Influence of photoperiod on growth in Baltic salmon parr (Salmo salar L.) with special reference to the effect of precocious sexual maturation

Individually tagged 2-summer-old Baltic salmon parr (Salmo salar L.) from Ume River (63°50′ N, 20°05′ E), Sweden, were exposed to photoperiods of either 6 h light – 18 h dark (LD 6:18) or 20 h light – 4 h dark (LD 20:4) during autumn. Fish subjected to natural day length (nLD) served as controls. Sexually immature female and male fish exposed to LD 20:4 grew more rapidly than salmon exposed to LD 6:18 and natural photoperiod. Sexually maturing male parr subjected to LD 20:4 grew more slowly than immature fish in the same photoperiod and did not differ significantly in growth rate compared with maturing males in LD 6:18 and nLD.Precocious male parr subjected to LD 6:18 and nLD became ripe in the middle of October while mature fish in LD 20:4 exhibited a delay in the ripening.It is suggested that either the major androgen 11-ketotestosterone, or gonadotropin (GTH), or both, inhibit photoperiod-stimulated growth in precocious males.