Seasonal changes in the levels of 11‐oxotestosterone and testosterone in the serum of male salmon, Salmo salar L., and their relationship to growth and maturation cycle

Levels of 11‐oxotestosterone (17 β‐hydroxyandrost‐4‐ene‐3, 11‐dione) and testosterone in the blood serum of individually marked adult male Atlantic salmon held in captivity, were measured by radioimmunoassay at approximately monthly intervals for periods of up to 18 months. In addition to peak concentrations of both hormones shown by all the maturing fish at the time to full sexual maturation during October and November, a majority of maturing fish also showed a significant elevation of 11‐oxotestosterone during the early months of the year. The possible involvement of this early elevation of 11‐oxotestosterone in controlling the mitotic multiplication of spermatogonia is discussed. Weight and length increases expressed as specific values G W and G L and weight to length relationships for the maturing males for each sampling period are presented and compared with those of non‐maturing fish.