Science Response 2021/038
Pre-season Run Size Forecasts for Fraser River Sockeye (Oncorhynchus nerka) and Pink (Oncorhynchus gorbuscha) Salmon in 2021
Context
Fraser River Sockeye stocks have been experiencing historically low productivity in recent years, making quantitative forecasting challenging. Forecasts for these stocks have been prepared with Bayesian models and presented as probability distributions, in order to capture some of this uncertainty. These distributions generally represent the range of survival the stocks have exhibited historically, although we have used environmental covariates for several stocks, in an attempt to capture recent declines in productivity. Environmental drivers are incorporated into forecast models for 13 out of 19 major Sockeye stocks. In addition, we have employed the use of sibling models for nine stocks; using productivity observed in 2020 returns to inform 2021 returns of older age-classes from the same cohort. For five major stocks, including Chilko, we identified a significant disconnect between the productivity implied by forecasting models, and productivity observed in recent generations, which has declined significantly, relative to earlier generations. For these stocks we use a naïve model that applies average productivity (recruits-per-spawner) from the last two generations to brood year spawners.
The Fraser River Sockeye total return is forecast to be 1.33 million fish, at the median level (80% prediction interval:313,000 - 5,496,000). The forecast is dominated by Summer Run stocks, which are forecast to contribute 1.05 million fish to this forecast. Returns are expected to be dominated by age-4 fish, making up 91% of expected returns (Table 3). Across all run-timing groups, 78% of the forecast return is made up of stocks that spawn above the Big Bar landslide. In addition to the quantitative forecast for 2021, we have compiled supplementary freshwater and marine data to provide some qualitative context to the forecast. These indicators provide mixed signals, with freshwater conditions appearing marginally better for the primary cohort returning in 2021 (except for Quesnel; the stock forecast to contribute the most fish to the 2021 return). Marine conditions have remained very warm, which are generally unfavourable for Fraser Sockeye prey quality. If Fraser Sockeye productivity stays within the range observed in the previous generation (2017-2020 returns), then returns should fall closer to the level seen at the 0.25 posterior quantile (P25), of 624,000 fish.
Fraser Pink Salmon represented a forecasting challenge in 2021, since the juvenile data from 2020, which would have generally been used to run the forecast, was not available due to the COVID-19 pandemic. Therefore, a new set of models were considered using retrospective analysis. The chosen model incorporated the effects of temperature, Fraser discharge during outmigration, and sea-surface salinity on productivity. All three environmental covariates are consistent with lower-than-average productivity, and our chosen forecast model for 2021 is predicting productivity near historically observed lows. The forecast return of Fraser Pink salmon in 2021 is 3.01 million (80% prediction interval: 1,701,000 - 5,375,000). This forecast should be used with caution, since it is based on an escapement time-series that spans several different enumeration methodologies.
This Science Response results from the Regional Science Response Process of December 18, 2020 and January 22, 2021 on the Pre-season abundance forecast for Fraser River Sockeye and Pink Salmon returns in 2021.
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