Oxygen uptake across the gills has been proposed as a bottleneck in the oxygen transport cascade of pelagic fish with high aerobic requirements. However, most investigations of this topic have relied on intermittent blood sampling, which only provides a snapshot in time of blood oxygenation, so large knowledge gaps remain.
We present a novel method with exciting potential for the study of oxygen uptake limitation in fish. By implanting oxygen sensors into the dorsal aorta of the highly athletic rainbow trout (Oncorhynchus mykiss, n=25), arterial oxygen tension (PaO₂) immediately downstream of the gills could be monitored continuously. We combined this with whole animal respirometry at two distinct water flow regimes within the respirometers (10 and 20 l min⁻¹) and hypothesised that increased flow rates would enhance arterial oxygenation and maximum metabolic rate (MMR) following exhaustive exercise at 20°C. The Pao2  recordings were validated by assessments of dorsal aortic PaO2  and O2  content from intermittent blood samples collected from a separate group of fish.
Intriguingly, PaO₂ closely tracked the minor water oxygen deviations associated with flush and seal cycles during respirometry. Pilot tests exposing the fish to <20% water oxygen saturation confirmed that fish only loosely defend their PaO₂ via hyperventilation and other measures. High water flow did not significantly affect MMR or PaO₂ recovery times, suggesting gill oxygen uptake was not limited following exercise. We highlight research avenues that our new approach has uncovered and discuss how remaining knowledge gaps surrounding gill oxygen uptake dynamics of fish can be addressed.

Erik Sandblom

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