Marine and atmospheric heatwaves are intensifying worldwide, exposing intertidal communities to increasing thermal stress. Yet the severity and ecological consequences of such stress are difficult to predict due to high spatial and temporal variability of temperatures in intertidal sediment. Using high-frequency temperature records, we quantified thermal exposure across emersion gradients at two contrasting intertidal flats within the Oosterschelde tidal inlet (the Netherlands). Threshold-based and cumulative metrics captured exposure to above optimal, sublethal and lethal temperature zones relevant for benthic infauna. Sediment temperatures frequently exceeded thresholds, but patterns differed sharply between locations. Under the same atmospheric conditions, a modest (similar to 2 degrees C) difference in daily mean water temperature produced markedly different heating patterns, amplifying sublethal stress in the lower intertidal at the location with warmer water. Here, elevated water temperatures prolonged infauna exposure during heatwaves by similar to 6 h day(-1) of sublethal and similar to 15 h day(-1) of above-optimal conditions. Lethal exposure, however, was confined to the upper intertidal and showed little variation between locations. The results demonstrate that thermal stress in intertidal sediments are determined from interactions between emersion duration and local environmental drivers, particularly water temperature, and that the areas most affected by cumulative sublethal stress may not align with areas experiencing lethal conditions. As coastal waters continue to warm, these drivers are likely to extend sublethal heatstress into lower intertidal zones, reshaping community composition and ecosystem function. Our findings highlight the need to account for both lethal and cumulative sublethal exposure, as well as location-specific conditions, to better predict vulnerability under climate change.

Reef-building bivalves such as mussels and oysters are ecosystem engineers that strongly modify sediment properties by attenuating water flow and accumulating fine biodeposits. Yet, despite advances in understanding reef-sediment dynamics, it remains partially resolved how reefs and their environment interact to control sediment characteristics within and beyond reef boundaries. We explored how the extent and spatial scale of the sediment modification depend on the prevailing environmental conditions (e.g., bathymetry, orbital and flow velocity) and reef characteristics (e.g. size, density) in the intertidal Dutch Wadden Sea. We sampled sediment silt and total organic matter content at three sediment depths (0-1 cm, 9-11 cm, 25-28 cm) on twelve reefs from their centre up to 400m beyond their borders, and used linear models to assess relationships. We found that within reefs, organic matter and silt content were higher compared to the surroundings, and that denser reefs had a greater impact, with only a marginal influence of environmental conditions. Effects on sediment properties beyond reef boundaries were found to extend up to 100m at the surface, but were detected up to 200m in deeper layers, with total organic matter and silt content decreasing nonlinearly with distance. These effects were more pronounced around larger reefs and correlated negatively to hydrodynamic conditions that increase sediment resuspension, such as orbital and flow velocity. Overall, our findings highlight that intertidal bivalve reefs modify their environment well beyond their physical borders. However, the scale of the extended-engineering footprint is context dependent as it varies with reef size and density, and current and wave action.

Global climate change increasingly exposes intertidal ecosystems to extreme weather events, reshaping biodiversity patterns and altering community assembly dynamics. While many studies have examined the effects of individual stressors, few have explored the interactive impacts of multiple, co-occurring stressors on the comparative survival and establishment potential of native versus introduced species. The current study examines how simultaneous fluctuations in temperature and salinity affect the survival and burrowing behavior of two ecologically and economically important intertidal bivalves: the native Cerastoderma edule and the introduced Ruditapes philippinarum. Using a mesocosm experiment across four salinity levels and five temperature regimes, we found that R. philippinarum exhibited significantly lower mortality than C. edule at temperatures exceeding 35 degrees C, particularly under low-salinity conditions (10 and 20 PSU). In contrast, burrowing behavior did not differ significantly between the two species, suggesting that their ecological functions may persist despite differential survival. Our results show that compound stressors widen the mortality gap between native and introduced species more than single stressors alone. Integrating these findings with conceptual modeling and field data, we propose the concept of 'disturbance-driven establishing windows', critical periods during which compound extreme events preferentially facilitate the establishment of introduced species with broader environmental tolerance. These insights enhance our understanding of species turnover and community dynamics in intertidal ecosystems facing intensifying climate extremes.

