Finalised Assessment of the South African anchovy resource using data from 1984 – 2011: results at the posterior mode

The operating model (OM) for the South African anchovy resource has been updated from that used to develop OMP-08 given five more years of data, a revised time series of commercial catch and November survey proportion-at-age 1 estimates provided by a new approach. A Beverton Holt stock recruitment relationship is used, marginally supported by the AICc model selection criterion over a Ricker stock recruitment relationship. Time-invariant natural mortality is assumed at 1.2year-1 for both juvenile and adult natural mortality; an increase from that assumed for the OM from which OMP-08 was developed, with the change made because of a better fit to the data and avoidance of the questionable implication that the recruit survey detects a greater proportion of the recruits than the November survey detects of the adult biomass. There has been a decrease in recruitment residual standard deviation and in recruitment autocorrelation for this updated OM compared to the values used in previous OMs. The impact of this on the appropriate choices of a risk definition and threshold for the new OMP to be developed needs to be considered. The resource abundance has dropped below the historic (1984-2010) average, with a model-estimated spawner biomass of 1.2 million tons in November 2011, following 2 years of below average recruitment. Only four out of the past 13 years have produced below average recruitment. The harvest proportion over the past 11 years has not exceeded 0.13