GONADAL DEVELOPMENT AND SEX STEROIDS IN A FEMALE ARCTIC CHARR BROODSTOCK

Gonadal development and plasma levels of sex steroids were investigate d in female Arctic charr at 3-week intervals over a 12-month period. C irculating levels of oestradiol-17 beta (E-2), testosterone (T) and 17 ,20 beta-dihydroxy-4-pregnen-3-one (17,20 beta-P) were measured by rad ioimmunoassay, and gonadal status assessed through histological examin ation and measurement of gonadosomatic index (GSI) and frequency distr ibution of oocyte size-classes. Gonadal recrudescence during March-Jul y was characterized by modest but insignificant increases in plasma le vels of E-2 (2-4 ng ml(-1)) and T (2-5 ng ml(-1)) and recruitment of o ocytes into yolk accumulation. Only a small and insignificant rise in GSI and no apparent increase in oocyte diameter occurred during this p eriod, indicating that the rate of yolk formation and oocyte growth wa s low. Following transformation from stage V (peripheral yolk granule stage) to stage VI (yolk granule migration stage) in late July, the vi tellogenic oocytes entered a phase of rapid growth which resulted in a marked rise in GSI until ovulation commenced in late September. Gonad al growth during this period was accompanied by increases in plasma le vels of E-2 and T which peaked at 11+/-1 (mid-August) and 71+/-5 ng ml (-1) (late September), respectively. The levels of both steroids dropp ed rapidly during final maturation and ovulation: followed by a surge in plasma levels of 17,20 beta-P which peaked at an average of 74+/-17 ng ml(-1) in early October. All three steroids returned to basal leve ls within a month after ovulation, and all steroids, except E-2, remai ned low until March of the following year. A slight increase in E-2 de tected in February and March during the second season may have been as sociated with recruitment into vitellogenesis of a new generation of o ocytes. It is suggested that the abrupt increase in vitellogenesis in late July may reflect a condition-dependent decision to proceed with m aturation, once the energy reserves have been repleted during spring-e arly summer. (C) 1997 The Fisheries Society of the British Isles.