Benchmark workshop on selected flatfish stocks (WKBFLATFISH)

The objective of this benchmark process was to propose and evaluate assessment methods and the data upon which they depend for the four stocks: Anglerfish (anf.27.3a46), and sole in Subarea 4 (sol.27.4), Division 7.a (sol.27.7.a) and Divisions 8.a-b (sol.27.8ab). Anglerfish was previously assessed using data limited methods (category 3), sole in subarea 4 using a statistical catch at age model (AAP) and sole in 7a and 8.a-b both assessed using extended survivor analysis (XSA). Landings, discards, survey data and commercial catch and effort data for all stocks were extensively reviewed and collated during the data workshop. For all stocks, revisions were made to the data preparations and methodology behind the derivation of the stock biomass indices, stock and catch weights, maturity data and natural mortality. All the terms of reference were covered during the data workshop and an agreement was reached on the data for to use for an assessment, projections and reference points of each stock reviewed. This was the first benchmark workshop in thirteen years for sole in 7a and eleven years for sole in 8ab. Considerable changes were made to the input data, assessment model, and consequently reference points for both stocks. For these two stocks, SAM was proposed as assessment method while Stock Synthesis (SS3) was proposed as the assessment method for sole in subarea 4 and anglerfish in subareas 4 and 6, and subdivision 3a. For sole in subarea 4, the model addressed and fixed the retrospective issue that was present in the previous assessment. For all models, the diagnostics, model fit, and retrospective model runs revealed no substantial patterns. Sensitivity runs were conducted to investigate the sensitivity to key model assumptions and the workshop reviewed and agreed on the final model configuration. The workshop further reviewed and agreed on the methods used to calculate reference points and forecasts and on the proposed reference points. For sole in 7a, the workshop encountered some issues when determining the stock recruitment relationships based on the ICES guidelines. Therefore, different scenarios were explored considering MSY reference points and their robustness tested using a MSE framework.