Time spent in distinct life-history stages has sex-specific effects on reproductive fitness in wild Atlantic salmon

In species with complex life cycles, life history theory predicts that
fitness is affected by conditions encountered in previous life history
stages. Here, we use a four-year pedigree to investigate if time spent in
two distinct life history stages has sex-specific reproductive fitness
consequences in anadromous Atlantic salmon (Salmo salar). We determined
the amount of years spent in fresh water as juveniles (freshwater age, FW,
measured in years), and years spent in the marine environment as adults
(sea age, SW, measured in sea winters) on 264 sexually mature adults
collected on a river spawning ground. We then estimated reproductive
fitness as the number of offspring (reproductive success) and the number
of mates (mating success) using genetic parentage analysis (>5000
offspring). Sea age is significantly and positively correlated with
reproductive and mating success of both sexes whereby older and larger
individuals gained the highest reproductive fitness benefits (females:
62.2% increase in offspring/SW and 34.8% increase in mate number/SW;
males: 201.9% offspring/SW and 60.3% mates/SW). Younger freshwater age was
significantly related to older sea age and thus increased reproductive
fitness, but only among females (females: -33.9% offspring/FW and -32.4%
mates/FW). This result implies that females can obtain higher reproductive
fitness by transitioning to the marine environment earlier. In contrast,
male mating and reproductive success was unaffected by freshwater age and
more males returned at a younger age than females despite the reproductive
fitness advantage of later sea age maturation. Our results show that the
timing of transitions between juvenile and adult phases has a sex-specific
consequence on female reproductive fitness, demonstrating a life-history
trade-off between maturation and reproduction in wild Atlantic salmon.