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    This report presents the ongoing work and initial testing for the operating models (OM) utilized for the 2023 management strategy evaluation (MSE) analysis of four commercially important fish stocks (pikeperch, perch, bream, roach) within the IJssel/Markermeer complex. In recent years, these stocks have witnessed a concerning decline prompting the Ministry of Agriculture, Nature, and Food Quality to take management decisions based on the best available scientific information. With the goal of long term sustainability while allowing the highest possible commercial catches, an MSE analysis of the IJssel/Markermeer stocks was conducted in 2019/2020. Following the results of the previous MSE, an update of the OM’s is essential to optimize the outcomes of both past and current management objectives and to signal the need adjustments when needed.--These operating models provide virtual representations of stock population dynamics enabling simulation testing for various management procedures (MPs). Through this approach, it becomes possible to predict (with certain levels of uncertainty) future stock populations and conduct risk assessments of current management policies. The OM’s use data from scientific surveys, commercial catches and estimated catches from birds with the purpose of achieving adequate fish stocks for commercial benefit while ensuring sufficient nourishment for bird populations. Commercial and bird catches were modelled in the OM’s as separate ‘fleets’ with differing levels of selectivity at age for both catch sources for the different stocks. Age structured models were constructed with spawning stock biomass used to predict annual changes in recruitment.--OM’s were conditioned using the available data inputs as well as three sets of priors (initial biomass, level of stock depletion, the steepness of the stock recruitment relationship) to set up a pool of possible trajectories for the stocks to best explain historical populations while providing the basis for future projections. These stock trajectories underwent several rounds of selection to choose the output which best represented historical catches and biomass trends.--The pikeperch OM showed a considerable level of uncertainty regarding the current stock status. The recent increase in commercial catches was only explained by a considerable rise in fishing mortality, though relatively high recruitment in recent years has been suggested from the open water survey. Several alternative OM’s were also given consideration for this stock. The bream OM relied on a smoothened biomass index from the open-water survey to project the OM over the historical period, which, after selection of stock trajectories, showed a stock which has declined significantly since the 1990’s. Fishing pressure for this stock, was seen as currently well above Fmsy, although lower than the highest period between 2007-2010. Initial future projections for this stock under different levels of exploitation were also explored.--The perch OM suggests a gradual decrease in spawning stock biomass (SSB) in recent years, though these results change depending on two alternative scenarios regarding high or low bird catches. Fishing pressure for this OM is seen as currently above Fmsy and higher than historical levels. Potential future scenarios under different fishing pressures were also investigated for this stock. The roach OM, like with bream, uses a smoothened biomass index from the open-water survey. The OM for this stock, however, is currently ongoing with no projection of historical SSB levels or fishing pressure.--This analysis faced challenges due to limited information and conflicts between catch and survey data which posed difficulties for models to reconcile. These conflicts left the roach OM, in particular, unresolved. To address these issues, plans are underway for an external review and more rigorous investigation of the data sources and implementation of the OM’s scheduled for early 2023. Additionally robustness tests for management procedures are also planned to ensure the reliability and effectiveness of the models.

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    The exemptions under the landing obligation (LO) for undersized plaice and sole in the 80 mm beam trawl fishery expire in 2023. The European Commission has indicated that new scientific substantiation is needed to maintain the exemption. Here we evaluate the conservation strategies of adjusting the minimum conservation reference size ("MCRS”, currently 27 cm for plaice and 24 cm for sole) to 25 cm for both species and increasing the mesh size from 80 to 90 mm. This study analyzes data on annual landings and discard estimates from 2019 to 2021 and uses computational simulations to assess the potential effects of changing MCRS and mesh sizes on discards, landings, catches and the economic consequences. Our results show that a 25 cm MCRS for plaice could decrease discarding by 13 to 18% while increasing landings by 28 to 34%. A MCRS increase to 25 cm for sole was predicted to increase discards by 10 to 24% while reducing landings by 5 to 9%. Increasing the mesh size to 90 mm reduced total catches for both species (16% and 25% for plaice and sole, respectively) and largely negated changes caused by MCRS. Both species exhibited decreased discards with the larger mesh size simulations (plaice: 10 – 48%; sole: 32 – 64%) with sole landings being reduced (8 – 25%). Increasing the mesh size versus a shift in MCRS may have a more prominent effect on the discards of sole and plaice, however, potential changes to landings must be considered as well when evaluating potential economic and environmental consequences. Economic analyses show that the change in MCRS for both sole and plaice in combination does have a small effect on total income for the main groups of vessels targeting these two species. The decrease in sole landings is almost completely compensated by the increased landings from plaice resulting in a total reduction of landings value of 2 and 0% for euro cutters and large beam trawlers respectively. The reduction in sole landings resulting from an increase in mesh size to 90 mm had an adverse effect on the landings value for both types of vessels and their economic performance. Total value of landings decreased by 9% for both euro cutters and large beam trawlers, causing lower or negative profitability. We assume that landings of other species would also be affected by the mesh size increase, though the analysis suggests a limited impact (5% reduction). It is evident that lower sole landings would continue to affect revenue in the ensuing years after increasing the mesh size. However, considering the degree of uncertainty surrounding the future costs and earnings of large beam trawlers and euro cutters, the economic consequences from reduced landings and the possibilities of the fishing fleet to cope with such changes remains unclear.

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    This report presents the work that was carried out in 2022 to develop a new set of candidate management strategies for the pikeperch, perch, bream and roach fisheries in the IJsselmeer – Markermeer complex. The management procedures proposed here are based on stock indicators that can be derived from the two main sources of data available for these stocks: 1) changes in abundance obtained from the scientific survey, and 2) landings quantities and composition from the commercial fisheries within the complex. The status indicators derived from these data are used in harvest control rules, set to propose a total allowable catch for the following year. These new developments build upon the experience from the previous Management Strategy Evaluation analysis (Tien et al. 2020a&b), and extends the set of options available. This report provides a brief demonstration of the application of the updated management tools on the pikeperch and bream operating models.