Seasonal variation in the profile of sex steroids and histological testicular development of brook trout (Salvelinus fontinalis Mitchill) during the annual reproductive cycle in Tasmania

Abstract

Early maturation in brook trout males has been a constraint for commercial production of this species in Tasmania. However, control of maturation in males can be achieved by manipulating photoperiod (duration of light phase). To design an appropriate light treatment regime, an assessment of the annual reproductive cycle was initially required under ambient photoperiod as the annual reproductive cycle of male brook trout under ambient Tasmanian conditions of light and temperature was unknown. Here we describe the seasonal variations in testicular development and plasma profiles of testosterone (T), 11-ketotestosterone (11-KT) and oestradiol-17β (E2) during the second year in a male brook trout cohort held under ambient light conditions. Brook trout males were recruited for maturation in December when a reduction in daylength presumably stimulated testicular development. Increases in levels of T, 11-KT and gonadosomatic index (GSI) values were observed soon after the summer solstice, corresponding with maturation. The highest GSI mean value (3.44 ± 0.11%) was observed in April when mature males could be manually stripped of milt and such males were present until July. Profiles of sex steroids and histological development observed in this study were finely synchronised with variations in seasonal photoperiod changes. Photoperiod before recruitment during December should be controlled to manipulate maturation in brook trout males during their second year.