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The marine benthos has been largely studied through the use of response traits that characterise species vulnerability to disturbance. More limited has been the specific use of effect traits that represent other species descriptors and that express ecosystem functions. On the sea floor, the benthos is a key ecosystem-engineering component for which functions can be relevantly derived from effect traits. This study provides a typology of sea floor functions based on an extensive data compilation of effect traits. We classified 812 benthic invertebrate species from the northeast Atlantic by 15 effect traits expressing substratum alteration and habitat creation. Cluster analysis identified 15 species groups that represented various epi- or endobenthic functions. Beyond function−habitat specificity, we show that soft sediment species exhibited broader functional niches in the trait space that increase multi-functionality, and were endowed with rare combinations of traits that expanded the functional extent of the species assemblage. As a consequence, soft sediments can host a higher functional diversity than hard substrata because a wider range of above- and below-substratum activities are possible in soft bottoms. Based on response traits documented for the same species and used to express vulnerability to natural or humaninduced disturbance, we then show that vulnerability within sea floor functions can be considerably variable. This can be a consequence of the independence between the evolutionary nature of response traits and the contingent engineering abilities of benthic species through effect traits. The paper provides theoretical and utilitarian clarifications on this trait dichotomy. Benthic invertebrate · Effect trait · Ecosystem engineering · Ecosystem function · Functional group · Functional diversity · Vulnerability

The populations of native iguanas in the Caribbean Lesser Antilles are threatened by the wide occurrence and spread of non-native iguanas. Until recently, competitive hybridization was not believed to threaten the Saba Green Iguana, a subpopulation of Iguana iguana (Linnaeus, 1758) from the island of Saba. However, the arrival of non-native iguanas has put the native population at risk, leading to a change in the conservation status of the Saba Green Iguana to Critically Endangered, according to guidelines from the International Union for the Conservation of Nature. Here, we generated the complete mitogenome of the Saba Green Iguana using Oxford Nanopore long-read technology. The mitogenome is 16,626 bp long and has 13 protein-coding genes, 22 tRNA genes, 2 rRNA genes, and a control region (1194 bp). Noteworthy, this is only the second published mitogenome for the Iguana iguana species complex, despite the known high intraspecific genetic variation., Sample collection was possible during fieldwork that was financially supported by the Ministry of Agriculture Nature and Food Quality (LNV) as part of their "BO" policy support research program with Wageningen University and Research under the project BO-43-117-006 and project number 4318100346-1.

What makes an experience sustainable? Can polar cruise tourism in the High Arctic ever be sustainable? If so, under what conditions? These questions guide this report as it explores tensions embedded in the concept of ‘sustainable polar cruise tourism’. These matters are of particular importance in Svalbard, with a rapidly developing tourism sector being an important part of the Svalbard economy. The report is based on a participatory workshop held on-board a recent expedition to the Svalbard archipelago, the so-called SEES expedition (the Scientific Expedition Edgeøya Svalbard). Conducted as a combined scientific and touristic expedition, the SEES expedition offers opportunity to explore the meaning(s) of sustainability among environmentally aware visitors representing key actor groups on Svalbard. The purpose of the report is thus to contribute to ongoing conversations on the paradoxes of sustainable polar tourism by sharing experiences and perspectives on what sustainability can mean from a passenger point of view.

The Arctic marine ecosystem is changing fast due to climate change, emphasizing the need for solid ecological baselines and monitoring. The polar cod Boreogadus saida functions as a key species in the Arctic marine food web. We investigated the stomach contents of polar cod from the northern Barents Sea using DNA metabarcoding with the mitochondrial cytochrome c oxidase I gene in parallel with classical visual analysis. Arctic amphipods and krill dominated the diet in both methods. Yet, metabarcoding allowed for the identification of digested and unidentifiable prey and provided higher taxonomic resolution, revealing new and undiscovered prey items of polar cod in the area. Furthermore, molecular results suggest a higher importance of barnacles and fish (presumably eggs and pelagic larvae) in the diet than previously recorded. Parasites and, in 6 cases, other prey items were only visually identified, demonstrating the complementary nature of both approaches. The presence of temperate and boreal prey species such as northern krill and (early life stages of) European flounder and European plaice illustrates the advection of boreal taxa into the polar region or may be indicative of ongoing borealisation in the Barents Sea. We show that a combination of visual analysis and metabarcoding provides complementary and semi-quantitative dietary information and integrative insights to monitor changing marine food webs.

