Monthly variations in the profile of sex steroids and ovarian development of brook trout (Salvelinus fontinalis Mitchill) during the annual reproductive cycle in Tasmania

Control of maturation in all-female salmonid stock can be achieved by manipulating photoperiod. This initially requires an assessment of the annual reproductive cycle under ambient photoperiod. This study therefore describes the seasonal variations in ovarian development and plasma profiles of oestradiol-17 and testosterone during the second year of a population of female brook trout. It was found that fish recruited for maturation following the summer solstice in December. Shortening of photoperiod likely stimulated the brain-pituitary-gonadal axis for vigorous progress of ovarian development, as indicated by increasing levels of oestradiol-17 and gonadosomatic index values. The highest gonadosomatic index value (18.26 +/- 1.1%) was observed in June. During July, eggs were released from mature females by manual stripping. Total fecundity was observed as 4266 +/- 341 oocytes per mature fish. Profiles of sex steroids observed here were found to be finely entrained within the succession of seasonal photoperiod. This finding indicates that maturation suppression by photoperiod manipulation should be instigated before initiation of oocyte recruitment that commenced during December.