HISTOPATHOLOGY OF RAINBOW-TROUT (ONCORHYNCHUS-MYKISS) AFTER LONG-TERM EXPOSURE TO BIOLOGICALLY TREATED BLEACHED KRAFT MILL EFFLUENT IN EXPERIMENTAL STREAM CHANNELS

Outdoor experimental streams were used to determine the effects of effluent on histopathology of rainbow trout (Oncorhynchus mykiss). Studies were conducted for 10-11 mo using biologically treated bleached kraft mill effluent at concentrations ranging from 0.5 to 2.0 mg.L-1 of effluent BOD5 (1.3-5.1% v/v) and for 42 mo at 0.5 BOD5 addition (1.5% v/v). Twenty different tissues from randomly selected fish were examined at the end of each exposure period. Lesions or tissue changes observed in fish from both control and effluent-treated streams were of primary or secondary parasite-induced etiology, a condition typical of natural streams. There was an absence of neoplasia over the range of concentrations. Hematocrit, leucocrit, and liver somatic index remained normal throughout the course of effluent exposure. These data corroborate the lack of effluent effects as determined from measurements of trout growth, survival, production, and reproduction.