Abundance, Survival, and Life History Strategies of Juvenile Chinook Salmon in the Skagit River, Washington

To identify potential actions for conserving Chinook Salmon Oncorhynchus tshawytscha in the Skagit River, Washington, we used a 16-year time series of streamflow data, adult escapement, and out-migrant abundance to understand how out-migrant abundance and life history diversity were related to spawner abundance and incubation flows. Three freshwater rearing strategies were distinguished based on body size at out-migration: fry (≤45 mm FL), subyearling parr (46–100 mm FL), and yearling smolts (>100 mm FL). Density-independent and density-dependent processes were hypothesized to influence survival in sequence, with density-independent mechanisms operating during incubation and density-dependent mechanisms operating between emergence and out-migration. A model selection process compared spawner–recruit models with and without different incubation flow metrics. Density-independent models that included measures of flow duration and magnitude were strongly supported (Akaike's information criterion [AIC] difference ≤ 3). Sustained flow events of moderate magnitude (1-year recurrence interval) were an equivalent if not better predictor of freshwater survival than short-duration flow events of high magnitude (peak flows). A second model selection process evaluated density dependence of each life history type. The composition of out-migrants (fry, subyearling parr, and yearling smolts) was a density-dependent function of spawner abundance. Fry out-migrant abundance was density independent, and subyearling parr out-migrant abundance was density dependent. Neither model was supported for yearling smolts. At least one out-migrant life history, subyearling parr, should benefit from continued restoration of freshwater habitats in the Skagit River system. Factors contributing to the yearling smolt life history will benefit from additional study.

Received October 12, 2014; accepted February 3, 2015