1. 1.

    The effects of thermal acclimation on whole blood oxygen affinity were examined in the freshwater blackfish Gadopsis marmoratus.

  2. 2.

    Oxygen equilibrium curves for 20°C-acclimated fish were shifted to the right of curves obtained for 10°C-acclimated fish when determined at both 20°C and 10°C. Oxygen equilibrium curves obtained for solutions of stripped haemoglobin prepared from blood of 20°C- and 10°C-acclimated fish did not show the differences observed for whole blood.

  3. 3.

    Thermal acclimation did not alter the number, migration rates, or relative amounts of the five electrophoretic forms of haemoglobin present in blackfish blood.

  4. 4.

    The intraerythrocytic concentration of nucleoside triphosphates was higher in the 20°C-acclimated fish than in 10°C-acclimated fish, while the whole blood haemoglobin concentration was lower in the 20 °C-acclimated fish. These differences gave NTP: Hb4 molar ratios of 1.68 for the 20°C-acclimated fish and 1.32 for 10°C-acclimated fish. The effects of nucleoside triphosphates on oxygen affinity were similar for stripped haemoglobins of both acclimation groups.

  5. 5.

    The change in NTP:Hb4 molar ratios with acclimation temperature acts to enhance oxygen unloading to the tissues rather than oxygen uptake at the gills at the higher acclimation temperature. As the waters inhabited by the blackfish retain high oxygen tensions at 20°C, these changes in blood oxygen affinity could be considered adaptive if they were associated with elevated rates of oxygen-dependent metabolism at the higher temperatures.

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