Contribution of Subyearling Migrant Coho Salmon (Oncorhynchus kisutch) Life Histories to Spawning Populations on the Southern Oregon Coast

Early coho salmon life history has been described as consisting of a single strategy: freshwater residence for the first year of life followed by yearling smolt migration to sea during second spring of life.  However, during the last decade evidence began to accumulate suggesting that subyearling migrant coho salmon could represent an alternative life history. The goal of our study was to document the return of subyearling migrant coho salmon as spawners to confirm this possibility.  We used PIT tags to identify individual fish as they returned to two streams on the Oregon coast. In the 2010 run the proportion of spawners that had migrated to the estuary as subyearlings was 2.5 times higher than that of fish that smolted as yearlings.  Fork length (FL) at estuary entrance was reconstructed based on changes in Sr:Ca and Br:Ca in spawner otoliths.  Four main life histories were identified based on this: migrating fry (< 60 mm FL), migrating parr (60 - 70 mm FL), migrating parr that returned to freshwater before leaving as yearlings (“nomads”), and yearling smolts (> 70 mm FL).  Overall, 30 to 42% of the spawning run consisted of fish with evidence of estuarine residence during their first year.