Movement of juvenile weakfish Cynoscion regalis and spot Leiostomus xanthurus in relation to diel-cycling hypoxia in an estuarine tidal tributary

Fish movement and the spatial and temporal dynamics of hypoxia determine hypoxia exposure and the effect of poor water quality on nursery habitat function. Although water quality criteria for dissolved oxygen (DO) are well defined, hypoxia exposure of juvenile estuary-dependent fishes in situ is largely unknown. Thirty-one juvenile weakfish Cynoscion regalis and spot Leiostomus xanthurus were implanted with acoustic tags. Fish were acclimated for 5 d to either DO saturation or diel-cycling hypoxia (cycling between 11.0 and 2.0 mg O(2)l(-1)). Fish were released during summer into Pepper Creek, Delaware, an estuarine tributary. A logistic generalized additive model with generalized estimating equations was used to determine which environmental covariates significantly discriminated between movement types. Individual fish tracks were overlain on spatiotemporal contour plots of DO that highlight behavioral avoidance thresholds determined in the laboratory. Most models showed that DO, tide, and the spatial DO gradient were significant predictors of movement. Saturation-acclimated fish generally avoided DO < 2.8 mg O(2)l(-1) by increasing swimming speed and using tidal flow to facilitate escape. An exception to tidally aided movement occurred when a flooding tide appeared to trap smaller fish in spatially extensive hypoxia. Diel-cycling hypoxia acclimated weakfish appeared to have a lower avoidance threshold of similar to 1.4 mg O(2)l(-1). Downstream movements were far more frequent than upstream movements, possibly because late stage juveniles were moving out of the estuary. The relationship between tributary residency and the spatial extent of hypoxia suggests that there is a tradeoff between risk of hypoxic exposure and energetic benefits of remaining in productive areas.