On-site bioassays were conducted at the furthest upstream pulp mill on the Fraser River in British Columbia. Uncontaminated river water was used to dilute treated effluent as discharged from the final diffuser pond. A single cohort of juvenile (8-10gm) chinook salmon (Oncorhynchus tshawytscha) was divided into an hypoxic group receiving 65% oxygen saturated water and a normoxic group receiving ambient 88% oxygen saturated water. Both groups were exposed over a period of 30 days to effluent concentrations of 2%, 4%, 8%, and 16%, while the controls received uncontaminated river water. This range of concentrations spanned those encountered by wild juvenile salmon overwintering in the upper Fraser River mainstem. The blood when analyzed by flow cytometry showed significant concentration-dependent clastogenic damage in both the normoxic and hypoxic groups starting at the 4% concentration. A concentration-response curve was determined from the hypoxic data set. Genetic implications of mutagenic damage to natural populations of chinook salmon are discussed along with the utility of the flow cytometer in detecting genotoxic damage.

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