NRC Research Press, National Research Council of Canada
Abstract
The seasonal changes in gonadosomatic index (GSI) and strum testosterone, I I-ketotestosterone, and 17&estmdiol levels
were measured in adult feral brown bullheads, Ictalurus nebulosus Lzsueur. The maximum GSI of both male and femalz brown
bullheads was consjderably lower than that of most other teleostean species investigated. lo 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 throu_ghout
May and June (during the prespawning and spawning periods). Peaks of serum testosterone and serum 11-ketotesrosterone
leveb were evident in mid-April and late May to June, and in mid-April and mid-May, respxtively. In females here was a
rapid increase in GSI during May, when the ambient water temperamre reached 16°C. The peak GSI was evident in mid- to
late-May and had declined by early June. Peak serum testosterone and I 1-ketotestosterone levels were evident .in mid-April
and again in late May. wbereas peak 17P-estradiol levels were fou.nd in mid-May and mid-June. The peak serum teswsterone
levels in females were 4.5-fold higher than in the males, whereas che I I-ketotestosterone levels were similar in males and
females
