Distribution and population genetic structure of river- and sea-type sockeye salmon in western North America

Abstract– In contrast to the well-known lake-type sockeye salmon, two additional anadromous life-history types have been recognized within the species: 'river-type' sockeye salmon whose juveniles spend 1 or 2 years in off-channel river habitats prior to migrating to sea, and sea-type sockeye salmon that initially rear in similar river habitats yet migrate to sea as underyearlings. Persistent populations of river-/sea-type sockeye salmon occur in small numbers throughout the species'range in North America but are usually associated with glacier-fed rivers. We found published and unpublished records showing that riverine-spawning sockeye salmon occur in 11 rivers in western Washington, USA, that don't have access to juvenile lake-rearing habitat. Evidence of persistent spawning was strongest for the Nooksack and Skagit rivers in northern Puget Sound. We analyzed allozyme frequency differentiation in 26 laketype and 12 river-/sea-type populations of sockeye salmon in North America, ranging from northern Puget Sound, Washington (including 3 in the Nooksack and Skagit rivers) to northern Southeast Alaska. Across this 2000 km range, river-/sea-type sockeye salmon showed very little genetic differentiation between populations, much less than that displayed by the highly divergent lake-type sockeye salmon. Genetic similarity among river-/sea-type sockeye salmon in this study is likely a result of common ancestry and a high level of historical gene flow among river-/sea-type sockeye salmon populations.