Abundance and Distribution of Precociously Mature Male Spring Chinook Salmon of Hatchery and Natural Origin in the Yakima River

Precocious maturation of artificially propagated male Chinook salmon Oncorhynchus tshawytscha has the potential to alter abundance and distribution of males in freshwater and thereby influence ecological and genetic interactions with other fish in the natural environment. Between 1999 and 2007, the Cle Elum Supplementation and Research Facility has produced and released into the upper Yakima River basin of Washington an annual average of 124,573 males that mature precociously. We investigated the abundance and distribution of precociously mature male spring Chinook salmon of hatchery and natural (wild) origin during the spawning season (4-7 months after hatchery release) in the Yakima River. We Counted the number of precocious males on the spawning grounds while snorkeling during the peak of spawning and electrofished to determine abundance and distribution of precocious males away from redds. We also collected Chinook salmon to determine percent precocity and size and age distributions. The number of precocious hatchery males on redds was less than 0.05% of the total number of fish released, and they were significantly less abundant on redds than precocious wild males. Between 1999 and 2007, the mean annual abundance of precocious age-1 hatchery males observed on the spawning grounds was 22 fish (range, 0-78). Precocious hatchery and wild males were both found throughout the spawning range during the spawning season, but significant differences in distribution between origins were detected. Precocious hatchery males were proportionately more abundant in the most downstream sampling reach and less abundant in a tributary with no hatchery facilities, In addition, most precocious hatchery males were found downstream of spawning areas during the spawning season. It appears that many precocious hatchery males migrate downstream from release and fail to migrate back to the spawning grounds. or they die within the Yakima River before spawning. Thus, precocious mate Chinook salmon resulting from hatchery production in the Yakima River do not contribute favorably to harvest and may pose ecological risks to other taxa, but most of these fish have it low probability of contributing genes to future generations.