Comparative assessment of cortisol in plasma, skin mucus and scales as a measure of the hypothalamic-pituitary-interrenal axis activity in fish

Cortisol, the end product of the hypothalamus-pituitary-interrenal (HPI) axis, has been traditionally measured in blood as indicator of stress in fish, however, the degree of invasiveness inherent to blood collection is not always possible or desirable. Instead, cortisol measurement in skin mucus is far less invasive, but as blood, this method potentially provides only a brief window of information of the HPI axis activity. The newly described method of cortisol measurement from scales may serve as a long-term, integrated measure of the HPI axis activity in fish. While skin mucus and scales cortisol measurement present practical and conceptual advantages, there are still several unclear issues related to their biological relevance that need deeper study. Accordingly, we aimed to evaluate whether skin mucus and scales cortisol levels can be reliably used as stress indicators by subjecting fish to prolonged, continuous stressful conditions. The present study demonstrates that the measurement of cortisol in skin mucus reflect circulating cortisol concentrations when fish are responding to stress with an intense activation of the HPI axis. Results also revealed that cortisol content in scales strongly correlates to circulating cortisol levels in chronically stressed fish. Besides, we provide further support that scales cortisol assessment offer a retrospective measure of the past stress experience in fish. While this study provides a good basis for future research applying the methods presented, our results open the question of whether these matrices have additional sources of cortisol other than blood, and the route of incorporation or diffusion. Further knowledge about the general robustness and stability of scales cortisol in fish subjected to prolonged stress would largely help strengthen the interpretation of hormone fluctuations in this matrix.