Female colour and male choice in sockeye salmon: implications for the phenotypic convergence of anadromous and nonanadromous morphs

Divergent mate choice patterns between populations may lead to premating isolation and speciation. However, similar or identical mate choice patterns may also constrain phenotypic divergence, although not necessarily genetic divergence, across populations. We examined spectral characteristics necessary for male mate choice of abstract female models in sockeye salmon, Oncorhynchus nerka, in experiments conducted on the breeding grounds and under controlled conditions in the wild. Males preferentially selected and spawned with models with a red hue, the predominant colour of females. Small changes in wavelength, saturation and brightness affected preference. Sexually naïve males showed the same preference as experienced males, suggesting that a preference for red hue might be innate. We discuss these results in relation to female colour, the transmission environment and the visual physiology of salmon to evaluate further the possible role of a pre-existing bias for red in the evolution of red breeding colour in nonanadromous (kokanee) salmon. Our results suggest that a sensory bias can account for the re-emergence of red in kokanee, after its loss during the initial development of a freshwater morph.