Managing Uncertainty: Forecasting Ocean Abundance of Klamath River Fall-Run Chinook Salmon (Oncorhynchus tshawytscha)
In 2011, when assessing the environmental impacts of allowing a minimal fishery during years of lower Chinook salmon abundance, the National Marine Fisheries Service noted that Pacific salmon preseason forecasts of ocean abundance, although variable, were unbiased over the long-term, with underestimates offsetting overestimates (PFMC 2011). Recent error in preseason forecasts of a critical Pacific salmon stock, the Klamath River Fall Chinook (KRFC), suggests that they are no longer unbiased. This project reviews the accuracy of forecasts of KRFC ocean abundance between 1985 and 2021 and arrives at four principal findings. First, overforecasts of ocean abundance have increased in frequency and magnitude. Second, KRFC salmon are maturing earlier in their life cycle. Third, increased maturation rates in ages 2 and 3 fish are predictive of increased error rates in ages 3 and 4 fish, respectively. Fourth, by using more recent brood years, we were able to achieve more accurate forecasts. These findings can inform potential modifications to the KRFC salmon forecasting model. They also highlight that salmon management approaches must be able to adapt to future changes in maturation timing.