Science Response 2014/040
Pre-season run size forecasts for Fraser River Sockeye (Oncorhynchus nerka) salmon in 2014
Context
Pre-season run size forecasts of returning Fraser River adult Sockeye salmon in 2014 were requested by Fisheries and Oceans Canada (DFO) Fisheries Management. Forecasts are used for pre-season planning purposes and for in-season management. They are most useful early in the migration of each stock-group before in-season test fisheries provide adjustments to the run size estimates. Forecasts are produced by DFO as stipulated in Annex IV, Chapter 4 of the Pacific Salmon Treaty and are presented by stock and run timing group.
To capture inter-annual random (stochastic) uncertainty in Fraser Sockeye returns (largely attributed to variations in stock survival), forecasts are presented as standardized cumulative probabilities (10%, 25%, 50%, 75%, and 90%), rather than as single deterministic point estimates. The 50% (median) probability level is the mid-point of the forecast distribution, indicating a one in two chance that Fraser Sockeye returns will be at or below these values, assuming that stock survival is similar to past observations. A stock’s forecast probability distribution represents the range of survivals it has exhibited historically. Forecast values at the lower probability levels represent lower stock survivals and, conversely, values at the higher probability levels represent higher stock survivals. Since not all stocks will exhibit similar survival, the forecast distribution for total Fraser River sockeye salmon will likely over-estimate total returns, particularly at the high probability levels. It is therefore more appropriate to reference individual stock forecast distributions, versus the total Fraser Sockeye forecast.
To capture the structural uncertainty in the forecasts, which refers to uncertainty in the model forms (i.e. Ricker, Power, Larkin, etc.) selected to describe the stock-recruitment relationships, alternative forecasts using different model forms are presented for each stock.
The 2014 forecasts will be particularly uncertain since the exceptional brood year escapements observed in 2010 for particular stocks (Scotch, Seymour, Harrison, Late Shuswap, and Portage stocks) require the extrapolation of forecast models beyond their observed stock-recruitment data range. However, juvenile (fry and smolt) data for various key stocks in the 2010 brood year (Shuswap and Chilko) provide evidence for density-dependent compensation due to the record high spawner abundances. The juvenile data, therefore, provide support for model forms that predict overcompensation at high spawner abundances.
This Science Response resulted from a DFO Canadian Science Advisory Secretariat (CSAS) meeting of contributors (January 2014) on ‘pre-season abundance forecasts for Fraser River Sockeye Salmon returns in 2014. The 2014 forecast relies on past CSAS processes and publications (Cass et al. 2006, DFO 2006, 2007, 2009, 2011, 2012, Grant et al. 2010, Grant and MacDonald 2012, MacDonald and Grant 2012).
To support the 2014 Fraser Sockeye forecast an additional CSAS Science Response meeting occurred January 2014 to summarize data on fish condition and/or survival from the 2010 spawners and their offspring.
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