Stimulatory and inhibitory effects of testosterone on testes in Atlantic salmon male parr

Immature 1-year-old Atlantic salmon Salmo salar parr were implanted with Silastic capsules of diVerent sizes filled with testosterone (T). Testosterone had both positive and negative eVects on testicular weights, spermatogenesis and steroidogenesis. The positive eVects: higher incidence of males with enlarged gonads, spermiation, and high plasma levels of 11-ketotestosterone (11-KT) and 17,20‚-dihydroxy-4-pregnen-3-one (17,20‚-P), were most pronounced in males treated with small T capsules. The negative eVects: suppression of gonadal development and depressed plasma levels of 11-KT and 17,20‚-P compared with mature controls, were most evident in fish treated with large T capsules. ? 1995 The Fisheries Society of the British Isles