Spring 2022

Document Type

Master's Thesis (Open Access)

Degree Name

Master of Science (M.S.)


Moss Landing Marine Laboratories


Estuaries serve numerous important ecosystem roles, including providing critical nursery habitat for juvenile fish. However, due to eutrophication and climate change, estuaries experience highly variable dissolved oxygen (DO) levels and hypoxic conditions. Though hypoxia negatively impacts juvenile fish, there are physiological compensatory mechanisms fish may utilize to mitigate its effects. This study examines the effects of hypoxia on two ecologically and economically important flatfish species in Elkhorn Slough on California’s central coast: juvenile English sole, Parophrys vetulus, and juvenile speckled sanddabs, Citharichthys stigmaeus. I measured metabolic rate, ventilation rate, and hematocrit, as well as biochemical indicators of hypoxia (hypoxia-inducible factor, HIF-1α, and L-lactate) and oxidative stress (superoxide dismutase, SOD), following an ecologically relevant, 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. I found that both standard and maximum metabolic rate, as well as aerobic scope, decreased as DO level declined for English sole. For both species, ventilation rate increased as DO level decreased, likely as a mechanism to bolster oxygen supply. Both species also exhibited a significant increase in anaerobic activity (measured as elevated L-lactate in muscle tissue) at low DO levels (1.5 mg/L O2 for English sole and 2.0 mg/L O2 for speckled sanddabs). Overall, all these factors can lead to or indicate decreased survival and fitness of juvenile flatfish in hypoxic conditions, with lower survival potentially reducing population size and fishery success. Evaluating thresholds for tolerance of hypoxia may allow us to predict these changes, as well as determine areas of suitable nursery habitat and targets for estuarine restoration. Many responses to hypoxia were observed only at very low DO levels (i.e., 2.0 or 1.5 mg/L O2). These flatfish species appear to have a higher tolerance for hypoxic conditions than other teleost fishes and may be able to withstand many of the environmental hypoxia events observed in Elkhorn Slough, provided DO levels do not drop below lethal thresholds. Species-specific differences were also found. Although hematocrit, SOD, and HIF-1α did not exhibit significant relationships with DO, hematocrit and SOD were both higher in speckled sanddabs. Additionally, for two metrics measured, ventilation rate and L-lactate, speckled sanddabs exhibit a nonlinear response (highest values at mid DO levels), in contrast to a more linear response for English sole (highest values at low DO levels), suggesting responses to hypoxia may be employed at different DO thresholds for the two species. Finally, greater interindividual variation in responses to decreasing DO in English sole, compared to speckled sanddabs, suggests that English sole are more significantly impacted by DO level, and potentially less tolerant of hypoxic conditions. Interspecific variation could potentially alter the relative distribution and abundance of the two species in this nursery habitat, while interindividual variation could potentially drive evolutionary change toward greater hypoxia tolerance.