Data from: Eliminating variation in age-at-spawning leads to genetic divergence within a single coho salmon population

Most coho salmon (Oncorhynchus kisutch) in Washington State spawn at three years of age, creating the potential for three temporal populations or “broodlines” at each spawning site. This is generally prevented by a portion of males in each spawning site that mature and reproduce as two-year olds, resulting in population structure in which the geographic component is stronger than the temporal component. Quilcene Hatchery selected against latest returning coho salmon by excluding all but the earliest returning fish from its broodstock for an unknown number of generations, and restricted gene flow among broodlines by excluding two-year old males for 27 generations. The resulting hatchery population exhibited three distinct broodlines which returned in alternating years: an “early” broodline which arrived one month before the wild fish; a “late” broodline which arrived at the same time as the wild fish; and a “middle” broodline which arrived in between these two. We evaluated temporal and geographic components of population genetic structure in coho salmon from Quilcene Hatchery and several other sites from Puget Sound and the Strait of Juan de Fuca using 10 microsatellite loci. Genetic diversity at Quilcene Hatchery was lowest in the early broodline and highest in the late broodline. Divergence among the three broodlines at Quilcene Hatchery was greater than that observed among broodlines at any other site, and was also greater than divergence observed between any of the sites. This apparent reversal of the relative magnitudes of temporal and geographic components for this species emphasizes the importance of variable age-at-maturity in shaping population genetic structure.