Effects of pulp mill effluents and restricted diet on growth and physiology of rainbow trout (Oncorhynchus mykiss).

Juvenile female rainbow trout was exposed for 4.5 months (June to October) to two dilutions of untreated and activated sludge treated whole mill effluent from a pulp mill producing bleached ECF pulp. Two controls were used, on fed ad libitum and a second receiving 0.5% feed of the body weight. All effluent exposed groups were fed ad libitum. Mean weight of the fish was measured monthly. At the end of the experiment a number of physiological and biochemical parameters were analyzed in order to establish the physiological status of the exposed fish in comparison with unexposed fish that obtained ad libitum or restricted amount of feed. The fish exposed to treated effluent grew significantly more than ad libitum control fish until August, whereupon growth retarded in fish exposed to the lower effluent dilution (400 v/v). The growth of fish exposed to untreated effluent did not deviate significantly from the control fed ad libitum. The results from the hematological analysis clearly showed that fish fed restricted amount of feed deviated significantly in most parameters compared with the control fed ad libitum. Fish exposed to treated effluent showed a response pattern similar to that of the control fed restricted amount of feed, whereas the fish exposed to untreated effluent showed a response pattern that did not deviate from that of the ad libitum control. The metabolic parameters suggested that fish exposed to treated effluent had a higher metabolic demand than ad libitum control and that the energy allocation at the end of the experiment was directed to processes other than growth. The responses on hematology were mainly a consequence of the increased energy demand and were not primary effects. The implications of using feed related parameters at field studies are discussed.