Behavioral responses of juvenile weakfish (Cynoscion regalis) to diel-cycling hypoxia: swimming speed, angular correlation, expected displacement, and effects of hypoxia acclimation

Areas of low flushing and high nutrient loading in shallow estuaries are susceptible to diel-cycling hypoxia and also represent important nursery habitat for juvenile estuary-dependent fishes. Swimming speed, angular correlation, and expected displacement were measured in juvenile weakfish (Cynoscion regalis) in response to diel-cycling hypoxia (cycling between 7.0 and 0.4 mg O-2.L-1). Saturation-acclimated (7.0 mg O-2.L-1) weakfish exhibited an active response by increasing swimming speed (to a maximum at 2.8 gm O-2.L-1) and angular correlation (to a maximum at 1.4 mg O-2.L-1) as dissolved oxygen (DO) decreased, after which weakfish exhibited a passive response and both swimming speed and angular correlation decreased by similar to 50% and 70%, respectively, at 0.4 mg O-2.L-1. Weakfish acclimated to hypoxia (cycling between 2.0 and 11.0 mg O-2.L-1 for 10 days) did not vary swimming speed during decreasing DO or DO recovery (increasing DO) and had an overall swimming speed 46% lower than saturation-acclimated weakfish at 7.0 mg O-2.L-1. At the end of DO recovery, saturation- and hypoxia-acclimated weakfish had recovered 60% and 80% of their initial swimming speeds, respectively. The relationship between previous hypoxia exposure and behaviour may be an important determinant of habitat utilization in estuarine nursery areas impacted by diel-cycling hypoxia.