Ambient Oxygen Tension and Transition to Anaerobic Metabolism in Three Species of Freshwater Fish

Abstract

Rainbow trout (Salmo gairdneri), bluegill (Lepomis macrochirus), and brown bullhead (Ictalurus nebulosus) were exposed to a gradual (over several hours) reduction in dissolved O2. Muscle and liver lactic and pyruvic acid concentrations were measured at 7 environmental PO2. The response was an increase in anaerobic metabolism of the resting fish at a threshold PO2 that varied with the species and acclimation temperature. A lower anaerobic threshold occurred after acclimation to cold in rainbow trout and possibly bluegill; cold-acclimated brown bullhead did not metabolize anaerobically when held in water for 20 min with an O2 tension near 0. The anaerobic thresholds correspond reasonably well to the relative positions of the blood O2 dissociation curves for the 3 species. Only in the trout does the anaerobic threshold found in this study compare to the critical O2 tension (PC; PO2 below which aerobic metabolism becomes dependent on PO2 in the water). PC is far higher in the bluegill and brown bullhead than the anaerobic threshold, suggesting a reduction in overall energy expenditure in these species at rest when in water of low dissolved O2.