SERUM CORTISOL RESPONSE TO HANDLING STRESS AND THE EFFECT OF CORTISOL IMPLANTATION ON TESTOSTERONE LEVEL IN THE TILAPIA, OREOCHROMIS-MOSSAMBICUS

A tropical fish species (Oreochromis mossambicus) was used to study cortisol response to handling stress. Unstressed fish were found to have very low (< 10 ng/ml) serum cortisol levels. Stressed fish showed a rapid and significant rise in cortisol levels within 4 min. The peak level, reached within 30 min of stress, was dependent on the severity of the stressor, being lower (about 64 ng/ml) in fish disturbed by netting, and higher (about 119 ng/ml) in fish captured and confined. Removal of the stressor resulted in a rapid return of cortisol to a normal level. Cortisol cocoa-butter implants were successfully used to elevate and maintain cortisol at a level characteristic of a stressed fish for 19 days. Cortisol implants of 1.5 and 3.0 mg/g body weight were found to depress testosterone levels significantly over the study period.