Changes in gonadosomatic index (GSI), hepatosomatic index (HSI), oocyte development and plasma
levels of steroid hormones were studied during the reproductive cycle of the sweep Scorpis lineolatus.
GSI values for both sexes were maximal in March and April. During this period the size distribution
of oocytes showed a predominance of advanced stages of vitellogenesis, whereas males were completing
spermatogenesis. However, staging data suggest that spawning may not begin until April. HSI was
maximal 2 months prior to the beginning of gametogenesis in both sexes. Plasma concentrations of
oestradiol-17β in females increased rapidly from February, reached a peak (2.3 ng mL-1) in association
with the end of vitellogenesis in March, and returned to low concentrations in May and June. Plasma
concentrations of 17α,20α-dihydroxy-4-pregnen-3-one (17,20βP) also peaked in March in females, but
17,20βP was generally not detectable in males. Plasma testosterone concentration peaked along with
other steroids in March in females but showed no significant change in males. Plasma ll-ketotestosterone
(1 1KT) concentration was maximal in males before spawning (February), but was not elevated
during the spawning period itself. Changes in oestradiol-17β and testosterone in association with
vitellogenensis in females are consistent with those reported for other teleosts, whereas changes in
17,20βP, testosterone and 11-ketotestosterone in males are different from those previously reported.
Effect of oral contraceptives on plasma testosterone concentration.