Rainbow trout (Salmo gairdneri), bluegill (Lepomis macrochirus), and brown bullhead (Ictalurus nebulosus) were exposed to a gradual (over several hours) reduction in dissolved oxygen. Muscle and liver lactic and pyruvic acid concentrations were measured at seven environmental [Formula: see text]. The response was an increase in anaerobic metabolism of the resting fish at a threshold [Formula: see text] 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 oxygen tension near zero. The anaerobic thresholds correspond reasonably well to the relative positions of the blood oxygen dissociation curves for the three species. Only in the trout, however, does the anaerobic threshold found in this study compare to the critical oxygen tension (PC; [Formula: see text] below which aerobic metabolism becomes dependent on [Formula: see text] in the water). PC is far higher in both the bluegill and brown bullhead than the anaerobic threshold which suggests a reduction in overall energy expenditure in these species at rest when in water of low dissolved oxygen.Key words: anaerobic metabolism, lactate/pyruvate ratios, muscle, liver, rainbow trout, Salmo gairdneri; bluegill, Lepomis macrochirus; brown bullhead, Ictalurus nebulosus
The Transition between Branchial Pumping and Ram Ventilation in Fishes: Energetic Consequences and Dependence on Water Oxygen Tension