Daily cycles in serum cortisol levels in the goldfish: effects of photoperiod, temperature, and sexual condition

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Abstract
The daily fluctuations in serum cortisol levels, measured by radioimmunoassay, in goldfish were examined. Male and female goldfish with regressed or immature gonads ('regressed' fish), female goldfish undergoing ovarian recrudescence ('maturing' females), and female goldfish with oocytes that had completed vitellogenesis ('mature' females) were exposed to environmental conditions of either 16 h light 8 h dark (16 L:8 D) and 12 °C, 16 L:8 D, 21 °C, 8 L:16 D, 21 °C, or 8 L:16 D, 12 °C. Blood samples were taken from individual fish in the groups and a 24-h cycle in serum cortisol levels, based on 4-h intervals, constructed. Significant daily fluctuations were found under most environmental conditions at each sexual condition, most commonly with one daily peak, but in some cases with 2 or 3 daily peaks. The 'mature' females tended to have the lowest levels of serum cortisol and fluctuations of the smallest magnitude. The 'maturing' females tended to have the highest levels and fluctuations of the greatest magnitude. The 'regressed' fish had levels similar to the 'maturing' females under warm conditions and had low levels with relatively little fluctuation under cold conditions.

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