This study aims at describing the spatial distribution – and its temporal variations - of discarding intensity (i.e. expected weight of discards for a standard trawl haul) for the 6 main species discarded by the Dutch beam trawl fisheries. For each species, the spatial distribution (quarterly maps for the period 2013 to 2017) is estimated using statistical models that take spatial and temporal correlation into account, which also allowed to test for the effect of a number of factors related to geography, environment, fishing practices and operational aspects on discarding. The data used to fit those models came from the observer trips and self-sampling program conducted at Wageningen Marine Research and from discards sampling trips conducted by the fishing industry. As by-product, the models provide descriptors of the temporal and spatial scales at which the discards of a given species are structured. The distribution of the expected discards per haul for dab was highly variable from quarter to quarter, with generally high discarding intensity in front of the southern coast of the Netherlands in quarter 1, a discarding intensity which is high on the German bight and low in front of the Dutch coast in quarter 3, and variable distributions for quarter 2 and 4. For plaice, the distribution was more stable, with high values consistently observed in the south of the area (between the south of the Netherlands and England), with occasional hot spots on the German bight. For sole, discards were not observed on the north-western part of the area, and a hotspot of sole discarding was found consistently in front of the southern coast of the Netherlands, occasionally expanding towards England or to the northern coast of the Netherlands. Discarding of turbot first occurred with a low intensity along the coast from Belgium to Germany. After the fourth quarter of 2015, high discarding started to occur, first limited to the small area in the southern North Sea, but progressively expanding to a larger area in the southern and central part of the North Sea, while discarding intensity remained low in the northern part of the area and in front of England. The distribution discarding intensity for whiting was highly variably, characterised by hotspots suddenly appearing for most years in the fourth quarter, and disappear in the following first quarter. Discarding of rays occurred mainly in the western part of the area, especially in front of southern England, with an increasing level since the fourth quarter of 2016. The distributions observed and their variability were further discussed in the light the available information on the distribution and migration of the species and on the management measures potentially influencing discarding.

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.