Sensitivity of Blood Cell Counts in Juvenile Coho Salmon (Oncorhynchus kisutch) to Stressors Including Sublethal Concentrations of Pulpmill Effluent and Zinc

Transfer of fish from optimal to high-temperature, crowded holding conditions decreased white blood cell-thrombocyte (WBC-T) counts markedly on 12–96-h exposure. Red blood cell (RBC) counts were increased only slightly at 12 and 48 h. During 28 days acclimatization following transfer to simulated stream compartments, WBC-T counts returned to stock values within 2–4 days, following an initial decline. RBC counts were not altered appreciably.The decline in WBC-T counts of acclimatized fish to a high sublethal concentration of pulpmill effluent was greatest at 24 h. Mean RBC counts were decreased over 24–96 h, but differences were significant at 24 h only. In acclimatized fish exposed to pulpmill effluent, WBC-T counts were decreased (P < 0.001) at concentrations between 0.2 and 0.9 of the 96-h LC50. A 24-h exposure to 15% vol/vol of this effluent had considerably less effect on the WBC-T count when the effluent had been detoxified by foam fractionation.Under similar experimental conditions, exposure to sublethal levels of zinc depressed mean WBC-T counts proportional to concentration, although differences were only significant at 0.5 LC50 and greater. RBC counts were unchanged.The WBC-T response provides a reasonably rapid and sensitive method for measuring stressful levels of pulpmill effluents to salmon. Declines in WBC-T counts are attributable to reduced numbers of circulating small lymphocytes, which could result in decreased resistance of stressed fishes to disease.