Understanding environmental predictor variables of species occurrence may contribute to conservation management. In this paper we test the use of a spatial binomial model, estimated with the combined INLA-SPDE method, to relate the probability of occurrence of the mussel Mytilus edulis in the subtidal part of the western Dutch Wadden Sea in the period 1992–2022, to a range of environmental and human-impact related variables. Salinity and orbital velocity appeared to be the most important driving variables, and maximum probability of occurrence was predicted at intermediate salinity levels around 22 PSU and low orbital velocity. Mussel occurrence was also lower in the shipping lanes that are regularly dredged. One of the hypotheses is that at lower salinity physiological stress occurs, but that at higher salinity levels predation limits the occurrence. The spatial structure of the unexplained variation is described by a Gaussian field, but it remained unclear what the type of underlying explanatory mechanisms could have been that some areas had much lower probability of occurrence than expected on the basis of environmental conditions. Further understanding of these observed patterns, for example by including temporal dynamics or experimentally testing settlement limitations, could benefit future decision making for conservation management.

The availability of mussel seed is a critical aspect in mussel farming. Since 2009, the Dutch mussel sector has been transitioning from wild seed fishery to suspended seed collectors (Seed Mussel Collectors, or SMCs). Collector systems using either ropes or nets as settlement substrate are placed in Oosterschelde Bay, the Wadden Sea, and the North Sea annually. We analyzed detailed harvest data from 2010 until 2022, to investigate the efficiency of different systems, identify differences between years and areas, and assess how production can be optimized. Additionally, numerical density, biomass, and shell lengths of mussels from 0.375mm shell length were recorded on SMC ropes at one SMC location during a full growth season to evaluate biomass-density relations and assess the process of self-thinning on the ropes. Total harvest of SMC mussel seed increased over the period 2010–2022, from 8.0 ×106kg to 21.0 ×106kg fresh weight. Harvest per unit substrate was remarkably stable over the years across sites, with a lower mean in Oosterschelde Bay (~2.56kgm-1) than in the Wadden Sea (~3.28kgm-1). Ropes were found to provide a greater yield per unit area than nets, but nets are less labor-intensive to use. Occurrence of density-dependent growth on the ropes was indicated by the allometric relation between mussel biomass and mussel density. A positive relation between density and growth rate suggested that competition increased with growth rate. In the growth data covering a full SMC season, we first observed a rapid numerical increase as newly settled mussels continued to grow into the measured size range. This was followed by a period of rapid numerical reduction and increasing biomass, indicating self-thinning. Finally numerical reduction stabilized and biomass increase accelerated coupled with comparatively slower shell length increase. The self-thinning occurred between approximately 2.3mm and 11.6mm mean shell length. Our analysis of 12 years of production data shows that SMC seed is a robust and annually more reliable alternative to wild capture fishery as a seed provisioning resource for mussel culture. Production per unit substrate does not appear to be easily amenable to further improvement. Production per unit area showed no indication of overstocking on the scale of the SMC plots, suggesting that production gains could be made by increasing substrate density.

Bivalves play a key role in coastal ecosystems by supporting food web, modifying habitats, and their economic value for fisheries. Many bivalve species are under pressure, showing large variations in population sizes and distributions, with climate change and human activities considered as important drivers. The Dutch North Sea hosts high densities of bivalve species, dominated by the cut trough shell Spisula subtruncata, with strong interannual variations and a patchy distribution. To explore the causes of this variation, data of an extensive long-term spatial benthic monitoring program (1995-2021) was analysed using a Bayesian spatio-temporal hurdle model. We considered indicators related to human activities, biological processes, climate change and habitat preference as explanatory variables for the observed long-term temporal and spatial variations. Results revealed that medium sediment grain size was key determinant of S. subtruncata occurrence and density. Increasing sea water temperatures during winter and the post-settlement phase positively affected annual population densities, while strong north-westerly winds led to lower densities. These climate change related factors had an overall positive effect on this species in the region. Human activities like shellfish dredging and sand nourishment had no measurable impact. However, shrimp and flatfish beam trawling overlapped with S. subtruncata occurrence and were negatively related to densities, suggesting higher beam trawling intensity in these areas may negatively impacts densities. Overall, the effects were stronger at medium to finer sediments where the highest densities occurred, indicating a strong habitat-dependent effect. Despite identifying multiple drivers, unexplained annual variation suggests other not included factors like predation pressure, also play a role. More detailed studies on the combined effects of climate change-driven environmental stressors and human activities are needed to fully understand the population dynamics. This knowledge is essential for developing more adequate fisheries and coastal management strategies to sustain biodiversity.