The international Wadden Sea is an important flatfish nursery. Information from the Dutch Wadden Sea indicates that the flatfish nursery function of the area has been affected during the last decades. Increased seawater temperature has affected settling, habitat suitability for and growth performance of the various flatfish species. Settling of plaice, flounder and to a lesser extent sole larvae occurs earlier nowadays. In the 1960s, 0-, I-, II- and III-group plaice were present, but since 2000, II-group has disappeared and densities of I-group have decreased. For juvenile flounder, II-group almost disappeared, and for dab, a decline in densities of all age groups was observed from the 1990s onwards. Summer temperatures exceed the optimum for the cold-water species (plaice, flounder and dab) with increasing frequency, level and duration. Only for 0-group sole, the period with optimal growth conditions has become longer and has resulted in increased growth. Mortality rates in 0-group plaice have increased, coinciding with an increase in water temperatures and an increase in the abundance of predators. The decrease in density of juvenile plaice and dab in the Wadden Sea has not affected recruitment to North Sea stocks, suggesting that other areas have taken over part of the nursery function. The predicted increase in seawater temperature in the next decades will continue to improve the conditions for sole. The temperature tolerance of plaice and dab and to a lesser extent flounder will further reduce their scope for growth and may ultimately result in their disappearance from the Wadden Sea.

Thousands of oil and gas structures have been installed in the world's oceans over the past 70 years to meet the population's reliance on hydrocarbons. Over the last decade, there has been increased concern over how to handle decommissioning of this infrastructure when it reaches the end of its operational life. Complete or partial removal may or may not present the best option when considering potential impacts on the environment, society, technical feasibility, economy, and future asset liability. Re-purposing of offshore structures may also be a valid legal option under international maritime law where robust evidence exists to support this option. Given the complex nature of decommissioning offshore infrastructure, a global horizon scan was undertaken, eliciting input from an interdisciplinary cohort of 35 global experts to develop the top ten priority research needs to further inform decommissioning decisions and advance our understanding of their potential impacts. The highest research priorities included: (1) an assessment of impacts of contaminants and their acceptable environmental limits to reduce potential for ecological harm; (2) defining risk and acceptability thresholds in policy/governance; (3) characterising liability issues of ongoing costs and responsibility; and (4) quantification of impacts to ecosystem services. The remaining top ten priorities included: (5) quantifying ecological connectivity; (6) assessing marine life productivity; (7) determining feasibility of infrastructure re-use; (8) identification of stakeholder views and values; (9) quantification of greenhouse gas emissions; and (10) developing a transdisciplinary decommissioning decision-making process. Addressing these priorities will help inform policy development and governance frameworks to provide industry and stakeholders with a clearer path forward for offshore decommissioning. The principles and framework developed in this paper are equally applicable for informing responsible decommissioning of offshore renewable energy infrastructure, in particular wind turbines, a field that is accelerating rapidly.

Intertidal mudflat systems are shaped by geological processes and an interplay of hydrodynamics, sediment availability and ecological processes. All around the world these systems are affected by relative sea level rise (RSLR), changing climate and by human activities such as sediment nourishments, dredging, hydrological engineering and bottom trawling. These kinds of perturbations cause changes in morphology and sediment composition which may cause shifts in the composition, spatial distribution and productivity of benthic communities. We analyzed the spatial and temporal variability of the sediment grain size of almost 1200 km2 intertidal flats in the Dutch Wadden Sea in the period 2009-2019. The large scale coverage was achieved by yearly grid sampling at more than 4000 stations. Spatial paneldata models were used to analyse changes in median grain size and mud content between years and to estimate the effects of resuspension due to wind and the accumulation of silt during summer. We show that between years the sediments of the intertidal flats changed synchronously throughout the study area and that the flats became coarser during the period 2011-2015 and muddier again between 2015 and 2019. The system wide changes and the absence of clear local deviations leads to the hypothesis that a large scale factor like RSLR, variability in the composition of suspended sediment in the North Sea, changing microphytobenthic productivity, or sediment nourishments in the coastal zone were causally involved in the coarsening of intertidal flats. Our data and analysis provides a base for further scientific enquiry but longer time series on higher temporal resolution of both sediment data and the physical and ecological environment are required. Models simulating the environment may provide further insight into possible development of sediment composition of the intertidal flats of the Wadden Sea.

