Seasonal levels of pituitary and plasma gonadotropin in male and female Atlantic salmon parr.

The annual cycle of precocious testes development was studied in the male Atlantic salmon parr. Pituitary and plasma gonadotropin (GtH) levels were related to stages of testicular activity and the precocious male values were compared with those of immature males and also with those of female parr which fail to undergo sexual development during presmolt freshwater residency. Plasma GtH increases during gonad development of precocious males. Whereas immature male and female parr pituitary GtH remains low throughout the year, precocious male values vary more than 100-fold rising quickly with the onset of spermiation. A lengthy period of spermiation matches a prolonged span of elevated GtH levels which begin falling during gonad involution and the stage of spermatogonial proliferation. It is concluded that dramatic changes in synthesis and release of pituitary GtH underlie precocious sexual development of the male Atlantic salmon parr.