Metabolic responses to hypoxic challenge as indicators of sublethal environmental stress in fish populations

The effects of different environmental histories on metabolic processes in sunfish were determined by comparing the responses of individuals during hypoxic challenge tests. Patterns of metabolic responses to hypoxic challenge detected the effects of prior exposure conditions on physiological processes in these fish. Prior exposure conditions altered the relationship of routine metabolic rate (RMR) and dissolved oxygen concentration (DO) during progressive hypoxia under identical conditions in an automated respirometer. Exposure of bluegill, Lepomis macrochirus, to treatment combinations of temperature and food availability in laboratory tanks for two weeks caused significant differences in multiple response parameters representing various aspects of the RMR:DO relationship. Exposure differences in pH (5 vs 7) failed to cause significant differences in response among groups. A multivariate response model improved the ability to detect and visualize even complex differences among groups. In a second experiment, the mean critical oxygen concentrations (COC) for redbreast sunfish, Lepomis auritus, collected from polluted field sites, were significantly lower than for individuals collected from a reference site. Differentiation of polluted and reference sites by COC was consistent with other indicators of fish and aquatic system health measured previously, but polluted sites were not ranked consistently among themselves by these indicators. In a third experiment, only the routine metabolic rate differed among groups of cold-acclimated bluegill (15°C) held at test conditions (25°C) in laboratory tanks for differing times prior to testing. Metabolic responses to hypoxia were able to reveal metabolic differences caused by prior exposure conditions in the laboratory or at field sites. Metabolic response parameters indicated significant differences consistent with the presence or absence of a stressor, industrial pollution, among field sites.