Male size effects on fertilization success: lack of evidence in chinook salmon spawning under experimental conditions

Anadromous salmonid females exhibit indicators of mate choice based on male size. Direct benefits to females of mating with larger males have not been identified for semelparous Pacific salmon, Oncorhynchus spp. We tested the null hypothesis that females forced to spawn naturally in a stream channel and artificially (gametes removed manually) with males about half their body mass would experience egg fertilization rates similar to that of females forced to spawn with males of about equal mass. Fertilization rates did not differ significantly between large- and small-male pairs. The fertilization rates were also very similar for eggs deposited naturally and those that we fertilized artificially. Therefore, fertilization success does not appear to be the mechanism responsible for female mate choice based on male size. Benefits of females mating with larger males probably have only indirect (i.e., genetic) benefits to a female's offspring, as suggested by previous authors.