Physiological responses of rainbow trout in a pulp and paper mill recipient during four seasons.

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.