Factors Influencing the Relative Fitness of Hatchery and Wild Spring Chinook Salmon in the Wenatchee River, Washington

Understanding the relative fitness of naturally spawning hatchery fish and the causes of any fitness differences compared to wild fish has become an important issue in the management and conservation of salmonids throughout their range.  We used a DNA-based parentage analysis to measure the relative reproductive success of hatchery- and natural-origin spring Chinook salmon in the natural environment. Size and age had a large influence on male fitness, with larger and older males producing more offspring than smaller or younger individuals.  Size had a significant effect on female fitness, but not as large as the effect of size on male fitness.  For both sexes, run time had a smaller but still significant effect on fitness, with earlier returning fish favored over later returning fish.  Spawning location within the river, and tributaries, had a significant effect on fitness for both males and females. On average, hatchery-origin fish of both sexes produced about half the juvenile progeny per parent when spawning naturally than did natural origin fish despite having a much larger spawning population.  Hatchery fish tended to be younger and return to lower areas of the watershed than wild fish. These factors, along with size and return time, explained some of their lower fitness compared to wild fish.