Seasonal changes in serum testosterone, 11-ketotestosterone, and 17β-estradiol levels in the brown bullhead, Ictalurus nebulosus Lesueur

Abstract

The seasonal changes in gonadosomatic index (GSI) and serum testosterone, 11-ketotestosterone, and 17β-estradiol levels were measured in adult feral brown bullheads, Ictalurus nebulosus Lesueur. The maximum GSI of both male and female brown bullheads was considerably lower than that of most other teleostean species investigated. In males, the GSI began to increase in April concomitant with an increase in water temperature from 3 to 6 °C. The maximum GSI levels were evident throughout May and June (during the prespawning and spawning periods). Peaks of serum testosterone and serum 11-ketotestosterone levels were evident in mid-April and late May to June, and in mid-April and mid-May, respectively. In females there was a rapid increase in GSI during May, when the ambient water temperature reached 16 °C. The peak GSI was evident in mid- to late-May and had declined by early June. Peak serum testosterone and 11-ketotestosterone levels were evident in mid-April and again in late May, whereas peak 17β-estradiol levels were found in mid-May and mid-June. The peak serum testosterone levels in females were 4.5-fold higher than in the males, whereas the 11-ketotestosterone levels were similar in males and females.