Measuring water-borne cortisol in convict cichlids (Amatitlania nigrofasciata): is the procedure a stressor?

Cortisol is an important indicator of health and behavioral state in fishes, and is produced in response to stressors including confinement, handling and social conflict. An inherent difficulty in measuring circulating cortisol is the implementation of invasive procedures that can be potent stressors. Recent studies show that cortisol can be reliably quantified from fish holding water by placing individuals in a small beaker for a predetermined collection period. We investigated whether convict cichlid fish (Amatitlania nigrofasciata) mount a significant stress response to beaker confinement and whether they habituate to the collection method. We also determined the relationship between plasma and water-borne cortisol, and changes in cortisol release rates following handling and cortisol administration. Initial beaker exposure induced high cortisol release rates but cichlids quickly habituated after 3-4 exposures. We revealed significant positive correlations between plasma and water-borne cortisol, and marked increases in water-borne cortisol release rates after cortisol injection that persisted for between 4 and 24 h, depending on the dosage. In conclusion, we provide convincing evidence for the utility and validity of the water-borne collection method to measure cortisol release rates in convict cichlids.