Termination of puberty in out-of-season male Atlantic salmon smolts.

Environmental conditions are known to contribute to the phenotypic plasticity in the age of sexual maturation of Atlantic salmon (Salmo salar). Here, we report on an observation of out-of-season male Atlantic salmon initiating puberty as pre-smolts (jacks) but failing to complete maturation as post-smolts. Jacks were identified based on elevated plasma 11-ketotestosterone (range, 3-12 ng/ml) and the occurrence of type B spermatogonia in January 2017. However, these males failed to show running milt as post-smolts at the expected time in May 2017. Subsequently, 6 out of the 21 (32%) suspected "terminated jacks" went on to become grilse, whereas only 1 of the 22 (5%) males that showed no signs of initiating puberty in January became grilse in December 2017. Therefore, "terminated" jacks were more likely to mature as grilse than the males that remained immature. Why these pubertal pre-smolt males did not complete maturation is unclear but could be related to the transfer of fish from conditions of warm water and long days, risk factors for early maturation, to conditions of cold water and short days, which are expected to delay the age of maturation. We provide a description of the conditions under which male Atlantic salmon appear to have terminated the process of sexual maturation.