1. 1.

    Ram ventilation has been demonstrated in rainbow trout, Salmo gairdneri. The swimming speed eliciting the transition in mode of ventilation increased with declining ambient water oxygen tension (PwOO2).

  2. 2.

    The mode of ventilation of fish swimming at a constant speed could be altered by controlled variations in PwOO2.

  3. 3.

    Oxygen consumption (V·OO2) decreased by 10.2% when rainbow trout, swimming at a constant speed, shifted from active to ram ventilation. This difference can be ascribed to a lowering of the energetic cost of active ventilation as well as improved drag characteristics.

  4. 4.

    Water velocity required for the transition from active to ram ventilation in the resting sharksucker, Echeneis naucrates, increased with decreasing PwOO2.

  5. 5.

    The results show that water oxygen tension is an important stimulus for setting the ventilatory mode in rainbow trout and sharksucker.

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