Gill morphometry and hematology of juvenile chinook salmon chronically exposed to treated (elemental chlorine-free) bleached kraft pulp mill effluent at low temperature to simulate winter exposure

We used an on-site mobile laboratory to assess the effects of pulp mill effluent on juvenile (~0.9 g) chinook salmon, Oncorhynchus tshawytscha (Walbaum in Artedi, 1792), that normally overwinter under ice in the upper Fraser River at Prince George, British Columbia. After a 28-day exposure at 0.4–2.7 °C to secondary-treated bleached kraft mill effluent (TBKME; 0%, 2%, 4%, 8%, or 16%) under both normoxic and hypoxic (~67% air saturation) conditions, hematocrit (Hct), leucocrit (Lct), and differential white blood cell counts were determined and gills were examined for gross and fine morphometry. TBKME had no effect on either Hct or Lct, but the highest TBKME concentration (16%) increased lymphocyte numbers, reduced secondary lamellae numbers, and increased the blood–water diffusion distance of secondary lamellae. Given the relatively high TBKME concentration, compared with actual discharges to the river, and the small physiological changes observed, it appears that chronic winter exposure of juvenile chinoo...