Abstract
The content of thyroid and sex steroid hormones in the blood of immature and precocious trout Salmo trutta at the final stage of the formation of early maturing fishes (age 1+, 2+) in the population was determined. The formation period of early maturing males varies in different years in Alatsoya River (Karelia). Fish can reach sexual maturity at the age of 1+ or 2+. Immature and precocious trout at the age of 1+ and 2+ do not differ in the free and total triiodothyronine and free thyroxine content. Immature females and males aged 1+ and 2+ also do not differ in the level of sex steroid hormones. In contrast to immature brown trout, precocious males aged 2+ are characterized by an increased content of testosterone and a reduced level of estradiol-17β in the blood. It has been established that the final stage of the formation of early maturing fishes in the population is characterized by a weak involvement of the thyroid gland in the maturation process of males. Also, it was shown that the rate of conversion of testosterone to estradiol-17β in their blood was a significant decrease (4 times). This transformation in all studied females and males of brown trout is associated with their body length. The rate of formation of estradiol-17β in fish was increased with increasing of body length.
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