The nature of hematological response in fish

Hematological status was examined in rainbow trout,Oncorhynchus mykiss, held for 3–4 weeks under temperature, photoperiod and P_O2 conditions approximating those of their winter, spring and summer habitats. The most striking change observed was in red cell population composition. In winter fish mature cells were predominant; juvenile and developing erythrocytes characterized spring and summer animals. Hemoglobin, hematocrit and both mean erythrocytic volume and hemoglobin were modestly lower in spring and summer than in winter fish. Red cell numbers were not significantly affected. These observations suggest that avoidance of viscosity-based increases in circulatory work cost is more advantageous than elevation of blood O₂-carrying capacity. Although hemoglobin isomorph profiles were significantly altered, there is little evidence that such changes are of critical adaptive importance. Given presumed age-based reduction in gas transport effectiveness, the replacement of mature and senescent cells by more metabolically-competent juvenile cells appears to be the pivotal event in hematological response. Leucocyte counts were significantly elevated in spring and summer as compared to winter fish. Lymphocyte/heterophil ratios declined from 8.27 in winter fish to 3.13 in summer trout. Thrombocyte, monocyte, eosinophil and basophil abundances were little changed.