DNA parentage analysis reveals inter-annual variation in selection: Results from 19 consecutive brood years in steelhead trout

Questions: Given costs and trade-offs, can selection be consistent or strong in animal populations? Is selection on body size and arrival timing, and opportunities for selection, related to indicators of breeding competition (breeding density and sex ratio)? Methods: Our data set consisted of genetic, phenotypic, and demographic data taken from adults of a small wild population of steelhead trout (Oncorhynchus mykiss). We measured the lifetime reproductive success of adults by matching breeding adult offspring to their parents using genetic data and exclusion-based parentage assignment. We calculated the opportunity for selection and estimated selection coefficients using regression analysis. Results: Selection more often favoured large males and females than small ones, although the strength of selection operating on body size varied among years. Selection on arrival date varied widely in shape, direction, and strength among years. Male-biased sex ratios and greater male breeding density increased the opportunity for selection on males. Higher female breeding density increased the opportunity for selection on females. Sex ratio and breeding density were unrelated to the strength or direction of selection on male or female body size and arrival timing. Selection always favoured large male body size in years when the sex ratio was male biased, but varied greatly in direction among years when the sex ratio was female biased.