Size Dependent Spawning Site Selection in Yakima River Spring Chinook Salmon

Variation in mean size and age at maturity in salmonids resulting from environmental and anthropogenic influences may have important consequences for the reproductive success of individuals and conservation of natural populations. The mechanisms by which size and age at maturity influences salmon population viability are poorly understood. As part of a multi-year analysis of the patterns of homing and spawning by natural origin and hatchery-reared spring Chinook salmon in the upper Yakima River, Washington, we examined whether size and age at maturity influenced the spatial distribution of spawning adults. The final choice of spawning location within a watershed involves a complex tradeoff between homing to the natal site, spawning habitat selection, and mate choice. From 2002-2010, we comprehensively surveyed the spawning area of Yakima River spring Chinook salmon and GPS mapped and recorded origin, gender, age and length (POH) data for every carcass recovered (n=15,414). Spawning location for both natural origin and hatchery fish within the watershed was strongly influenced by size. Specifically, consistent differences in adult size were observed in two tributaries and the mainstem Yakima River. For hatchery fish, the patterns of size-specific spawning could not be attributed to juvenile rearing history (e.g., imprinting) because all fish shared a common juvenile rearing environment. These results suggest that size differences between individuals influences spawning site selection by spring Chinook salmon (although other explanations will be discussed). If hatchery rearing practices result in variation in size or age of maturity between natural origin and hatchery fish, one unexpected consequence might be spatial segregation of hatchery and wild fish spawning in the wild.