Storm-induced erosion events may alter the diversity of tidal flat communities by selecting species that can better tolerate such disturbances. Introduced and invasive species are highly adaptable to a wide range of abiotic characteristics, and this adaptability may make them better able to withstand erosion events. With a novel flume method, we compared the ability of two bivalve species to resist storm-induced erosion: Cerastoderma edule, a native species to the Scheldt estuary in the Netherlands, and Ruditapes philippinarum, an introduced species that is successful in the Netherlands and worldwide. We used three sediment erosion rates to simulate storms of increasing severity. At the 10.6 and 15.9 cm h(-1) sediment erosion rates, all R. philippinarum were surfaced, whereas only half C. edule were surfaced. However, after being brought to the sediment surface, C. edule were more readily transported by currents and waves than R. philippinarum due to differences in their shell shape. We concluded that the two bivalve species had different strategies to avoid mortality by severe storm erosion: C. edule avoided being surfaced and R. philippinarum avoided being transported. In this case, it appears that extreme storms favor the specific adaptations of a native species over the broad adaptability of a non-indigenous one. Indeed, C. edule may be more likely to survive moderately extreme storms than R. philippinarum, though the most extreme storms would be equally devastating to both species.

Samenvatting 1 Inleiding 1.1 Achtergrond Mosselconvenant 1.2 Achtergrond VISWAD 1.3 De gesloten gebieden 1.4 Monitoring van de gesloten gebieden 1.4.1 Onderzoeksvraag en globale opzet 1.4.2Bestaande monitoring 1.4.3

To understand the consequences of anthropogenic and environmental changes for wildlife populations, it is important to study how individuals differ in their sensitivity to environmental change and whether this depends on individual characteristics. An individual's reproductive performance may provide an integrative, unidimensional proxy of an individual's characteristics. In this study, we define an individual's characteristics by three such reproductive states, namely successful, failed and non-breeders in the previous year. We used a 16-year dataset of individually marked breeding Eurasian Oystercatchers Haematopus ostralegus to examine the interannual fluctuations in reproductive success and survival among breeding states, and their state-dependent sensitivity to environmental conditions. Environmental conditions included available biomass of the main prey species of breeding Oystercatchers (Ragworm, Baltic Tellin and Lugworm), tidal height, which reflects one of the main causes of nest loss (flooding), and conditions that may impact the energetic requirements during incubation, such as temperature. We also included environmental variables measured in winter, including available biomass of the main winter prey species (Blue Mussel and Common Cockle) along with factors that may affect food availability and energetic requirements for homeostasis, namely bivalve weight loss, windchill, winter severity and precipitation. Breeding birds that were successful the previous year had higher survival and were more likely to remain successful, in comparison with failed or non-breeders. The effects of environmental conditions acted in the same direction on reproductive success but had opposite effects on survival among the three breeding states, especially for windchill and Blue Mussel biomass. The contrasting state-dependent effects of the environment on survival thus averaged out when examining consequences for lifetime reproductive nest success (LRnS); instead, LRnS was largely influenced by environmental conditions acting upon reproduction. Our study indicates that an individual's previous breeding state provides an integrative measure of heterogeneity in individuals' sensitivity of reproduction and survival to environmental change. Incorporating previous breeding state as a source of individual heterogeneity in population modelling may improve predictions of future population dynamics in a rapidly changing world.

Aan Wageningen Marine Research is gevraagd om, in samenspraak met de projectgroep, een advies op te stellen voor een eenduidig protocol waarmee jaarlijks (gedurende de transitieperiode) een bevissingsvolgorde kan worden bepaald voor mosselbanken in de laatst gesloten gebieden. Dit protocol dient rekening te houden met, en invulling te geven aan, de criteria die zijn meegegeven in het addendum bij het ‘mosselconvenant’ [notitie 2 van het addendum bij de kamerbrief “toekomst mosselconvenant Waddenzee” (Schouten, 2020)]. Een belangrijk onderdeel hiervan is het in kaarten weergeven van potentiële visgebieden met de volgorde waarin deze kunnen worden opengesteld binnen de overeengekomen randvoorwaarden. Daarnaast dient het een plan van aanpak te bevatten voor de jaarlijkse uitwerking, inclusief specificatie en planning van de benodigde informatievoorziening. Dit is van belang omdat besluitvorming over de selectie plaats moet vinden in de beschikbare tijd tussen de bestandsschatting en opstellen van het visplan door de PO Mosselcultuur.

