Behavioral Thermoregulation of Atlantic Croaker Under Hypoxic and Normoxic Conditions

Eutrophication has led to seasonal hypoxia in the Chesapeake Bay mainstem during warmer months, with unknown physiological and behavioral consequences for demersal fishes such as Atlantic croaker. We therefore assessed the influence of oxygen concentrations on croaker temperature preferences using an experimental shuttlebox system that simultaneously tracked fish, acquired data, and digitally controlled experimental conditions in response to movements of the subject. Following overnight acclimation to the chamber, thermoregulatory behaviors of croaker were recorded during 24 h experimental trials at normoxia (> 90% 0₂ saturation), mild hypoxia (~ 50% 0₂ saturation), and severe hypoxia (~ 25% O₂ saturation).  Croaker thermoregulated over a broad temperature range (17-27 C) under normoxia, preferring 25 C.  Fish preferred cooler waters under decreasing oxygen conditions, with bimodal preferenda of 17 and 22 C under moderate hypoxia, and a unimodal preference for 17 C under severe hypoxia. Temperature preferences exhibited diel differences under normoxic conditions but not during exposure to hypoxia.  Behavior of Atlantic croaker can be strongly driven by ambient oxygen conditions, with fish actively selecting the coolest waters available under the most severe hypoxia.