Benchmark Workshop for Flatfish stocks in the North Sea and Celtic Sea (WKFLATNSCS)

The WKFlatNSCS benchmark was convened to evaluate the appropriateness of data and methodsto determine stock status for four sole stocks; North Sea (Sol.27.4), eastern English Channel(Sol.27.7d), Bristol Channel, Celtic Sea (Sol.27.7fg) and southwest of Ireland (Sol.27.7h–k); andone turbot stock, Skagerrak and Kattegat (tur.27.3a).For sole in the North Sea a new index of stock abundance was derived, combining data fromBelgium, Germany and The Netherlands (two survey series using different vessels). An updatedcategory 1 assessment was agreed, continuing to use the Aarts and Poos model previously employed.Reference points were calculated using Eqsim and the forecast settings agreed.For sole in the eastern English Channel, the main objective of the benchmark was to resolve anissue with the plus group in the French landings and commercial landings per unit of effort data.The catch data were revised several times between the data evaluation workshop and the benchmarkmeeting, and at the benchmark meeting, it became clear that there were issues that couldnot be addressed at that time concerning:1. whether the methodology used to estimate ages for length classes where no samples forageing had been taken was appropriate; and,2. whether the calculations of effort being used to raise the sampled discards was beingcalculated appropriately and consistently between the sampled fleet and the total fleet.As a result it was not possible to evaluate whether the catch-at-age data were appropriate for theassessment, or to evaluate the performance of an assessment. A further process following thebenchmark meeting will be set up to complete this work.For Sole in Bristol Channel, Celtic Sea, maturity assumptions and average stock weights-at-agewere revised. An updated category 1 assessment was developed, reducing the reliance on commercialtuning series, and moving to a statistical catch-at-age model (SAM). Reference pointswere calculated using Eqsim and the forecast settings agreed.For Sole in the southwest of Ireland, no appropriate method for evaluating the stock status andtrends was found, due to the sampling only covering a small part of the total fishery, which isnot considered to be representative of the whole area. The Workshop agreed to use category 5 toprovide advice for this stock.For Turbot in Skagerrak and Kattegat, a synthesis of work on stock boundaries within Division3a was presented, indicating that turbot in this area may be part of two stocks, the North Sea andBaltic Sea. A combined index from five surveys (BTS, BITS, IBTS and two Danish national surveys)was used as a tuning index for a SPiCT biomass dynamics model to determine stock status.The Workshop identified the need for future work:• to provide a basis for catch advice for sole in 7d this year;• to collect more sample data from 7h and to identify whether sole in 7h are connected tothose in 7e or 7fg by genetics and movement for a future benchmark;• to investigate whether the management boundaries for turbot in 3a are appropriate basedon current understanding of stock boundaries for a future benchmark.