Science Advisory Report  2012/074

Pre-season run size forecasts for Fraser River Sockeye and Pink Salmon in 2013

Summary

Fraser River Sockeye

• For Fraser Sockeye, quantitative and qualitative indicators of stock productivity (recruits-per-spawner) explored to-date have not reduced forecast uncertainty and remain an active area of research.
• Fraser Sockeye forecasts have been particularly uncertain in recent years due to the systematic declines in productivity exhibited by most stocks, which culminated in some of the lowest productivities on record in the 2005 brood year (2009 four year old and 2010 five year old returns). Subsequently, total stock productivity has improved.
• For the 2013 forecast, models were selected for each stock based on their relative ability to predict true returns over the full stock-recruitment time series. The performance of each model was compared to a suite of forecast models, which excluded the recent productivity models introduced in 2010.
• A sensitivity analysis was also conducted to compare the 2013 forecasts with forecasts produced using top ranked models evaluated over the low productivity period (brood years 1997 to 2005), which include recent low productivity models introduced in 2010.
• To capture inter-annual random variability in Fraser Sockeye productivity, forecasts are presented as standardized cumulative probabilities (10%, 25%, 50%, 75%, and 90%).
• The 2013 forecast indicates a one in ten chance (10% probability) the total Fraser Sockeye return will be at or below 1,554,000 (lowest observed on this cycle) and a nine in ten chance (90% probability) it will be at or below 15,608,000, assuming productivity is similar to past observations. The mid-point of this distribution (50% probability) is 4,765,000 (there exists a one in two chance the return will be at or below this value). The four year old percentage of the total forecast is 90%, and ranges from 13% to 100%, depending on the stock. The 2013 forecast is larger than the 2012 forecast, attributed to higher escapements in the 2009 versus 2008 brood year.
• Consistent with management changes made in 2012 by the Fraser River Panel, Raft, Harrison, and North Thompson miscellaneous stocks were included with the Summer Run timing group, due to changes in their migration timing.
• Summer Run stocks, particularly Chilko and Quesnel, contribute 78% to the total return forecast, whereas Late Run (12%), Early Summer (5%) and Early Stuart Run stocks (4%) each contribute considerably less. The Harrison 2013 forecast is particularly uncertain, and returns have a higher likelihood of falling outside the forecast distribution.
• The total forecasted 2013 Fraser Sockeye return largely falls (up to a three in four chance, based on past observations) below the cycle average (8,579,000), due to the below average 2009 and 2008 brood year escapements for most stocks. Conversely, there is a one in four chance the return will be above the cycle average, if Fraser Sockeye productivity falls at the high end of past observations. If low productivity conditions resume, returns could be considerably lower, based on a sensitivity analysis forecast that ranges from 523,000 to 5,419,000 at the 10% to 90% probability level.

Fraser River Pink

• For Fraser Pink Salmon, the forecast ranges from 4,794,000 to 17,111,000 fish at the 10% to 90% probability levels. The median (50% probability) forecast of 8,926,000 Pink Salmon is below the long-term (1959-2011) average return (12,580,000). The Fraser Pink forecast originally presented to CSAS was generated by the power model without the sea-surface salinity environmental covariate, due to the unavailability of salinity data at the time of the review. The original forecast was higher than the forecast presented in the current paper, and ranged from 6,881,000 to 27,687,000 returns at the 10% to 90% probability levels, with a median (50% probability) forecast of 14,010,000 returns. Fraser Pink forecasts are highly uncertain given the changes to the return estimation methods through time.

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