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

Abstract

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 O2·L–1). Saturation-acclimated (7.0 mg O2·L–1) weakfish exhibited an active response by increasing swimming speed (to a maximum at 2.8 mg O2·L–1) and angular correlation (to a maximum at 1.4 mg O2·L–1) as dissolved oxygen (DO) decreased, after which weakfish exhibited a passive response and both swimming speed and angular correlation decreased by ~50% and 70%, respectively, at 0.4 mg O2·L–1. Weakfish acclimated to hypoxia (cycling between 2.0 and 11.0 mg O2·L–1for 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 O2·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 behavior may be an important determinant of habitat utilization in estuarine nursery areas impacted by diel-cycling hypoxia.