Life History Pattern Diversity, Movements, and Habitat Use of Juvenile Coho Salmon in the Grays River Estuary, Washington State, USA

Sub-yearling, coho salmon (Oncorhynchus kisutch) downstream migrants or “nomads” represent an alternative life history to the “stream-type” freshwater-rearing yearling smolt life history that was traditionally thought to dominate coho salmon populations.  Recent studies suggest that nomad coho salmon may contribute substantially to an adult population.  We investigated the downstream migration patterns, estuarine habitat use, and life history composition of juvenile coho salmon in the Grays River watershed and estuary, a tidal freshwater sub-estuary to the Columbia River estuary, in the Pacific Northwest USA.  Over the duration of two cohorts (2008-2009), we used mark-recapture, diet and growth, and scale analyses to document the composition, outmigration chronology and estuarine ecology of juvenile coho salmon moving from their freshwater spawning habitats, through their transition to rearing habitats in the tidal freshwater estuary, and to their ultimate emigration to the broader Columbia River.  We also used scale pattern analysis to compare observed juvenile life history composition with that of returning adults. Catch data shows bimodal migrations of subyearling nomad coho salmon into the estuary in the spring and summer and suggests that early springtime migrant nomads rear in the estuary for an extended period in of time.  Nomads used a variety of estuarine habitats, but were most frequently caught within off-channel habitats including tidal freshwater forested wetlands and emergent wetlands. Scale pattern analysis showed that nomads have significantly higher growth rates than their subyearling counterparts who remain and rear in freshwater upriver habitat.  We observed similar life history patterns among years, suggesting that these patterns are relatively stable, even if the benefits of any particular pattern may vary.  Scale pattern analysis of adult scales verified evidence of a variety of juvenile life history patterns.   The presence of available and productive wetland habitat in the lower reaches of the river may allow for expression of these nomad life history patterns.  In addition, a diversity of upriver habitat, including beaver ponds, may also allow for expression of a variety of life history patterns.   This diversity could provide the Grays River coho salmon population with greater resilience to changing environmental conditions and greater sustainability into the future.  These results point to the importance of the preservation, conservation, and restoration of a diverse network of interconnected habitat throughout a watershed.