Breeding structure of steelhead inferred from patterns of genetic relatedness among nests

Ten polylmorphic microsatellite loci were used to infer kinship among steelhead Oncorhynchus mykiss sampled from nest sites in the Hamma Hamma River, Washington. Fertilized eggs were removed from 8 of 11 redds (i.e., individual nests within redds) surveyed in 1998, tagged with unique identifiers, and reared until adulthood as part of a conservation program. On average, individuals sampled from the same redd were more closely related than individuals sampled from different redds, and a tree generated from the pairwise relatedness estimates shows clear clustering of individuals by redd. Despite this general pattern, there was substantial overlap in the pairwise relatedness estimates within and among most redds. Of the eight redds, seven contained more than four alleles at a minimum of one locus and five contained more than six alleles, indicating that most redds were the result of matings by more than a single pair of parents. In total, the eight redds were created by a minimum of 21 parents, which we inferred to consist of at least 5 females and 16 males based on the location and timing of redd construction. The male-biased sex ratio is probably due to matings by either male resident trout or precocial steelhead part with female steelhead. The results of this study indicate that although multiple matings by both sexes are common in this population, the variance in reproductive success is much higher for males than females.