Scale cortisol reflects the acute but not the chronic stress status in green sunfish (Lepomis cyanellus)

Scale cortisol has the potential to be a useful biomarker of chronic stress in fish, but the dynamics of scale cortisol accumulation and release are not well understood. In this study, we assessed the dynamics of plasma and scale cortisol accumulation and clearance in green sunfish (Lepomis cyanellus) in response to acute and chronic stressors. While sunfish exposed to 21 days of unpredictable chronic stress (UCS) mounted an attenuated acute plasma cortisol response to the daily stressor on day 21 and had reduced liver and body conditions, their scale cortisol content was either unchanged or reduced. In contrast, exposure to an acute stressor increased scale cortisol content 30 min post-stressor in naïve fish but had no effect when preceded by the 21-day UCS protocol. We also examined the effects of an acute stressor on scale cortisol content in goldfish (Carassius auratus) to identify potential species differences in scale cortisol dynamics. Interestingly, although an acute stressor increased scale cortisol content 30 min post-stressor and declined 24 h later in both species, only sunfish scale cortisol levels returned to baseline by 24 h post-stressor. Together, our results indicate that scales in green sunfish can accumulate and clear cortisol rapidly following an acute stressor and do not reflect long-term accumulation of cortisol from the plasma during chronic stress. As such, we conclude that scale cortisol content is not a biomarker of chronic stress in green sunfish.