Contribution of Diverse Juvenile Life Histories to the Return of Chinook Salmon in an Oregon Coastal Basin, 2004-06

We examined variations in the juvenile life history of fall-spawning Chinook salmon, Oncorhynchus tshawytscha, for evidence of change in occurrence and duration of estuarine residency and migration patterns following the removal of dikes from 145 ha of former salt marsh in the Salmon River estuary (Oregon).  The absence of fry migrants in the estuary during April through June in 1975-77—a period that precedes restoration of any of the diked marshes—and the extensive use of marsh habitats by fry and fingerlings April–July, 2000-02 indicate that wetland restoration has increased estuarine rearing opportunities and breadth of residence for juvenile Chinook salmon. We used otolith daily increment counts and strontium:calcium ratios of juvenile Chinook to compare date and size at emigration from freshwater, growth, and estuarine residence with that obtained from mark-recapture field studies. More than 75% of fish captured at ocean emigration had entered the saline portion of the estuary during the summer, and two-thirds of those spent more than a month in the saline estuary before capture. Field sampling established that early migrating fry (<50 mm) were abundant in the upper saline estuary marsh habitats, but few were found at the mouth of the estuary. Nonetheless, otolith reconstruction revealed that 17% of the ocean emigrants from Salmon River in 2000-02 had entered the estuary as fry. We further quantified the contribution of each juvenile outmigrant type in 2000-02 to the adult returns to Salmon River in 2004-06. All juvenile life history types, including fry migrants, were represented among returning spawners. The field and otolith results suggest that wetland recovery has expanded life history variation of Chinook in the Salmon River population by allowing greater expression of estuarine-resident behaviors.  The contributions of different segments of the adult population and diverse life history strategies of the juvenile Chinook may increase the resilience of the population to disturbance and natural climate cycles.