Soft-bottom beds of the blue mussel (Mytilus edulis) are of high ecological importance in intertidal and subtidal habitats. They create habitat, shelter and food for other organisms, and play a dominant role in energy flow and nutrient cycling. Intertidal beds are much better studied than subtidal beds. Though it is often assumed that subtidal mussel beds resemble their intertidal counterparts, major differences in factors driving recruitment, growth and survival can be expected. The aim of our study was to estimate survival chances of the mussel beds in the subtidal parts of the Dutch Wadden Sea in relation to environmental variables, and to compare the results with those obtained previously from the intertidal areas. We used data from a long-term annual survey, resulting in a survival analysis of 365 individual subtidal mussel beds. The average life span of subtidal mussel beds, once they have survived their first winter, was estimated at 2.3 years. This is lower than what was found in the intertidal (3.4 years) in a previous study. However, the survival of subtidal mussel beds in less-saline areas is comparable to survival of intertidal mussel beds, whereas survival of subtidal mussel beds in more-saline areas is significantly lower. The strong, significant effect of salinity is most likely an indication of an effect from starfish predation, since starfish (Asterias rubens) are virtually absent from the intertidal and their abundance is strongly reduced in the subtidal at lower salinities. Furthermore, the survival of individual beds is positively correlated with their size. This may be a direct effect of the bed size itself, or also an indirect effect of environmental factors that can affect the size of newly settled beds. A secondary aim was to compare two methods, based on different types of field data: 1) empirical point data and 2) estimated bed contours based on the point data and additional sources of information. Both methods give similar results. Advantages and disadvantages of both methods are discussed.

