D6 Oxygen Consumption Measurement

Topic

Fish oxygen consumption is the parameter measured by this reference method to quantify a fish's metabolic rate, the energetic cost of swimming, and its maximum aerobic capacity. The physical principle underlying this measurement is aquatic indirect calorimetry: since the oxidation of a fish's metabolic substrates consumes oxygen in known chemical proportions, measuring the rate of oxygen consumption allows one to infer the rate of metabolic heat production without needing to measure the heat directly.

To measure this oxygen consumption, one must first determine the dissolved oxygen concentration in the water surrounding the fish; this is done using either polarographic oxygen-detecting electrodes (of the type developed by Clark) or optical sensors that detect oxygen via fluorescence. Both sensor types achieve a precision ranging from one-hundredth to one-tenth of a milligram of oxygen per liter of water.

In a closed, static respirometer, the fish consumes the oxygen dissolved in the volume of water confined within the system; the rate at which the water's oxygen concentration drops over time is directly proportional to the fish's oxygen consumption. For this proportionality to be accurate, the respirometer's effective volume must account for the total water volume minus the volume occupied by the fish's body, as the latter does not form part of the available oxygen reservoir.

To measure oxygen consumption while the fish swims at known speeds, a swim-tunnel respirometer is used; this forces the fish to swim at a speed controlled by the water current generated within the tunnel. By repeating this measurement at various speeds, a curve relating oxygen consumption to swimming speed is constructed, and by applying Brett's incremental speed protocol to this curve, the fish's critical swimming speed is determined. Either of these two respirometers requires an additional correction: bacterial biofilm or other organisms present in the system also consume oxygen. Therefore, one must measure the system's oxygen consumption without the fish present and subtract that background value from the total recorded oxygen consumption to isolate the consumption actually attributable to the fish. Based on this corrected oxygen consumption—and by combining it with the water flow rate the fish passes over its gills during ventilation—it is also possible to estimate the efficiency with which the fish's gills extract oxygen from the water.

The expected pattern of oxygen consumption along the curve generated in a swim-tunnel respirometer is that it increases with swimming speed raised to a power of between two and three; consequently, swimming faster requires a disproportionately higher amount of oxygen. Within this curve, there is an optimal cruising speed where the fish's cost of transport reaches its minimum value; at the opposite end of the curve—the point of exhaustion—a clearly defined maximum oxygen consumption is reached, representing the same upper limit of aerobic capacity mentioned earlier as one of the parameters this method allows us to quantify.

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