Long-term effects of alternative hatchery mating practices and size selective fishing on age and sex composition of Chinook salmon populations returning to hatcheries

Experimental matings of hatchery Chinook salmon (Oncorhynchus tshawytscha ) demonstrate that age at maturity is an inherited trait: generally, older aged parents produce progeny that mature at older ages than those produced by younger aged parents. On natural spawning grounds, larger and older males generally dominate in behavioral contests with younger and smaller males and females prefer mates that at least match their body size. I therefore hypothesized that completely random mating of Chinook salmon at hatcheries, often recommended as a strategy to maximize genetic diversity, should lead to unintended long-term selection for younger age at maturity. Using an age- and sex-structured model incorporating matrices of age- and sex-specific maturation probabilities for each possible mating type, I explored the theoretical long-term consequences of alternative hatchery mating strategies. Model results showed that completely random mating does indeed result in long-term selection for younger age at maturity, even when jacks are excluded as spawners. If instead females are mated only with males that exceed their size, then long-term selection for young age at maturity can be greatly reduced. Using the same age- and sex-structured model, I calculated approximate equilibrium age and sex structures for wild population models that incorporated male competition as well as female choice. A wild population model incorporating size-dependent mating success produced an equilibrium age and sex composition close to the wild Chinook salmon population upon which it was based. The size-dependent mating strategy very crudely mimics the natural spawning behaviors of Chinook salmon and could be feasibly implemented at large-scale production facilities. The importance of adopting such a mating strategy is heightened by continuing size-selective ocean fisheries that shift the age structure of spawning populations toward younger ages, contributing to additional long-term selection for younger age at maturity.