Yukon River Chinook Run Timing Project, 2021 Post-Season Wrap-Up

The Yukon River Chinook salmon run arrived with close to average timing in 2021: The 50% point at Big Eddy drift was June 18th and the 50% point at Big Eddy set was June 22nd — both just slightly earlier than the pre-season forecast of June 23rd. An error of one to five days is within the historical performance of the pre-season forecast model.

In contrast with the general agreement of the pre-season forecast and observed run timing, early-season catches painted a different picture. By the forecasted 50% point (June 23rd), catches at Big Eddy drift indicated a substantially earlier run than forecast (Figure 1). At this point, we investigated a refit to the in-season catch data and were unable to come up with a reasonable refit considering historical model performance.

Figure 1: A mid-season look at estimated percent catch per unit effort (cyan) against modeled percent catch per unit effort (blue) for Yukon River Chinook salmon in 2021 through June 23rd. The estimated line deviates considerably from the modeled line, suggesting an earlier run than forecast.

We communicated the conclusion of the refitting process to managers and stakeholders using our in-season email list and indicated that we were still confident in the pre-season forecast. We continued daily email updates and catches sustained, indicating the early signal was not in fact an early run (Figure 2).

Figure 2: An end-of-season look at estimated percent catch per unit effort (cyan) against modeled percent catch per unit effort (blue) for Yukon River Chinook salmon in 2021.

This year’s forecast highlights the utility of objective tools for interpreting in-season catch data in the sense that a statistical forecast model fit to decades of catch and environmental data establishes a strong prior with which to compare in-season catch data. This year also highlights the value in performing long-term monitoring as ADF&G has done in the Lower Yukon Test Fishing project and at the sonar site near Pilot Station. Without data from these projects, this project would not be possible.

As always, we thank the Alaska Ocean Observing System for providing project support, including web hosting and data access. We also thank the Alaska Department of Fish & Game for the critical work they do managing the run and providing timely in-season updates every year.