O2 and CO2 transport in relation to ventilation in the Atlantic mackerel, Scomber scombrus

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Abstract
Adult mackerel actively ventilate their gills whilst swimming at low speeds (20–60 cm∙s−1; ca. 0.6–2.0 body lengths∙s−1) in a swim tunnel at 15 °C. There was a regular alternation of a buccal pump, with dominant positive pressure phase, and an opercular pump, with dominant negative pressure phase. The two pumps cooperated to produce a maintained differential pressure across the gills (buccal to opercular cavities) so that water would flow in this direction for most, if not all, of the respiratory cycle. At swimming velocities between 60 and 80 cm∙s−1 (2.0–2.6 body lengths∙s−1), mackerel were found to suspend their cyclic buccal–opercular pump and rely instead on ventilation of the gills by forward movement through the water column (ram ventilation). The transition from one type of ventilation to the other caused no major change in the oxygenation of arterial blood, though CO2 was eliminated readily at the higher swimming speeds despite its being produced in larger quantities. Variations in arterial acid–base relationships were largely due to the mackerel's response to capture and subsequent experimental procedures, and not to the change from cyclic to ram ventilation. It is concluded that arterial blood oxygenation and CO2 output are regulated within limits that suggest the gills are adequately ventilated at all swimming speeds.

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