Reproductive timing phenotypes in female salmon: true alternatives or extreme variants?

Kokanee salmon, Oncorhynchus nerka, can vary in their maturation status at arrival to spawning areas and in their subsequent behaviour. In Meadow Creek, British Columbia, typical ‘red-arriving’ females arrive with red nuptial coloration, spawn within a few days, and then defend their nests against digup by conspecifics. In contrast, atypical ‘silver-arriving’ females arrive with scales still present and delay maturation and spawning by 3–4 weeks. Our objective was to further describe the silver- and red-arriving phenotypes and to test the hypothesis that they are proxies for a dimorphism of life cycle variation. Colour was a good indicator of a female’s stage of maturity and senescence because silver-arriving females had larger guts, larger and fattier livers, and smaller eggs containing less lean mass than red-arriving females. Colour, and therefore maturity status, covaried with life history traits: silver-arriving females were younger (in 2008 and 2009) and smaller (in 2008) than red-arriving females. The percentage of silver-arriving females was higher in 2009 (11.4%) than in 2008 (2.2%), possibly because of poor conditions for growth in 2009. We also observed some silver-arriving males, but they were much rarer (0.4% in 2008 and 1.5% in 2009). Scores on the first discriminant function based on colour information from digital photographs distinguished silver-arriving and red-arriving females. At the population level, these scores were bimodally distributed, but the dip between modes did not clearly separate silver- and red-arriving females. Although silver- and red-arriving phenotypes represent female reproductive tactics for delivering parental care, they are not distinct alternatives and instead represent the extremes of life cycle variation in this population.