Abstract
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.
Highlights
There is a high degree of phenotypic plasticity in the age of sexual maturity in Atlantic salmon (Salmo salar)
This contrasted with experiment one, whereby males with high and low 11-KT only differed in body mass in January, but not at the expected time for completed sexual maturation in April/May
The growth profiles of these males with elevated 11-KT suggested they had initiated the early stages of puberty, as they were heavier with a higher body condition than those males with low 11-KT, the same pattern observed in the mature jacks from experiment two
Summary
There is a high degree of phenotypic plasticity in the age of sexual maturity in Atlantic salmon (Salmo salar). Wild males may enter puberty during the early freshwater life stage (parr) or as anadromous males following one (grilse) or more seawinters (Klemetsen et al, 2003). Mature post-smolts, or “jacks”, have been observed in wild Atlantic salmon (Klemetsen et al, 2003), but is a rare phenotype. Jacks are more frequently observed in domestic salmon populations (Imsland et al, 2014; Stefansson et al, 1993; Thrush et al, 1994). Males have been observed to enter puberty either directly before, or shortly after, seawater adaptation (Fjelldal et al, 2011; Melo et al, 2014). Jacks can occur during the natural spawning season (i.e. Stefansson et al, 1993) or out-of-season following photoperiod and/or temperature manipulation (i.e. Fjelldal et al, 2011)
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