Partial Migration of Puget Sound Coho Salmon (Oncorhynchus kisutch) Individual and Population Level Patterns

Abstract

Partial migration, the behavior pattern in which a portion of a population migrates while others do not, is a widespread phenomenon with ecological and evolutionary consequences. Most Coho Salmon (<italic>Oncorhynchus kisutch</italic>) from streams in the Puget Sound, Washington basin rear over the continental shelf or offshore waters of the North Pacific Ocean after leaving fresh water, but some rear in the semi-estuarine waters of Puget Sound and are termed residents.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 what factors might influence fish to adopt a resident migration pattern. To understand this migration pattern at the population level, we used coded-wire tag data to evaluate the effect of several factors on the tendency of Coho Salmon to remain resident in Puget Sound rather than migrating outside, and on catch location within Puget Sound. We found that location of origin, day of release, and year of release most strongly affected residency, with fish released later and from south Puget Sound the most likely to remain resident. These factors together indicate that environmental variation plays a strong role in resident migration pattern. Additionally, fish remaining resident were more likely to be recovered in the basin they were released from than in neighboring basins.To understand this migration pattern at the individual level, we investigated the patterns of movement by resident Coho Salmon in Puget Sound at a series of spatial scales using acoustic telemetry. Some residents were detected departing Puget Sound, though they rarely moved between the different basins of Puget Sound. Additionally there was strong evidence of movement to deep, offshore environments during day, and shallow, close to shore environments at night. Rather than a discrete behavior, we suggest that residence in Puget Sound by Coho Salmon is part of a continuum of migratory behavior patterns.