Sexual selection and breeding in Pacific salmon

Understanding natural breeding in Pacific salmon may offer insights for the management of these economically and culturally important fishes. I examined mating patterns by allowing Chinook salmon (Oncorhynchus tshawytscha ) to breed in experimental stream channels, and used genetic analyses to determine reproductive success for each fish. Phenotypic characteristics including body size and colouration had mixed effects on reproductive success, and were generally more important for reproductive success in males than females. Mating patterns were also affected by the genes of the major histocompatibility complex (MHC), as females mated with males that significantly increased the genetic diversity of their offspring at the MHC class IIB locus. Behavioural observations of spawning fish revealed that females directed aggression at more MHC similar males than expected by chance, providing a possible mechanism of female choice. However, male harassment of MHC similar females appeared to interfere with female mating preferences, as females that were the target of high levels of male aggression had lower than expected MHC diversity in their offspring. The potential for female skin carotenoid displays to be used in mate choice was examined by measuring carotenoid and retinoid concentrations in the skin, eggs, flesh and plasma of Chinook salmon. Skin and egg carotenoid concentrations were not correlated, which suggests that skin carotenoids do not advertise egg carotenoid resources to males, but may instead contribute to cryptic female colouration. The behavioural observations of spawning fish provided evidence for egg cannibalism, which contradicts the longstanding belief that Pacific salmon cease feeding when they return to fresh water to spawn. Analysis of stomach contents showed that multiple species of Pacific salmon consumed eggs during spawning, and feeding trials confirmed that mature fish could digest eggs. The final chapter compares hormone concentrations and behaviour in juvenile Chinook salmon produced by natural and hatchery breeding. Differences in the rearing environment affected cortisol concentrations and feeding rates, whereas androgen concentrations and aggression appeared to differ based on the breeding method itself. Together these findings suggest that natural breeding has important applications for managing Pacific salmon in the wild and in aquaculture.;Keywords. Sexual selection, mate choice, major histocompatibility complex, sexual conflict, carotenoid, retinoid, astaxanthin, hatcheries, aggression, egg cannibalism.