Testicular development in the sea trout (Salmo trutta morpha trutta L.) after sex differentiation, with a reference to precocious maturation

Testicular development was followed in juvenile sea trout (Salmo trutta morpha trutta L.) stocked in a river near Szczecin, Poland in 1992. Fish age was between 3 and 6.5 months post-hatch. Fish were sampled monthly. Sex-dependent differences in gonad structure and timing of their differentiation were observed after dissection under light microscope. In 3-month-old fry, when female gonads were differentiating (morphologically and cytologically), gonads of potential males remained undifferentiated. Development of a gonad into a male was primarily indicated by the formation of seminificrous tubules (lobules). In the sea trout under study, the lobules formed between the fourth and fifth month post-hatch (July-August) (fork length >5.6 cm). There were no significant differences in body fork length between fish with and without lobules, although mean length of the former was higher. Early spermatogenesis began once the type B spermatogonia appeared. The timing of their appearance differed widely among individuals. Type B spermatogonia were found for the first time in a 5-month-old male (late August). Spermatocytes and cells of subsequent stages appeared in ail incompletely matured 6.5-month-old male as 'attempted spermatogenesis' (fork length=8.8 cm). Most examined males remained immature, their germ cells not having passed the type A spermatogonium level. In 6.5-month-old alevins, no significant differences in fish size between individuals beginning spermatogenesis (stage II) and those at stage I were detected, although those at stage II were longer. As the male gonad structures were forming, the quantitative gonad parameters were gradually increasing, even when referring to the unit area; only the gonocyte size gradually decreased. Generally, each observed monthly or bimonthly difference was statistically significant (P<0.05).