| dc.description.abstract | Understanding the causes and consequences of variation in life-history traits is fundamental for managing natural populations in a changing environment. Atlantic salmon (Salmo salar) spend 1–5 years feeding at sea before reproducing in fresh water, with late maturation resulting in a large body size, but lower survival probability. In this thesis, I aimed at better understanding 1) the role of growth, genetics, and genotype–environment interactions in shaping variation in age at maturity in Atlantic salmon, and 2) how variation in age at maturity translates into variation in reproductive success. I analysed data on life history, growth, and genetics obtained from fish scales of over 200,000 fish, collected over 28 years in 179 Norwegian rivers. First, I found that variation in the genotype frequencies of two large-effect loci explained parts of the among-year variation in age at maturity within a single population. Furthermore, genetic effects on age at maturity varied among years, suggesting genotype–environment interactions. Second, based on data from 103 populations and 28 years, I tested whether relationships between growth and age at maturity could be used to predict changes in age at maturity following an abrupt decline in marine growth. I predicted a decrease in age at maturity following the growth collapse, but the opposite response was observed, suggesting a highly non-linear relationship between growth and age at maturity. Third, I found, using adult body size as a proxy for fecundity, that genotypes associated with late maturation gained more in fecundity by delaying maturation from one to two years at sea, compared to genotypes associated with early maturation. The observation of a similar pattern across 174 populations, with fecundity gain increasing with average age at maturity, suggests that the divergence among populations in average age at maturity and marine growth has a genetic basis, involving pleiotropic effects of two large-effect loci. Fourth, I and collaborators showed that the frequency of repeat spawning, for a given age at maturity, and the period between reproductive events, increased with average age at maturity across 174 populations. A lower loss in body mass between reproductions in late-maturing populations than in early-maturing populations, suggests an adaptive association between average age at maturity and frequency of repeat spawning. | en_US |
