Effects of Supplementation in Upper Yakima River Chinook Salmon

To promote recovery of natural salmonid populations, managers are utilizing hatchery supplementation programs to increase abundance of spawners on the spawning grounds. However, studies have provided evidence that captive breeding can result in domestication, demonstrated by lower fitness of hatchery-origin compared with natural-origin fish. Supplementation programs, therefore, typically use natural-origin broodstock in an effort to minimize long-term negative fitness impacts. Here we evaluated the upper Yakima River spring supplementation program for Chinook Salmon Oncorhynchus tshawytscha, which has broodstock comprised exclusively of unmarked fish presumed to be of natural-origin. Using 5 years of spawner data, we tested for effects of hatchery breeding and rearing on total adult returns and their individual reproductive success when spawning naturally. Our study revealed that supplementation increased overall abundance of fish spawning naturally on the spawning grounds. However, on average, compared with natural-origin spawners, hatchery-origin fish had reduced reproductive success, which also translated to reduced reproductive success in three out of five return years for natural-origin fish that spawned with hatchery-origin fish. As expected, body length and return timing were also significant predictors of reproductive success. However, more generations of data are needed to establish the extent to which reduced reproductive success is passed on to naturally produced progeny.