Searching for Oxygen: Dynamic Movement Responses of Juvenile Spot ( Leiostomus xanthurus ) in an Intermittently Hypoxic Estuary

The movement responses of juvenile fishes exposed to intermittent hypoxia mediate the effects of impaired water quality on estuarine nursery habitat function. Twenty-five juvenile spot ( Leiostomus xanthurus ) were implanted with hydroacoustic tags and tracked in the Neuse River Estuary (NRE), NC during multiple hypoxic episodes (dissolved oxygen, DO ≤ 2 mg L −1 ), while simultaneously monitoring environmental conditions at fish detection locations and at a representative mid-channel location. Mean swimming speed increased nearly ninefold under hypoxia, as fish traversed waters with low bottom DO over long distances (~ 10 km) for up to 35 h, before moving from the deeper main channel to shallow, nearshore oxygenated refuge habitats. Generalized additive models indicated that spot swimming speeds increased significantly with decreasing bottom DO and increasing depth, and were correlated with speed during the previous movement segment, though most (60–70%) of the variability in swimming speed remained unexplained. K -means clustering identified three behavioral modes: (1) slow swimming in deep water when DO was high throughout the NRE (normoxic behavior); (2) rapid and highly directed swimming that traversed deep waters with low bottom DO (hypoxia avoidance behavior); and (3) slow swimming in shallow, oxygenated waters while deeper waters remained hypoxic (refuge behavior). Despite comprising only 8.4% of the observed movements, hypoxia avoidance behavior resulted in highly conspicuous increases in swimming speed that led to large displacements of juvenile fish. The results help elucidate the specific behavioral responses of juvenile spot to intermittent hypoxia, as well as provide insight into the mechanisms by which variable DO conditions affect estuarine nursery habitat function.