Unnatural random mating policies select for younger age at maturity in hatchery Chinook salmon (Oncorhynchus tshawytscha) populations

We explored the long-term consequences of three mating regimes ((1) completely random, (2) completely random but excluding jacks (age 2 males), and (3) male length female length) on age and sex structure of wild and hatchery populations of Chinook salmon (Oncorhynchus tshawytscha). Regimes similar to regimes 1 or 2 are used at most salmon hatcheries, whereas regime 3 emulates the outcomes of natural spawning behaviors that favor larger males. Inheritance of age at maturity is captured in age- and sex-structured models via matrices of age- and sex-specific conditional maturation probabilities that depend on age and sex of parents. In unexploited populations, regime 1 leads to substantial long-term selection for younger age at maturity, an effect that is somewhat reduced by regime 2, but greatly reduced under regime 3. Equilibrium age and sex structures for wild and hatchery populations under regime 3 are similar to those of natural populations, whereas mating regime 1 generates age structure that is greatly shifted toward younger ages and jacks. To prevent unintentional selection for younger age at maturity, we recommend that large-scale hatcheries replace unnatural completely random mating regimes with mating regimes that emulate the outcomes of natural spawning behaviors.