Partial Migration and Diel Movement Patterns in Puget Sound Coho Salmon

Abstract

AbstractPartial migration, a term referring to populations in which only a fraction of the individuals migrate, is a widespread phenomenon among fishes. However, it is not always clear whether there are only two alternatives (migration or residency) or a continuum of movement patterns. For example, Coho Salmon Oncorhynchus kisutch are anadromous, and most individuals rear over the continental shelf or in offshore waters of the North Pacific Ocean; however, some Coho Salmon (known as residents) spend all or part of their marine lives within Puget Sound. The movements of residents are poorly documented, and it is unclear whether they ever leave Puget Sound and move to the coast of Washington and to what extent they move within Puget Sound. Accordingly, the goal of this study was to investigate the patterns of movement by immature Coho Salmon in Puget Sound at a series of spatial scales. We tagged 45 resident Coho Salmon in the central basin of Puget Sound with acoustic transmitters and detected their movements with fixed receivers in the Salish Sea. Seven individuals were detected as departing Puget Sound through the Strait of Juan de Fuca, but these fish did not differ in body size, origin (wild or hatchery), or tagging date from fish that remained in Puget Sound. The fish remaining as residents seldom moved between the marine basins of Puget Sound. Within the central basin, deeper/offshore sites had higher frequencies of detection and other indices of site use. Fish were more often present and moved more often at shallow sites close to shore during the night, whereas they were more often present and moved more at deep, offshore sites during the day. We suggest that rather than being a discrete behavior, residence in Puget Sound by Coho Salmon is part of a continuum of migratory behavior patterns.