The number of leucocytes and thrombocytes in the circulating blood of fishes seems to reflect more accurately than the number of erythrocytes the fish's reaction to acute stress, including that caused by pollution. To facilitate the estimation of leucocytes in fish blood, a new test is being introduced called leucocrit, which is based on the determination of the volume of packed leucocytes–thrombocytes expressed as a percentage of the whole blood. For stocks of juvenile coho salmon (Oncorhynchus kisutch) and rainbow trout (Salmo gairdneri) held in freshwater under a variety of conditions, mean leucocrit values varied from 0.55 to 1.91%. Variations in age, species, food ration, degree of crowding, and prior disturbances offish stocks were associated with these differences. Leucocrit and leucocyte–thrombocyte counts for both species were depressed from stock values after 96-h or shorter (rainbow) exposure to stressful (high-temperature crowded) conditions; values for these measures were positively correlated but uncorrected with hematocrit values. Hematological values for subpopulations of underyearling rainbow trout reared under summer or winter photoperiod conditions were unaffected by photoperiod and responded similarly to acute stress. Leucocrit values for stocks of coho (1.30 ± 0.27%) and rainbow (0.92 ± 0.12%) were depressed due to transfer or acute exposure to bleached kraft pulpmill effluent (BKME). A 24-h exposure to BKME after 2 days' acclimation caused consistent leucocrit depressions, with median effective concentrations of 0.28 of the 96-h LC50 value for coho and 0.16 LC50 for rainbow. Hematocrit showed less sensitive or consistent changes due to stress or effluent exposure than leucocrit. Key words: leucocrit, leucocyte–thrombocyte count, hematocrit, blood, acute stress, BKME, pulpmill effluent, sublethal, salmonid fish, applications

Underyearling Arctic grayling (Thymallus arcticus) from the Yukon River system were expsed for 4 d to suspensions of fine inorganic (≤ 250 g∙L−1) and organic (≤ 50 g∙L−1) sediment and for 6 wk to inorganic sediment (≤ 1000 mg∙L−1) under laboratory conditions. The test sediments were collected from an active placer mining area near Mayo, Yukon Territory. The exposures evoked sublethal responses but did not cause gill damage. Mortalities (10 and 20%) occurred only in experiments at 5 °C with inorganic sediment concentrations ≤ 20 g∙L−1. Six weeks of exposure to sediment concentrations > 100 mg∙L−1 impaired feeding activity, reduced growth rates, caused downstream displacement, colour changes, and decreased resistance to the reference toxicant pentachlorophenol, but did not impair respiratory capabilities. Stress responses (elevated and/or more varied blood sugar levels, depressed leucocrit values) were recorded after short exposure (1–4 d) to organic sediment concentrations as low as 50 mg∙L−1. Inorganic sediment strengths ≥ 10 g∙L−1 caused fish to surface. The lethal and sublethal responses of Arctic grayling to pentachlorophenol were similar to those determined for other healthy salmonid fishes.

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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.

Red blood cell counts and hematocrits of juvenile coho salmon were unaltered by 12-hr exposure to neutralized kraft pulp mill effluent, although hematocrits were decreased by exposure for 25 days. The number of circulating immature erythrocytes increased in effluent-exposed fish in both the 12-hr and 25-day exposures. The number of circulating small lymphocytes decreased markedly after 12-hr exposure. However, following the prolonged exposure, the number of small lymphocytes returned to normal, while the number of circulating neutrophils increased.The level of plasma glucose increased in fish exposed to effluent for 12 hr, and decreased in fish exposed for 25 days. Liver glycogen was not altered significantly during either exposure period.No pathological changes attributable to exposure to kraft pulp mill effluent were observed in the tissues examined, including the spleen, kidney, gill, interrenal gland, skin epithelium, and thyroid gland.Results are discussed in terms of a stress response.