Reduced daylength stimulates size-dependent precocious maturity in 0+ male Atlantic salmon parr

To study relationships between fish length, photoperiod and the onset of precocious maturity in 0+ parr during intensive rearing of Atlantic salmon (Salmo salar L.), 0+ juveniles of 55-89 mm length were individually tagged with Passive Induced Transponders (PITs) in May and exposed to reduced daylength (LD12:12) in May, June or July, or kept as control fish under continuous light (LD24:0). Relationships between the length of the fish and maturity were studied by the use of probabilistic maturation reaction norms. The incidence of mature males and the proportion of fish of lower modal group size in autumn were highest in the groups exposed to short days in May, and lowest under continuous light. In contrast with the expectation that high growth rates promote maturation, the future mature male parr were smaller than the immature males at the start of the experiment, and they also grew more slowly during the subsequent maturation process. Variability in condition factor was low until autumn, when the condition factor of maturing males rose well above that of immature males during autumn as the fish grew from ~ 80 to ~ 95 mm length. Reduced daylength increased the probability of the onset of precocious maturity in the fish smaller than, but not above a threshold length of ~ 90-100 mm (9 - 11 g). Intensively reared parr of 60-90 mm, and possibly also smaller fish, may be particularly sensitive to photoperiod manipulations that may influence the probabilities of fish adopting one of the three life-history alternatives, to enter lower or upper mode or to mature precociously.