A global concern for coastal ecosystems is the predicted rise in sea-level for which salt marshes must keep pace by increasing in surface elevation sufficiently. Variables that control this elevation change need to be identified to predict the adaptability of marshes to future sea-level rise. Many European marshes are grazed by livestock and these heavy grazers can biocompact the soil, a process often underestimated in studies assessing the long-term survival of marshes. We measured elevation changes for thirteen years in the field in grazed and non-grazed marshes. With a statistical model the most important factors controlling rates of surface elevation change were identified and provided the input for a mathematical model to study future elevation change of grazed and non-grazed salt marshes up to 2100 under three Sea Level Rise and sediment supply scenarios. We found that trampling by grazing cattle significantly reduced the annual rates of elevation gain from 11.9 mm yr-1 in the non-grazed marsh to 3.6 mm yr-1 in the grazed marsh. Next to biocompaction by livestock, precipitation deficit and extreme drought resulted in extra compaction. Our model results showed that cattle presence had a negative impact on the future adaptability of salt marshes to grow vertically for rising sea levels. Biocompaction reduced the total elevation change by 42% if the current linear SLR does not accelerate. For an accelerating and high SLR to 109 cm +NAP in 2100, biocompaction reduced elevation changes by 12% and the grazed marsh can no longer outcompete the rise in sea level from around 2050 onwards, compared to the non-grazed marsh. The grazed marsh will slowly drown but this will not lead to a significant change in vegetation composition yet. For an extreme SLR to 195 cm +NAP in 2100 the elevation changes in both the grazed and non-grazed marshes cannot keep pace with the rise in sea level and the marsh vegetation is expected to show regression to plants typical for a low marsh. A reduction in sediment supply will aggravate the effects of SLR and may result in highly increasing inundation frequencies and subsequent disappearance of the marsh vegetation.

Where and when animals forage depends on the spatio-temporal distribution and catchability of their prey. In dynamic environments, animals can repeatedly target areas that provide predictable availability of prey or may search for ephemeral conditions of high prey availability. However, how foraging behaviour is initiated in response to static versus dynamic environmental conditions is difficult to study, since both environmental data sources are often lacking. In this study, central-place foraging Sandwich terns were tracked using GPS loggers during foraging. Hidden Markov models showed that the probability of switching between transit and foraging was most strongly affected by the static variable sediment type. Wave period (a dynamic variable related to weather), salinity (a dynamic variable) and water depth (another static variable) affected the transition probability to a lesser extent. Cloud cover, wind speed and current speed were only included in lower ranked models. Air and water temperature were not included in any model. Consistent with the greater importance of static versus dynamic abiotic conditions, consistency between foraging trips of the same individual varied irrespective of tidal, diurnal or seasonal cycles, although trips made close in time within a season were slightly more similar than trips with a larger time gap. We suggest that Sandwich terns target broad areas with coarser sediments, where sandeels (Ammodytidae) are more common, and that weather variables may be related to prey visibility. Our study suggests that even in highly dynamic environments, static environmental variables may more strongly affect foraging behaviour of coastal seabirds than dynamic variables.

Binnen het mosselconvenant is afgesproken de bodemzaadvisserij in de Waddenzee stapsgewijs af te bouwen. Deze afbouw gebeurt in een tempo waarin alternatieve bronnen voor mosselzaad als uitgangsmateriaal voor de kweek kunnen worden ontwikkeld en een rendabele kweek mogelijk blijft. Mosselzaadinvangstallaties (MZIs) zijn daarvoor op dit moment het enige alternatief. Inmiddels zijn drie stappen gezet en is de vierde transitiestap in het najaar van 2022 geformaliseerd. Per stap is afgesproken dat het vangstverlies van mosselzaad gecompenseerd moet worden vanuit de invang met MZIs in de Waddenzee. Voor 2022 geldt hierbij de compensatie die gekoppeld is aan de derde stap en dient 14 miljoen kg (Mkg) mosselzaad vanuit MZIs voor de kweek in de Waddenzee beschikbaar te zijn. Voorliggend technisch rapport behandelt de resultaten van de oogst van mosselzaad in 2022 met de MZIs in de Oosterschelde, Voordelta en Waddenzee met als doel: • Inzicht te geven in de oogstresultaten in relatie tot de afspraken in het mosselconvenant. • Inzicht te geven in de geschiktheid van locaties voor MZIs en de daarbij gebruikte invangsystemen. In 2022 is in de Nederlandse wateren in totaal 21 Mkg (210 duizend mosselton) mosselzaad geoogst van de MZIs. Het merendeel hiervan, 16,9 Mkg, is ingevangen in de Waddenzee, waarvan 15,1 Mkg door transitiebedrijven en 1,8 Mkg door de voormalige experimenteerbedrijven. In de Oosterschelde is in 2022 2,9 miljoen kg en in de Voordelta 1,2 miljoen kg mosselzaad van de MZIs geoogst. Met de productie van 19,2 miljoen kg door transitiebedrijven, waarvan dus 15,1 miljoen kg in de Waddenzee is in 2022 ruim voorzien in de 14 miljoen kg die beschikbaar dient te zijn vanuit MZIs na de derde transitiestap. In 2022 is 74% procent van de vergunde MZI-kavels benut. In de Waddenzee ligt dat percentage (met 79% door de transitiebedrijven en 82% van het areaal van de voormalige experimenteerbedrijven) een stuk hoger dan in de Zeeuwse Delta (met 38% in 2022). De invangefficiëntie (oogst per meter substraat) was in de Waddenzee lager dan in voorgaande jaren, in de Zeeuwse Delta was de gemiddelde oogst per meter de hoogste sinds het begin van de metingen. De lagere invangefficiënte in Waddenzee wordt geweten aan een massale val van mosdiertjes op het substraat. Het hoge aandeel mosdiertjes op en tussen de mosselen heeft volgens de terugmeldingen ook geleid tot erg veel tarra in de oogst.

