Diversity in freshwater life history in spring and summer Chinook Salmon from the Fraser River, Canada

Objective We describe juvenile migration tactics for spring/summer-run populations of Chinook Salmon Oncorhynchus tshawytscha that spawn in tributaries of the Fraser River, Canada. Methods We trapped juveniles migrating from tributaries, and sampled juvenile Chinook salmon rearing in the Fraser River. Result In four tributaries, we found two dominant life history types within each population: a natal-stream-rearing variant in which juveniles spend a year in their natal stream before migrating to the sea as yearling smolts; and a fry migrant variant in which juveniles disperse downstream from spawning areas to the Fraser River main stem soon after emergence. In a fifth tributary, where flows are regulated, juveniles migrated as parr later in the spring. Juveniles colonized the Fraser River main stem in spring and were distributed throughout the accessible length of the main stem. Their relative density in nearshore habitats was similar to that observed in other rivers. Migrants spend up to a year in the main stem before smolting as yearlings. Conclusion The extensive year-round use of the Fraser River main stem appears to be unique among Chinook Salmon-producing watersheds, likely due to favorable environmental conditions in the main stem during the spring and summer months. The diversity of habitats used by these populations is an important consideration for the design of conservation programs that are intended to maintain or improve freshwater productivity.