Research Document - 2011/134

Pre-season run size forecasts for Fraser River Sockeye (Oncorhynchus nerka) and Pink (O. gorbuscha) Salmon in 2011

By S.C.H. Grant and B.L. MacDonald

Abstract

Salmon forecasts remain highly uncertain due to stochastic (random) variability in annual survival rates. Fraser Sockeye survival has been particularly uncertain in recent years due to the systematic declines in productivity exhibited by most stocks, and the extremely variable productivity of the past two brood years (2005 and 2006 brood years corresponding, respectively, to the 2009 and 2010 returns for most Sockeye). To capture inter-annual random (stochastic) variability in Fraser Sockeye survival, forecasts are presented as standardized cumulative probabilities (10%, 25%, 50%, 75%, 90%). For example, at the 25% probability level there is a one in four chance that Sockeye returns will be at or below the forecasted value, given survival is within long-term or recent average historical observed ranges. Alternative assumptions of Sockeye productivity are presented as separate forecasts: ‘Long-Term Average Productivity’ and ‘Recent Productivity (brood years: 1997-2004)’. The ‘Recent Productivity’ scenario is considered most plausible (CSAP Salmon Sub-Committee). The ‘Long-Term Average Productivity’ scenario is considered plausible but less likely. For Fraser Sockeye forecasts, under the assumption of ‘Recent Productivity’, there is a one in ten chance (10% probability) the Sockeye return will be at or below 1.0 million, and a nine in ten chance (90% probability) it will be at or below 12.1 million. The mid-point of this distribution (50% probability) is 3.2 million (there exists a one in two chance the return will be above or below this value assuming recent stock productivity). Under the assumption of ‘Long-Term Average Productivity’, there is a one in ten chance (10% probability) the return will be at or below 1.7 million, and a nine in ten chance (90% probability) it will be at or below 15.1 million. The 2011 forecast has a higher age-5 proportion (35-50% of age-4 + age-5 returns across stocks) than average (~20%), given the generally high brood year escapements for age-5 Sockeye, and the use of average (in the case of ‘Recent Productivity’ forecasts) to above average (for ‘Long-Term Average Productivity’ forecasts) 2010 age-4 productivities in forecasting the age-5 returns for some stocks. For Fraser Pink Salmon forecasts, based on the assumption of ‘Long-Term Average Productivity’, there is a one in ten chance (10% probability) the Pink return will be at or below 9.2 million and a nine in ten chance (90% probability) it will be at or below 37.5 million. The mid-point of this distribution (50% probability level) is 17.5 million. A recent productivity scenario was not produced for Pink Salmon, as they have not exhibited declines in productivity like Fraser Sockeye. The Fraser Pink forecast is highly uncertain because this forecast required extrapolation outside the range of observed data, given the record high fry abundance in 2010.

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