Does size matter? Fitness-related factors in steelhead trout determined by genetic parentage assignment

Estimates of reproductive success and the factors that affect it can be inferred from observations of behavior or data on life history traits (e.g., dominance hierarchies, clutch size). Genetic parentage analysis, however, can precisely determine the number of offspring attributable to individual parents. We coupled multilocus microsatellite genotype-based parentage assignment exclusion testing with morphological and behavioral data to test hypotheses concerning the effects of specific traits on fitness of naturally spawning steelhead trout (Oncorhynchus mykiss). With eight microsatellite loci (mean heterozygosity = 89%), we examined two parental generations (n₁ = 198, n₂ = 226) and their offspring as smolts (age 2: n₁ = 365, n₂ = 285) and as adults (n₁ = 81, n₂ = 77) to determine the effects of size, reproductive timing, and origin (hatchery or wild) on reproductive success. The mating system was complex; monogamy, polygamy, polyandry, polygyny, and polygynandry were all observed. Males had a greater variance in reproductive success than females, wild steelhead produced more offspring than hatchery steelhead, and the timing of reproduction had a significant role in production of smolt and adult offspring, especially for females. There was no indication that body size contributed to offspring production, in stark contrast to the literature and theory on the subject. A more complete knowledge of traits associated with fitness in salmonids is important for understanding salmonid ecology and evolution and crucial for the design of appropriate conservation programs.