Science Advisory Report 2010/031

Pre-season run size forecasts for Fraser River Sockeye salmon in 2010

Summary

• Most Fraser River Sockeye stocks have exhibited declining trends in productivity beginning as early as 1960. Exceptions to this trend include Late Shuswap (a stock that has not exhibited long-term systematic trends in productivity), which is expected to be the largest component of the 2010 total return. The 2009 return of adult Sockeye salmon was at record low productivity for a number of important stocks, including Quesnel and Chilko.
• Salmon forecasts remain highly uncertain and vary depending on the assumptions underlying the chosen forecast models.
• Uncertainty that is attributed to stochastic (random) variability in annual survival rates is communicated in the 2010 forecast paper through a series of forecasted values that correspond to standardized cumulative probabilities (10%, 25%, 50%, 75%, 90%). For example, there would be a one in four chance at the 25% probability level that the actual number of returning Sockeye will be at or below the forecasted value given the assumptions about future survival. This is different from previous years when the 25% probability level forecast indicated a one in four chance that the actual number of returning Sockeye would meet or exceed the forecasted value.
• Uncertainty that results from different assumptions about future survival (productivity), is assessed through the evaluation of three alternative scenarios that characterize the potential productivity of spawners in the 2006 brood year (returning as age 4 recruits in 2010):  “Long-Term Average Productivity”, “Recent Productivity (brood years: 1997-2003)”, and “Productivity Equivalent to the 2005 Brood Year” (2009 return year).
• The forecast approach that assumes that recent productivity will persist through to 2010 (“Recent Productivity” forecast), is recommended to be the most appropriate for pre-season planning. Based on this assumption, there is a one in four chance (25% probability) that the return of Fraser River Sockeye Salmon will be at or below 7.0 million and a three in four chance (75% probability) that it will be at or below 18.3 million. The total return forecast is dominated by Late Shuswap Sockeye (63% of the total forecast at the 50% probability level) and this stock has not exhibited long-term systematic decreases in productivity similar to most other stocks.
• “Recent Productivity” forecasts for each of the four run timing groups from the 25% and 75% probability levels are as follows: Early Stuart Run: 26,000 to 66,000; Early Summer Run: 374,000 to 1,601,000; Summer Run: 1,605,000 to 4,343,000; and Late Run 5,023,000 to 12,305,000. 
• The two other forecast scenarios “Long-Term Average Productivity”, and “Productivity Equivalent to the 2005 Brood Year” (2009 return year), are considered to be within the range of possible outcomes but are given less weight in the current assessment.  

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