Respiratory and Circulatory Responses to Hypoxia in Largemouth Bass and Smallmouth Bass: Implications for “Live-Release” Angling Tournaments

Arterial blood respiratory variables, ventilation rate, and cardiac output were examined in largemouth bass Micropterus salmoides and smallmouth bass M. dolomieu to compare their physiological responses to graded levels of hypoxia. Reduction in water Po-2 (Pwo(2)) from 150 to 45 torr (I torr approximate to 133.3 Pa) caused similar decreases in arterial Po-2 (Pao(2)) in both species, although total blood O-2 content was markedly higher in largemouth bass at the lower oxygen tensions. Curves for in vitro O-2 dissociation in normoxic fish indicated that largemouth bass blood had a higher affinity for O-2 than smallmouth bass blood. Severe hypoxia caused a significant increase in blood concentrations of catecholamines in smallmouth bass but not in largemouth bass. Increases in ventilation rate (54%) and decreases in cardiac output (27%) during hypoxia were also more pronounced in smallmouth bass than in largemouth bass. Arterial blood pH did not change during hypoxia exposure in largemouth bass but decreased significantly during severe hypoxia in smallmouth bass. The results of this study indicate that smallmouth bass are more sensitive to hypoxia than largemouth bass are. These results have important implications for situations in which these two bass species may be exposed to periods of hypoxia, such as during live-release angling events.