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 methods to 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); and one turbot stock, Skagerrak and Kattegat (tur.27.3a). For sole in the North Sea a new index of stock abundance was derived, combining data from Belgium, Germany and The Netherlands (two survey series using different vessels). An updated category 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 an issue 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 benchmark meeting, and at the benchmark meeting, it became clear that there were issues that could not be addressed at that time concerning: 1. whether the methodology used to estimate ages for length classes where no samples for ageing had been taken was appropriate; and, 2. whether the calculations of effort being used to raise the sampled discards was being calculated 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 the assessment, or to evaluate the performance of an assessment. A further process following the benchmark meeting will be set up to complete this work. For Sole in Bristol Channel, Celtic Sea, maturity assumptions and average stock weights-at-age were revised. An updated category 1 assessment was developed, reducing the reliance on commercial tuning series, and moving to a statistical catch-at-age model (SAM). Reference points were calculated using Eqsim and the forecast settings agreed. For Sole in the southwest of Ireland, no appropriate method for evaluating the stock status and trends was found, due to the sampling only covering a small part of the total fishery, which is not considered to be representative of the whole area. The Workshop agreed to use category 5 to provide advice for this stock. For Turbot in Skagerrak and Kattegat, a synthesis of work on stock boundaries within Division 3a was presented, indicating that turbot in this area may be part of two stocks, the North Sea and Baltic 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 to those in 7e or 7fg by genetics and movement for a future benchmark; • to investigate whether the management boundaries for turbot in 3a are appropriate based on current understanding of stock boundaries for a future benchmark