Seasonal endocrine changes in Baltic salmon, Salmo salar, immature parr and mature male parr. I. Plasma levels of five androgens, 17α-hydroxy-20β-dihydroprogesterone, and 17β-estradiol

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Abstract
Plasma levels of the androgens testosterone (T) and 11-ketotestosterone (OT), the progestin 17α-hydroxy-20β-dihydroprogesterone (17,20-P), and the estrogen 17β-estradiol (E2) were measured by radioimmunoassay in monthly samples from immature parr (mostly females) and mature male parr of Baltic salmon, Salmo salar. The parr belonged to the Umeälven stock, northern Sweden, and were kept at ambient photoperiod and temperature throughout their rearing period. The sampling began in February at the end of their 2nd year and continued until December. 11β-Hydroxytestosterone (OHT), 11β-hydroxyandrostenedione (OHA), and 11-ketoandrostenedione (OA) were measured at selected times only. The levels of OT, T, and 17,20-P were low in mature and immature fish from February until the summer; in the immature fish they remained low for the entire period. There were only minor differences in steroid levels between mature and immature fish before and during the first half of smoltification (May). This was also the case for OHT, OHA, and OA. In the mature fish, the levels of T and OT started to rise in June. T peaked in August–October, and OT in September–October. Both androgens dropped to a very low level in December. OT displayed the highest levels of all androgens in the reproductive season. 17,20-P rose to a high level in October, peaked in November, and was still high in December. E2 levels were relatively high in winter, declined during spring in both groups, and rose again in autumn. The poorer adaptability to seawater that has been observed in mature vs. immature fish could not be explained by differences in steroid levels at the time of smoltification.

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