Relationship between temperature, ovarian recrudescence, and plasma cortisol level in Tilapia aurea (Cichlidae, Teleostei).

Abstract

Abstract An RIA procedure for measuring cortisol in the circulation of Tilapia aurea is described and evaluated. Cortisol, chromatographically purified from the fish plasma, accounted for its total immunoreactivity. The correlation between observed and expected cortisol values in the accuracy test was r = 0.977. The within-assay variance was 10% and the between-assays variance was 18.9%. The sensitivity of the whole RIA procedure was 0.24 ng/ml when the dilution was 1:100 and 1.2 ng/ml when the dilution was 1:500. Female T. aurea collected in winter and kept at 17° had regressed ovaries, refused food, and their plasma cortisol levels at rest, measured after 1 and 2 months in captivity, were 74 ± 3.5 and 65 ± 6.1 ng/ml, respectively (mean ± SEM). Two weeks after their exposure to 28° the fish resumed feeding, the GSI had doubled, and the plasma resting cortisol level had decreased to 40 ± 4.0 ng/ml. The cortisol level remained low in the subsequent 11 days. Fish collected in the breeding season (April) had a resting cortisol level of 35 ± 4.0 ng/ml (mean ± SEM, n = 28). Nine days following ovariectomy no change could be found in plasma cortisol. Cortisol level in ovariectomized fish injected with estradiol-17β did not differ from that of oil-injected controls. In this nonmigratory, freshwater fish there is no relation between either the state of the ovary, or ovarian hormone, and the level of cortisol. The high level of plasma cortisol in fish exposed to low temperature may be attributed either to the slow rate of cortisol clearance or to the high activity of the interrenal tissue in the fasting fish.