Alternative reproductive phenotypes represent adaptive life-history responses to local environments. Hybridization

with domesticated conspecifics exposed to selection against one of the phenotypes could affect the plasticity and incidence of

alternative reproductive phenotypes within wild populations, potentially influencing individual fitness and population viability.

We addressed this hypothesis by undertaking a common-garden experiment on Atlantic salmon (Salmo salar), a species in which

males mature either as large, migratory anadromous individuals or as small, generally nonmigratory parr. Comparing one wild

population and two domesticated–wild hybrids (F1, wild backcrosses), we evaluated the incidence of parr maturity at three

different temperatures. Parr maturation probability exhibited a significant quadratic relationship with body mass. Early maturation

was absent in the coldest temperature treatment. Body-size maturation thresholds were higher in the warmest temperature

treatment relative to the intermediate temperature treatment, resulting in a similar incidence of maturation in both

treatments despite increased growth in the warmest temperature treatment. Although body-size thresholds for parr maturity

did not differ between crosses, F1 hybrids and backcrosses exhibited a lower incidence of maturity relative to wild fish (4.8%, 9.3%,

and 30.1%, respectively). Changes in the incidence of alternative maturation phenotypes resulting from temperature and

domesticated–wild hybridization could have negative fitness consequences for wild populations.