AbstractTissue levels of chlorinated dioxins and furans and the physiological and biochemical responses of fish were measured upstream and downstream of a bleached kraft mill in the St. Maurice River, Quebec. Relative to fish 10 km upstream of the source, white suckers (Catostomus commersoni) downstream showed evidence of chemical exposure, chemical metabolism, stress, and altered energy use. Levels of 2,3,7,8–tetrachlorodibenzo‐p‐dioxin and tetrachlorodibenzofuran in gutted whole fish averaged 23 and 260 pg/g immediately downstream of the plant and declined to 18 and 112 pg/g at 95 km downstream. The activity of hepatic aryl hydrocarbon hydroxylase (AHH) was 10X higher immediately downstream and 5X higher 95 km downstream. There were corresponding increases in relative liver size, hematocrit, serum glucose, serum protein, and fin‐ray asymmetries, with a pattern of response that matched enzyme induction. These responses pointed to a strong effluent effect and followed the patterns of tissue contamination by dioxins and furans and of water contamination by chlorinated phenolics and guaiacols. There were strong correlations between AHH induction and tissue contamination. Whereas lipid levels increased at all downstream sites, condition factor decreased at 95 km from the source, indicating that there may be disturbances in energy metabolism. Effects on sexual maturation, as shown by levels of serum hormones and gonad somatic index, were incon
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