Breeding Success of Hatchery and Wild Coho Salmon (Oncorhynchus Kisutch) in Competition

The divergence of hatchery fish in traits important for reproductive success has raised concerns about their ability to rehabilitate wild populations, and the threat that their inevitable straying poses to biological diversity through introgression. We therefore undertook a study of the breeding competition and success of sea-ranched hatchery fish placed in direct competition with wild fish. Experiments using wild and hatchery coho salmo (Oncorhynchus kisutch) were conducted within a controlled stream channel, allowing selective manipulation of breeding competition and density. Hatchery fish, particularly males, were competitively inferior to wild fish, being less aggressive and more submissive. As a consequence, hatchery males were denied access to ovipositing females; they partook in fewer spawnings, held more distal positions in spawning hierarchies, and attained only an estimated 62% of the breeding success of wild males. By contrast, competition did not appear to inhibit hatchery females as overtly as males. Hatchery and wild females exhibited similar levels of aggressive behavior, however hatchery females did suffer greater delays in the onset of breeding, failed to spawn larger proportions of their eggs, and lost more eggs to nest destruction by other females. They averaged an estimated 82% of the breeding success of wild females. There was thus a sex bias in the breeding disadvantage of hatchery fish, with males suffering more than females. Furthermore, the breeding disadvantage was density dependent with the relative success of hatchery to wild fish declining with increasing density. These results imply that hatchery fish have restricted abilities to rehabilitate wild populations, and may pose ecological and genetic threats to the conservation of wild populations.