Achtergrond Ieder voorjaar schat WMR de omvang van het kokkelbestand in de Nederlandse kustwateren. De bestandsopname in het voorjaar wordt geëxtrapoleerd naar het geschatte bestand op 1 september, welke wordt gebruikt als basis voor het visserijquotum. Hierbij wordt gerekend met gemiddelde waarden voor groei en sterfte gedurende de zomer. In de hete zomers van 2018, 2019 en 2020 was de sterfte uitzonderlijk hoog waardoor het bestand op 1 september is overschat. Hierdoor is ook het visserijquotum te hoog ingeschat, waardoor de minimum benodigde hoeveelheid kokkelvlees om scholeksters de winter door te helpen in gevaar zou kunnen komen. Daarom is door de vergunningverlener op basis van herbemonsteringen het visserijquotum naar beneden bijgesteld. Dergelijke extreme zomers kunnen we vaker verwachten als gevolg van klimaatverandering. Kennisvraag Het Ministerie van LNV heeft aan Wageningen Marine Research gevraagd om een advies te geven over mogelijke aanpassingen in de survey methodiek. Dit project is uitgevoerd binnen het KB-WOT Visserijprogramma (BAS-code KB-36-002-012). Onderliggende kennisvragen zijn: 1. Welke sterfte kan gezien worden als uitzonderlijk? Wanneer is de gemiddelde sterfte van 28% tussen 1 mei en 1 september niet meer representatief voor de werkelijk opgetreden zomersterfte? 2. Kan de inventarisatie verschoven worden naar het najaar, of korter voor aanvang van de visserij op 1 september, zodat het visserijquotum niet gebaseerd hoeft te worden op een extrapolatie?3. Hoe kan de werkelijk opgetreden sterfte het beste bepaald worden als basis voor een herziene bestandsschatting en bijstelling van het visserij-quotum? 4. Welke aanpassingen aan de surveymethodiek worden geadviseerd, alles overwegende? Uitzonderlijke sterfte Op basis van het 99% betrouwbaarheidsinterval rond de gemiddelde kokkelsterfte berekend uit historische datasets kan gesteld worden dat een zomersterfte hoger dan 50% zeker als uitzonderlijkgezien kan worden. De sterftepercentages waargenomen in de Waddenzee en Oosterschelde in de periode 2018-2020 waren duidelijk hoger dan 50%, en hoger dan in de voorgaande periode. In de Westerschelde worden vaker sterftepercentages hoger dan 50% waargenomen. Verschuiven van de survey Het verschuiven van de inventarisatie van kokkels naar het najaar is alleen zinvol als verschoven wordt naar een periode waarin geen sterfte meer op kan treden als gevolg van een hittegolf, dus vanaf 1oktober. Dit houdt in dat ook de aanvang van de visserij verschuift. Wanneer de huidige termijnen voor de berekening van het bestand en de vergunningverlening worden gehanteerd, kan aangevangen worden met de visserij per 1 februari. Geadviseerd wordt om dit met een maand op te schuiven naar 1 maart, na de winterperiode waarin nog kans is op verhoogde sterfte als gevolg van vorst en/of stormen.Verschuiving van de survey naar het najaar heeft nadelige gevolgen voor de praktische uitvoering van de surveys, en voor de statistische zeggingskracht van de tijdreeks. Voor uitvoering van het gehele pakket aan schelpdiersurveys in de Waddenzee zal meer tijd nodig zijn, naar schatting 2-3 weken. Bemonsteren in het najaar heeft als consequentie dat er schelpdierbroed in de monsters aanwezig is. In jaren met een omvangrijke broedval zorgt dit voor aanzienlijk meer werk, en de statistische zeggingskracht van de tijdreeks wordt lager. Ook betekent verschuiving naar het najaar een onderbreking van de tijdreeks die inmiddels 33 jaar bestrijkt, waardoor eveneens het statistische onderscheidingsvermogen afneemt. Hierdoor kunnen effecten van veranderingen in visserij- en/of natuurbeleid, en van grootschalige menselijke ingrepen, minder goed tot niet aangetoond worden. Omdat de huidige tijdreeksen voor tal van lopende studies worden gebruikt, wordt geadviseerd om een besluit tot onderbreking er van niet lichtvaardig te nemen, en als het even kan de survey dus niet te verschuiven. Rapportnummer CVO 22.021 5 van 43 Methodiek sterfte metingen Als de survey niet verschoven wordt zal in jaren met extreme sterfte een schatting gemaakt moeten worden van de omvang van de sterfte. Een bemonstering om de zomersterfte te herijken leent zich daarhet beste voor. Deze kan adaptief ingezet worden in de gebieden waar visserij plaatsvindt, zodra een extreme sterfte in het veld geconstateerd wordt.Advies Geadviseerd wordt om de voorjaarssurvey te handhaven, en dus niet te verplaatsen naar het najaar. Bij het optreden van een extreme zomersterfte zou dan in het najaar een sterfte-herijkingsbemonstering uitgevoerd moeten worden. Om meer inzicht te krijgen in het optreden van extreme sterfte in relatie tot zomerse temperaturen wordt aanbevolen om de komende jaren standaard ieder najaar een dergelijke bemonstering uit te voeren in de Waddenzee en Oosterschelde, en in de Westerschelde alleen indien daar een visserij gepland is. Gebaseerd op een power analyse en de relatie tussen het aantal monsterpunten en het betrouwbaarheidsinterval wordt geadviseerd om in de Waddenzee in het najaar minstens 300 punten te bemonsteren en in de Oosterschelde 225 punten. In de Westerschelde zou het volledige grid van 250 punten bemonsterd moeten worden in het najaar.

Shorebird populations, especially those feeding on shellfish, have strongly declined in recent decades and identifying the drivers of these declines is important for conservation. Changing food stocks are thought to be a key driver of these declines and may also explain why trends have not been uniform across Europe's largest estuary. We therefore investigated how winter population trends of Eurasian oystercatchers (Haematopus ostralegus) were linked to food availability in the Dutch Wadden Sea. Our analysis incorporated two spatial scales, a smaller scale focused on roost counting areas and food available to birds in these areas and a larger spatial scale of tidal basins. A novelty in our study is that we quantify the connectivity between roosting and foraging areas, identified from GPS tracking data. This allowed us to estimate food available to roosting birds and thus how food availability may explain local population trends. At the smaller spatial scale of roost counting areas, there was no clear relationship between available food and the number of roosting oystercatchers, indicating that other factors may drive population fluctuations at finer spatial scales. At the scale of tidal basins, however, there was a significant relationship between population trends and available food, especially cockle Cerastoderma edule,. Mortality and recruitment alone could not account for the large fluctuations in bird counts, suggesting that the site choice of wintering migratory oystercatchers may primarily drive these large fluctuations. Furthermore, the relationship between oystercatcher abundance and benthic food stocks, suggests winter shorebird counts could act as ecological indicators of ecosystem health, informing about the winter status of food stocks at a spatial scale of tidal basins.

