no pathogenic bacteria or vir uses were recorded in exposed and tap-water animals. Parasitological examin ation revealed a mild infestation with Gryodactylus sp. in all groups. Macr oscopically and histologically, only minor changes in gills, skin, and kidn ey of exposed animals were found when compared to fish kept in tap water. D egenerative and inflammatory reactions in the liver of exposed animals were the most prominent findings. Several brown trout caught in the River Lange te showed marked proliferative, degenerative and inflammatory lesions of gi lls, liver, and kidney. The results do not suggest that waste-water effects would explain the decrease of fish populations. However, it is conceivable that the effluents in combination with other factors in the river enhance the development of changes. (C) 1999 The Fisheries Society of the British I sles.