STUDIES ON STRESS, CORTISOL AND REPRODUCTION IN THE TILAPIA, OREOCHROMIS MOSSAMBICUS (PETERS)

Abstract

Female tilapia, Oreochromis mossambicus, were found to spawn regularly at 2 to 3 week intervals in the laboratory. Changes in ovarian steroidogenesis and growth over the reproductive cycle were consistent with those reported for other teleosts. Using male fish, the basal serum cortisol level was found to be low, usually less than 10 ng/ml; the level varied only slightly over a 24-hr period. When the fish were subjected to handling stress, a rapid rise in the serum cortisol level was observed. The rise was significant within 4 min and reached a peak 1evel within 30 min. The peak level, however, was dependent on the severity of the stressor, being lower (about 64 ng/ml) in fish subjected to a mild handling such as netting disturbances, and higher (about 119 ng/ml) in fish subjected to a severe handling such as capture and confinement. Removal of the stressor resulted in a rapid return of serum cortisol to the 'basal' level within 1 hr. The 'basal' cortisol level in female fish was more variable (up to 25 ng/ml); however, this was lower than the level induced by a mild stressor in the male fish. Using cortisol-cocoa butter implantation, a dose of more than 15 mg/10 g body weight of fish was found to maintain serum cortisol at the elevated stress level (more than 69 ng/ml) over a period as long as one reproductive cycle of the female. Such cortisol implantation in male fish was found to depress testosterone level over a 19-day study period. Implantation of cortisol in female was found to retard somatic and ovarian growth, and depressed serum T and E2 levels. The possible link between stress and mouth brooding in the effect on ovarian growth, as well as the possible link between somatic growth and ovarian growth were explored. The brooding females were found to have retarded ovarian growth and depressed serum testosterone and estradiol-17B levels, compared to non-brooding female, but their serum cortisol was not elevated to the stress level. Suppression of somatic growth by fasting the fish did not result in a retardation of ovarian growth. The role of cortisol in suppression of ovarian growth was investigated by an in vitro incubation techniques and cytological study. Cortisol was found to depress production of estradiol-17B by ovarian fragments in vitro. Ovarian fragments from cortisol-implanted fish were found to have depressed capacity to convert testosterone to estradiol-17B. The pituitary of cortisol-implanted fish was also affected and showed lower steroidogenesis-stimulating activity. Ultrastructural examination of the pituitary gland of cortisol-implanted fish revealed evidence suggestive of depressed synthesis as well as release of gonadotropin. The various effects of cortisol observed in the present study and the mechanism of cortisol action in the suppression of oocyte growth are discussed.