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The toxicity of pulp and paper mill effluents was estimated by exposing rainbow trout to the effluents in situ during four seasons. The experiments were carried out using one control group and three test groups at different distances from the discharge. After a 10-day exposure mainly hematological parameters were determined from the caged fish. The caging site affected most of the 15 parameters analyzed (blood Hct, MCHC, erythrocyte, and leucocyte counts; plasma gluc, prot, urea, Ca2+, Mg2+). Some changes were detected even at the farthest caging site, 8 km from the discharge. The time of year strongly influenced the results. Seasonal variation exceeded the variation caused by caging site. In addition, the season influenced the responses of rainbow trout to the effluent in many parameters. None of the responses detected were observed in all the experiments. The only significant changes in the median values of the parameters from the caging experiments were a decrease in the plasma Mg2+ and an increase in the plasma K+ concentration.

The toxicity of effluents from a petrochemical industry center in southern Finland was tested by conducting bioassays on organisms from three different trophic levels. In fish tests, rainbow trout (Salmo gairdneri) were caged at the discharge site and simultaneously at a reference area. The only clear differences, among the measurements of 25 metabolic parameters, were observed in fish liver where activities of two detoxication enzymes were significantly increased in the exposed group. The water flea (Daphnia magna) was used both in acute (EC50) and long-term reproduction tests. No acute lethal toxicity was detected in any of the wastewater samples investigated. A combined effluent, however, caused a reduction in the reproduction rate with an EC50 of 3%. No mutagenic activity was observed with the Ames test (Salmonella typhimurium, strains TA 97, TA 98, and TA 100) in concentrated effluents, in sediment samples, or in liver samples from predator fish caught from the discharge site.