The use of spatial stream network models to evaluate the effects of rearing environment on wild coho life history to better inform pre-season forecasts

The proposed work will address Southern Panel Priority #3: Coho Biological Studies, with the specific objective of understanding the effect of climate change on salmon life history with benefit to pre-season forecasts. One of the simplest coho forecast methods involves a sibling regression model, which uses the abundance of returning males (i.e., jacks – fish that return to freshwater after spending only ~6 months at sea) as an indicator of adult returns. These models are based on an association between jack marine survival in the first few months at sea and adult marine survival the following year. The models rely on a stable age structure, an assumption that does not always hold. Marine survival of wild coho salmon in coastal Washington is measured at Bingham Creek, the site of a long-term life-cycle monitoring program operated by the Washington Department of Fish and Wildlife. Marine survival estimates inform pre-season forecasts, and are important in the determination of allowable exploitation rates in bilateral fisheries. Estimates of survival come from release and recovery of coded-wire tagged (CWT) wild coho marked as smolts during the outmigration period and recovered as adults from fisheries and a weir trap. Preliminary analysis established an inverse relationship between rearing streamflow rates and jack survival.

The proposed work will expand on these observations and evaluate the effects of rearing temperature on jack rates. The goal is to make predictions about age-at-maturity by utilizing spatial stream network models of stream temperature (“Thermalscapes”) for the Grays Harbor coho population. The models can be used to hindcast and forecast rearing temperature based on variations in air temperature and streamflow, which allows for analyzing historical data in addition to predicting jack rates under future climate scenarios.