Physiological sensitivities to hypoxia differ between co-occurring juvenile flatfishes

Coastal hypoxia threatens estuarine nursery habitat for juvenile demersal fishes worldwide, and is expected to intensify with climate change. This study examines the physiological tolerances of hypoxia in two ecologically and economically important flatfish species that co-occur in a highly eutrophic estuary and biodiversity hotspot on California's central coast, the Elkhorn Slough. Juveniles of English sole (Parophrys vetulus) occur less frequently in areas of the slough where dissolved oxygen (DO) ≤ 4.0 mg/L O₂ compared to speckled sanddabs (Citharichthys stigmaeus), suggesting that English sole may be more sensitive to hypoxia. We exposed both species to an ecologically relevant, acute six-hour exposure to six DO levels ranging from ambient to severely hypoxic: 8.0, 6.0, 4.0, 3.0, 2.0, and 1.5 mg/L O₂ and measured known physiological and biochemical indicators of hypoxia stress. As DO declined, metabolic rate and aerobic scope decreased for English sole, while anaerobic activity (measured by L-lactate) and ventilation rate increased for both species. Biochemical responses to hypoxia were observed only at very low DO levels (≤ 4.0 mg/L O₂), indicating that both species appear to have a higher tolerance for relatively short-term hypoxia than other teleost fishes. Speckled sanddabs increased ventilation rate and anaerobic activity at higher DO levels than English sole, suggesting that earlier onset of compensatory mechanisms may contribute to the greater relative hypoxia tolerance in sanddabs. Examining the link between physiological and ecological thresholds for hypoxia tolerance will help determine suitable nursery habitat areas and management targets for estuarine restoration, and aid in predictions of fishery success under eutrophication and climate change.