1. 1.

    Gill ventilation (Vg), extraction of O2 from the ventilatory current andOg-uptake (Vg) were measured in three groups of undisturbed flounders naturally buried in sand. Two groups were acclimated to normoxic water(Pi, O2 120 mmHg). One was studied in normoxic water, the other during acute exposure to hypoxic water (Pi, O2 = 30 mmHg). A third group was acclimated to and studied in hypoxic water.

  2. 2.

    Gill ventilation was measured directly using an electro magnetic flow meter. The flowprobe, fitted to a plastic funnel, was placed over the fish's mouth without making physical contact with the fish. Exhaled water for calculation of O2-extraction was siphoned from another plastic funnel placed over the upper operculum of the flounder.

  3. 3.

    Acute hypoxia exposure of normoxic fish caused nearly a doubling inventilation volume. In flounders acclimated to chronic hypoxia and studiedin hypoxic water the ventilation volume reached a value approximately 3.5 times that of normoxic fish.

  4. 4.

    O2-extraction averaged 76% in normoxic flounders. During acutehypoxia the value declined to 52%, the same O2-extraction was obtained in the chronically hypoxic fish.

  5. 5.

    VO2 for the normoxia acclimated fish in normoxic water was 0.45 ml O2.kg−1.min−1 declining to 0.12 ml O2.kg−1.min−1 during acute exposure to hypoxia. VO2 of hypoxia acclimated fish in hypoxic water was 0.24 ml O2.kg−1.min−1

  6. 6.

    When compared in hypoxic water, the hypoxia acclimated fish show anunchanged O2-extraction of 52% in spite of a nearly two-fold increase inventilation relative to that of the normoxia acclimated fish. Possible causes for this may be a left shift of the O2-Hb dissociation curves and an increasein the transfer factor for O2 over the gills in the hypoxia acclimated fish.

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