Effect of Channel Morphology in Sustaining Population Diversity in Chum Salmon Via Hyporheic Flow

Two wild chum salmon (Oncorhynchus keta) populations, which spawn in September/October and November/December, respectively, and are designated early- and late-run populations, are found in Hokkaido, northern Japan. Timing of the seaward migration of chum salmon fry is generally synchronized with snowmelt runoff, suggesting the existence of a mechanism that offsets the variation in time of spawning of the two populations by timing of seaward migration. This study attempts to clarify how the two populations are sustained sympatrically by focusing on temperature within the redds, hyporheic flow and channel morphology. Redds of the early-run population were dominated by downwelling and were located mainly at the upper end of a gravel bar. Redds of the late-run population, however, were dominated by upwelling and were concentrated at the lower end of a gravel bar. Whereas temperatures within redds of the early-run population dropped to approximately 0°C in winter, those of the late-run population remained stable at 4–6°C during winter. Consequently, the variation in accumulated temperature units within redds between the two populations decreased considerably by spring. These results suggest that channel morphology, which mediates the interaction between surface and sub-surface water, is a significant determinant in maintaining population diversity in chum salmon.