Glucocorticoid and behavioral variation in relation to carbon dioxide avoidance across two experiments in freshwater teleost fishes

Organismal responses to stressors can be influenced by several internal and external factors including physiological condition and inherent behavioral type. Carbon dioxide (CO ₂ ), a known stressor for fish, is naturally increasing in fresh water, and has been proposed as a non-physical barrier to prevent invasive fish movement. Intraspecific differences in how fish respond to CO ₂ challenges have been noted, with some individuals responding at low partial pressures of CO ₂ (pCO ₂ ), and others responding at higher pCO ₂ . Sensitivity to pCO ₂ may play a role in avoidance behaviors with respect to CO ₂ barriers and may predict how fish respond to naturally occurring CO ₂ challenges. We sought to determine the role that both physiological condition (i.e., elevated cortisol) and personality (i.e., boldness) play in influencing behavioral responses. To accomplish this goal, a shuttle box assay was used to determine the pCO ₂ that elicited avoidance in cortisol-injected or non-injected largemouth bass (Micropterus salmoides), as well as bold or shy bluegill (Lepomis macrochirus). Cortisol-injected largemouth bass shuttled at 45% higher pCO ₂ than control fish, but personality of bluegill had no effect on shuttling. It appears that an individual’s cortisol level can affect CO ₂ avoidance, likely mediated through the effects of cortisol on acid–base balance at the gill, or through the effects of cortisol on coping styles. Our finding has important implications for how fish respond to either natural or anthropogenically-driven changes in CO ₂ , as stressed fish with high cortisol would appear to be more tolerant of elevated CO ₂ , independent of personality type.