Differences in oxygen consumption and critical oxygen levels of five stream fishes

Abstract

Metabolic rate represents an integrated measure of a fish’s physiology, particularly as it relates to stressors. A metric that is commonly used to determine the hypoxia tolerance of fishes is the determination of animal oxygen consumption rate (MO2), which is thought to reflect the ability of an organism to extract oxygen from the environment to maintain routine metabolic rate as dissolved oxygen (DO) decreases. We used respirometry to quantify the influence of two abiotic factors as potential stressors on stream fishes: temperature and dissolved oxygen. We determined standard oxygen consumption rates and the critical oxygen level (DOcrit) of 5 fish species, one from the Tennessee River Basin (Cottus carolinae) and 4 from the Mobile River Basin (Percina nigrofasciata, P. palmaris, Etheostoma jordani, and Cyprinella venusta). Combining intermittent flow with static respirometry allowed us to measure oxygen consumption as a function of dissolved oxygen concentration, as well as the DOcrit, at three different temperatures (20°, 24°, and 28°C for E. jordani, and C. venusta, 20°, 22°, and 24°C for C. carolinae, P. nigrofasciata, and P. palmaris). Linear and segmented regression analysis was used to determine if the oxygen curves had a DOcrit. Oxygen consumption patterns varied among species, C. venusta showing typical oxyregulating while the other species exhibited oxyconformation. Respiration increased with temperature for all species except C. carolinae which showed a declining trend. DOcrit did not change significantly with temperature among species, but generally remained constant with temperature. However, C. carolinae was again an exception, with the DOcrit showing a decline as temperature increased. The results of my study show that current management plans that maintain DO levels above 4 mg O2•L-1 likely not adequate to satisfy the oxygen demands of all the stream fishes tested. A DO level above 7 mg O2•L-1 would be required for C. carolinae which unlike the other species was not tolerant to low oxygen and temperature > 20°C. Although the minnow and darters were tolerant to low DO, downstream regulation of DO should also include periods of non-generation as well, as this could result in rapid declines of DO and increased temperatures well below the tolerances of these stream species.