Stock-Specific Migrations of Juvenile Coho Salmon Derived from Coded-Wire Tag Recoveries on the Continental Shelf of Western North America
John F. T. Morris, Marc Trudel, Mary E. Thiess, Ruston M. Sweeting, Joseph Fisher, Susan A. Hinton, Emily A. Fergusson, Joseph A. Orsi, Edward V. Farley, Jr., and David W. Welch
Abstract.—A conceptual model of juvenile coho salmon Oncorhynchus kisutch migration from Oregon, Washington, the Columbia–Snake River system, British Columbia, and southeast Alaska was derived using coded-wire-tag data from juvenile salmon surveys conducted between 1995 and 2004. Over this 10-year period, 914 coded-wire-tagged (CWT) juvenile coho salmon were recovered. In general, the migratory behavior of juvenile coho salmon observed in this study was consistent with previous studies showing that juvenile salmon generally undertake a northward migration and utilize the continental shelf as a migration highway. However, this study also revealed that both regional and specific river stocks of coho salmon from all parts of the North American coast are composed of fast components that take a rapid and direct migration in the summer to as far west as Kodiak Island, Alaska and slow components that migrate over a relatively short distance from their natal rivers and reside over winter on the shelf. The Columbia–Snake River system, coastal Oregon, and coastal Washington had the highest proportion of fast CWT migrants among regions. Furthermore, specific stocks within the lower Columbia River had the highest proportion of fast CWT migrants both within the Columbia–Snake River watershed and along the entire west coast of North America. Distances migrated along the shelf were positively correlated to size at capture during the summer for almost all regional stocks, indicating that fast-migrating juvenile coho salmon have faster growth rates. The widespread dispersion along the continental shelf as a consequence of a mix of slow and fast migrants and the subsequent offshore migration into different regions of the Gulf of Alaska may have been selected over evolutionary time scales. This strategy would have ensured the long-term survival of individual stocks by spreading the risk of mortality among oceanic regions.