Ieder voorjaar inventariseert Wageningen Marine Research in opdracht van het ministerie van Landbouw, Natuur en Voedselkwaliteit het kokkelbestand in de Waddenzee, Oosterschelde en Westerschelde. Op basis van de uitkomsten van de voorjaarssurvey wordt met behulp van groei- en sterftefactoren een schatting gemaakt van het kokkelbestand dat op 1 september aanwezig zal zijn (najaarsbestand). Op basis van dit najaarsbestand wordt door de vergunningverlener vastgesteld hoeveel kokkels opgevist mogen worden. In de berekening van het najaarsbestand wordt gewerkt met gemiddelde waarden voor groei en sterfte tijdens de zomermaanden. Aangenomen wordt dat in de periode van 1 mei tot 1 september 28% van de kokkels een natuurlijke dood sterft. In de zomer van 2018 en 2019 is echter grootschalige kokkelsterfte opgetreden in de Oosterschelde en Waddenzee. Vermoedelijk had dit te maken met het optreden van hoge temperaturen op de droogvallende platen waar kokkels zich ingraven en overtijen tijdens laagwater. In 2019 is dit project gestart met als doel het verbeteren van de schatting van het najaarsbestand op basis van de voorjaarsbemonstering. In 2020 is het doel aangepast naar het onderzoeken van zomersterfte onder kokkels in relatie tot de droogvalduur en gemeten temperaturen in het sediment. Om de relatie met temperatuur te kunnen onderzoeken, is het project gekoppeld aan een ander project (BO43 NAGW ZWD 2020, deelproject Hittestress) dat WMR uitvoert in opdracht van het Ministerie van Landbouw, Natuur en Voedselkwaliteit en waarbinnen onderzoek is gedaan aan het verloop van temperaturen in de bodem, in relatie tot droogvalduur, de luchttemperatuur, instraling door de zon en sedimenttype. In voorliggend rapport zijn de methoden die in 2019 en 2020 zijn toegepast voor het meten van sterfte in de Oosterschelde vastgelegd en de gemeten sterftecijfers gerapporteerd. Daarnaast is een literatuuronderzoek uitgevoerd naar wat bekend is over kokkelsterfte in relatie tot hoge temperaturen. In 2019 was de sterfte gemiddeld genomen voor de 1-jarige kokkels in de gehele Oosterschelde 5%, voor de 2-jarige 23% en voor de meerjarige 87%. In 2020 is kokkelsterfte over de zomer uitzonderlijk hoog gebleken. Met een sterfte van 95% onder de 1-jarige kokkels en 81% onder de 2-jarige kokkels, lag de sterfte onder 1- en 2-jarigen hoger dan voor meerjarige kokkels (sterfte van 68%) en hoger dan de gemiddelde zomersterfte in de Oosterschelde (28%, over een heel jaar is de gemiddelde sterfte ca. 60%). De locatie Roggenplaat vertoonde de laagste sterftecijfers, terwijl locaties Oesterdam en Dortsman beide hoge sterftecijfers vertoonden, ondanks hun geografisch contrasterende ligging, sedimentsamenstelling en daarmee samenhangende temperatuurverloop. Mogelijk heeft dit te maken met het aantal uur dat de temperatuur ’s nachts onder de 23°C kwam tijdens de hittegolf. Dit aantal uur ligt veel hoger op de Roggenplaat (202 uur) dan op de andere locaties (83 uur voor Dortsman en 134 uur voor Oesterdam). Met de huidige klimaatverandering zullen luchttemperaturen stijgen en worden extremere temperaturen en meer hittegolven verwacht. Als gevolg zal de watertemperatuur ook stijgen en mogelijk ook al eerder in de zomer zijn afkoelende werking verliezen. Daardoor zouden kokkels langer blootgesteld zijn aan voor hen suboptimale temperaturen, waarbij ze verminderd actief zijn en wat mogelijk hun fitness vermindert. Dit effect zal dan het grootst worden geacht in het oostelijke deel van de Oosterschelde, waar het water langer verblijft, eerder en hoger opwarmt en meer slibrijke sedimenten te vinden zijn. Met de huidige kennis is echter niet te voorspellen hoe de kokkelpopulatie in de Oosterschelde zal veranderen als gevolg van klimaatverandering.

