Relationship between effective population size, inbreeding, and adult fitness-related traits in a steelhead (Oncorhynchus mykiss) population released in the wild

Inbreeding is of concern in supportive breeding programmes in Pacific salmonids, Oncorhynchus spp, where the number of breeding adults is limited by rearing space or poor survival to adulthood, and large numbers are released to supplement wild stocks and fisheries. We reconstructed the pedigree of 6602 migratory hatchery steelhead (Oncorhynchus mykiss) over four generations, to determine the incidence and fitness consequences of inbreeding in a northwest USA programme. The hatchery maintained an effective population size, = 107.9 from F0 to F2, despite an increasing census size (N), which resulted in a decreasing Ne/N ratio (0.35 in F0 to 0.08 in F2). The reduced ratio was attributed to a small broodstock size, nonrandom transfers and high variance in reproductive success (particularly in males). We observed accumulation of inbreeding from the founder generation (in F4, percentage individuals with inbreeding coefficients Δf > 0 = 15.7%). Generalized linear mixed models showed that b...