Fish ladder installation across a historic barrier asymmetrically increased conspecific introgressive hybridization between wild winter and summer run steelhead salmon in the Siletz River, Oregon.

Managing intraspecific hybridization is crucial for maintaining a balance between inbreeding and outbreeding depression in winter and summer run steelhead (Oncorhynchus mykiss). While spatial distance is a common factor in maintaining reproductive isolation between these two ecotypes, physical barriers may also prevent hybridization, particularly in short river systems. To determine the effect of barriers and their removal on hybridization, we studied winter and summer steelhead populations in the Siletz River of northern coastal Oregon, which were historically separated by a physical barrier that was removed in the 1950s. We observed a large degree of admixture in the summer run population but little in the summer hatchery or wild winter populations, the former of which was established shortly after the removal of the barrier. This suggests that the high level of admixture in the wild summer run may be due to the removal of the barrier. We also found reduced genetic diversity in the wild summer run and in both hatchery populations. This highlights the need to balance inbreeding and outbreeding depression in hybridizing subpopulations.