AbstractPerch(Perca fluviatilisL.), roach(Rutilus rutilusL.) and bream(Abramis bramaL.) were collected from an area in the southern part of Lake Saimaa (Finland), which is polluted by effluents from the pulp and paper industry, and compared to fish caught at several upstream locations in the same lake system. Livers were sampled in the field, and polysubstrate monooxygenase (PSMO) activities and other parameters of the microsomal cytochrome P‐450 system, as well as conjugation enzymes, were later analyzed in the laboratory. The enzymes assayed included 7‐ethoxyresorufin O‐deethylase, pentoxyresorufin O‐dealkylase, benzoapyrene hydroxylase, UDP‐glucuronosyltrans‐ferase and glutathione S‐transferase. The influence of effluents discharged from the mill, which produces chlorine‐bleached kraft pulp, was seen in each fish species as elevations of liver PSMO activities. The content of total glutathione was increased in fish collected from the most polluted areas, but the changes in conjugation enzymes were not as clearly related to the pollution gradient. These results were compared to analyses from several control areas located at various distances upstream from the industry investigated. To assess the possible influence of another pulp mill, Baikalsk Cellulose Combine, hepatic PSMO activities ofCottus kessleriandParacottus kneri, two sculpins endemic to Lake Baikal (USSR), were also measured. In this case no effects were observed. In conclusion PSMO enzymes analyzed from stationary feral fish can serve as a tool for assessing and monitoring chemical contamination caused by pulp and paper
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