Cortisol does not increase risk of mortality to predation in juvenile bluegill sunfish: A manipulative experimental field study.

The hypothalamic-pituitary-interrenal (HPI) or stress axis in teleost fishes produces their primary glucocorticoid, cortisol. Although generally an adaptive response, prolonged HPI axis stimulation can impair organismal performance. Previous work has shown that stressed teleosts have higher mortality to predation than unstressed conspecifics, suggesting a role for HPI axis in modulating predator-prey interactions. Our current study investigated whether elevated cortisol levels altered the predation rate of a wild teleost fish, the bluegill sunfish (Lepomis macrochirus). Wild juvenile bluegill were given intraperitoneal implants of cocoa butter (i.e., sham), or cocoa butter containing cortisol or cortisol and the glucocorticoid receptor antagonist RU486. After 24 hr, fish were tethered along the bottom of the lake and their survival under natural predation was recorded following 24 hr. A subset of fish was used to validate the efficacy of cortisol implants in this setting. No treatment effect on survival was observed, suggesting that elevated cortisol has minimal involvement in mediating predator-prey interactions in this context. However, experimental fish may have demonstrated resiliency to physiological perturbations owing to the relatively acute duration of our experimental series, and negative effects might be manifested over a more chronic period.