Homeward Bound: In-River Movements of Adult Hatchery- and Natural-Origin Chinook Salmon in the Elk River, Oregon

Abstract Many adult hatchery-origin Pacific salmon Oncorhynchus spp. return to their natal river but do not enter the hatchery, instead spawning in the river, where they can have detrimental genetic and ecological effects on naturally reproducing wild populations. This phenomenon is especially well documented in Elk River (Oregon) Chinook Salmon O. tshawytscha based on previous analyses of coded wire tag recoveries. Here, we used radio tags to compare the movements of natural- and hatchery-origin Chinook Salmon (n = 11 and 15, respectively) to determine whether their behavior could explain the in-river straying patterns of hatchery salmon. Most of the tagged hatchery-origin (87%) and natural-origin (73%) adults initially moved upriver to the vicinity of the Elk River Hatchery. Most natural-origin adults then moved downstream of the hatchery entrance to spawn, but the movements of hatchery-origin adults were different. Hatchery-origin males and females had the highest and lowest variation in total distance moved, respectively. Our results are consistent with the hypothesis that while most hatchery-origin salmon return to the vicinity of the hatchery, some fail to enter, perhaps because the hatchery’s odors were not perceived as distinct from those of the river or (in the case of hatchery-origin males) because they stopped short of the hatchery after encountering spawning conspecifics. Thus, spawning distributions may not always reflect in-river movements of Chinook Salmon, and we encourage fisheries managers to continue investigating methods that reduce potentially harmful interactions between hatchery- and natural-origin fish.