Research Document - 2012/011
Pre-season run size forecasts for Fraser River Sockeye salmon (Oncorhynchus nerka) in 2012
By B.L. MacDonald and S.C.H. Grant
Abstract
Salmon forecasts remain highly uncertain, in large part due to the wide variability in annual salmon survival rates. For Fraser Sockeye in particular, quantitative or qualitative leading indicators of survival explored to date have not reduced forecast uncertainty and remain an active area of research. In the absence of leading survival indicators, Fraser Sockeye forecasts have been particularly uncertain in recent years, due to the systematic declines in productivity exhibited by most stocks, which culminated in the lowest productivity on record in the 2005 brood year (2009 four year old and 2010 five year old returns). Subsequently, productivity appears to have improved in the 2006 (2010 four year old returns) and 2007 (2011 four year old returns) brood years. A single forecast scenario is presented in 2012. Forecasts were produced using either recent productivity or long-term productivity (full time series) models, which were selected on a stock-specific basis based on their ability to predict true returns over the full stock-recruitment time series. Jackknife, rather than retrospective analysis, was used to generate a time series of forecasts for the model evaluation process, and a revised set of criteria and procedures were used in the model selection process. An additional sensitivity analysis was conducted to examine model performance over the most recent period of data (brood years 1997-2004), which for most stocks exhibited lower productivity. To capture inter-annual random (stochastic) variability in Fraser Sockeye survival, forecasts are presented as standardized cumulative probabilities (10%, 25%, 50%, 75%, and 90%). The 2012 forecast indicates a one in ten chance (10% probability) the total Fraser Sockeye return will be at or below 750,000, and a nine in ten chance (90% probability) it will be at or below 6.6 million, assuming survival is similar to past observations. The mid-point of this distribution (50% probability) is 2.1 million (there exists a one in two chance the return will be above or below this value). Summer Run stocks, particularly Chilko, Late Stuart and Stellako, contribute 67% to the total return forecast, whereas Early Stuart (5%), Early Summer (17%) and Late Run stocks (11%) each contribute considerably less. The forecasted 2012 Fraser Sockeye return falls largely (up to a three in four chance, based on past observations) below the cycle average (3.8 million). This below average return forecast is attributed to the well below average 2008 brood year escapements of the Early Summer and Late Run stocks. If Fraser Sockeye productivity returns to the low trend of recent decades, the 2012 return has the potential to be amongst the lowest observed on this cycle. Conversely, there is a small chance (one out of four) that returns will be above the cycle average if stock productivities fall at the high end of past observations. Due to low escapements in 2008 (which produce four year olds in 2012) relative to 2007 (which produce five year olds in 2012), the forecasted proportion of total four year old returns (~75%) is below average (82% average four year old proportion for all stocks combined, excluding Harrison). Four year old proportions ranged from 10% to 98%, depending on the stock.
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