mosselzaadvisserij en -kweek aan regels en afspraken ge bonden.De effecten van mosselkweek en de beheer maatregelen op de natuur worden al vele jaren onderzocht.In deze publicatie vatten we de belangrijkste bevindingen uit de onderzoeksrapporten en wetenschappelijke publicaties samen.

On 1-2 January 2019, an incident happened north of the Dutch Wadden Sea, in which the MSC Zoe lost part of her cargo. Among the cargo lost were plastic pellets of two different types: relatively large (several mm) high-density poly-ethylene pellets and small (0.5 mm) polystyrene granules. As part of the agreement between the Dutch government and MSC, the present study investigated whether significant damage is caused or likely to be caused to the EU protected habitats and species located in Natura 2000 sites, due to the incident with the MSC Zoe. The focus is on the detection of possible effects of ingestion of microplastics at population level. The study is based on many thousands of samples of the macrobenthos in the Wadden Sea (collected in the SIBES monitoring of NIOZ and the WOT shellfish survey of WMR) and the coastal North Sea (collected in the WOT shellfish survey of WMR), on bird counts in the Wadden Sea and North Sea (sea ducks, collected by Rijkswaterstaat and WMR) and bird counts of the Wadden Sea (collected by SOVON). All data span at least one decade. The species selected for analysis are flagship species that are protected through European directives and stand as a model for other species with similar trophic or habitat requirements, or with similar exposure to plastic particles. Only abundant species that can be analyzed with full statistical power have been selected. The best possible estimation of the spatial distribution of plastics lost by MSC Zoe did not permit to sharply delineate affected areas and use these as fixed factors in the analysis. An exploratory approach had to be used instead. The analysis looked for deviant spatial patterns between the year 2019 and the preceding (for birds also following) year and used expert judgement to evaluate whether any such patterns could be caused by plastic pollution from the MSC Zoe incident. The available data were modelled with a statistical model that took account of (1) physical cofactors (bottom shear stress due to currents and waves, grain size of the sediment, salinity) through non-linear GAM smoothers, (2) the type of statistical distribution of the data, including the number of zero observations, and (3) the spatial and temporal autocorrelations in the data. A Bayesian approach using R-INLA has been used. Possible effects of the Zoe incident were identified by comparing the unexplained (by physical factors) spatially structured variability in the populations between the samplings before and after the incident. All populations studied are characterized by relatively high variability in both space and time. Within the bird species, variability was very high for sea ducks (e.g. the Eider duck and the Scoter) and some waders, but more limited in shelducks and bar-tailed godwits. For none of the bird species, any important difference between the observations in early 2019 with those of 2018 or 2020 could be detected. Thus, no evidence for an effect of the MSC Zoe was found. Within the benthic species, recruitment processes that are highly variable from year to year and that dominate the population dynamics of the shellfish, were the prime cause of variability in the data set. Six species were each examined for abundance and biomass, yielding in total 12 analyses. Of these 12 analyses, four showed important differences between the spatial pattern in 2019 with that in 2018. In two cases, the cockle and the razor clam, this was clearly linked to a very high recruitment in 2018. In two other cases, the mussel and the Baltic tellin, the changes in spatial pattern concerned the appearance and disappearance of small-scale spatial hotspots. The important differences concerned only a few percent of the total number of comparisons. In half of the cases the values increased from 2018 to 2019. In the other half, they decreased. This was interpreted as part of the natural dynamics of these species. The study concludes that no evidence has been found for a negative influence of the MSC Zoe incident on dominant protected flagship species in the Wadden Sea and in the Dutch coastal North Sea.