Large-scale offshore wind energy developments represent a major player in the energy transition but are likely to have (negative or positive) impacts on marine biodiversity. Wind turbine foundations and sour protection often replace soft sediment with hard substrates, creating artificial reefs for sessile dwellers. Offshore wind farm (OWF) furthermore leads to a decrease in (and even a cessation of) bottom trawling, as this activity is prohibited in many OWFs. The long-term cumulative impacts of these changes on marine biodiversity remain largely unknown. This study integrates such impacts into characterization factors for life cycle assessment based on the North Sea and illustrates its application. Our results suggest that there are no net adverse impacts during OWF operation on benthic communities inhabiting the original sand bottom within OWFs. Artificial reefs could lead to a doubling of species richness and a two-order-of-magnitude increase of species abundance. Seabed occupation will also incur in minor biodiversity losses in the soft sediment. Our results were not conclusive concerning the trawling avoidance benefits. The developed characterization factors quantifying biodiversity-related impacts from OWF operation provide a stepping stone toward a better representation of biodiversity in life cycle assessment.

Highlights •A solid knowledgebase and governance process is required to develop policies for sustainable growth of offshore aquaculture in the North Sea. •Environmental goals largely define maximum sustainable production levels. For seaweed culture this is limited to the scale of several hundred km2. •Definition of realistic ambitions is a process rather than a fixed value and greatly benefits from a ‘learning by doing’ approach.

Aquaculture production in extensive shrimp ponds along mangrove coasts may be influenced by many factors such as pond size, pond management, species diversity of mangroves and aquaculture species used, as well as the position and abundance of mangroves in the ponds. A growing number of studies indicate the potential positive effects of mangrove presence on pond productivity. In this study we aimed to assess the influence of mangrove presence on pond production characteristics as observed in the south-western coastal region of Bangladesh where mangroves grow largely naturally in aquaculture ponds. Using structured interviews we quantified a large number of pond production parameters from mangrove-associated shrimp aquaculture in ponds with a wide range of mangrove coverage (0% - >50%) including total costs, revenues and net profits. The 435 ponds for which data were collected were distributed among 61 villages in four regional clusters. The regions differed significantly in terms of the species present, the total coverage of mangroves, and their position in and around the ponds. Mangrove presence in the ponds correlated strongly with pond productivity and farm profits. Mangrove presence was significantly associated with smaller ponds which had higher input costs and revenue per unit pond area than larger ponds but lower net profitability. Notwithstanding our large sample size, and contrary to our expectations, this study was not able to identify clear profit benefits of the association of mangroves with shrimp ponds. Among various possible explanations, we suggest that, as currently observed in Bangladesh, the presence of mangroves in aquaculture ponds yields little or at most sub-optimal financial benefits. Hence, our results seem to indicate that there is great scope for improving of mangrove-use in shrimp aquaculture. In that, one can expect the largest potential benefit of associating mangroves with shrimp culture in large ponds (“scale benefit”) in which mangroves are presently not being used. The great potential for mangroves to help boost pond productivity and bolster farm economic and environmental resilience clearly remains largely untapped. Our results suggest that further work is needed to help farmers better select appropriate mangrove species, the most suitable locations for planting mangroves and the best percentage of mangrove pond coverage to boost shrimp production. As our cross-sectional study only provides a baseline of mangrove association with shrimp ponds additional,properly controlled experiments following the “Before-after-control-impact (BACI)” design are also needed in order to be able to discern the true causal effects of mangroves on aquaculture production.

Shallow coastal zones are intensely used by humans but simultaneously are biodiversity hotspots, with a crucial role in the life cycle of many marine species. The competition for food or space between humans and protected wildlife intensifies under pressure of an increased need for marine resources for human consumption. For successful management it is important to establish the key components driving such conflicts of interest. Here we focus on the protected common scoter (Melanitta nigra), a sea duck wintering in coastal habitats that are rich in food, but also among the most disturbed marine systems worldwide. Due to the scoters’ shyness disturbance impacts the birds’ ability to forage and poses a conflict for balancing bird conservation and economics, including a fishery on its main bivalve prey Spisula subtruncata. In this study, we use an energy budget model to quantify the consequences of depth, currents and disturbance on scoter energetics and carrying capacity. Energetics were described using physical parameters and field data on food availability and disturbance. Results reveal non-linear relationships and a threshold value for when a scoter can no longer maintain its energy balance. This is caused by limited foraging time, rather than food availability. From a conservation perspective, this implies that a precautionary principle should be used, because there will be no warning when an area becomes unsuitable. In addition, the model was applied to study the effects of disturbance from different kinds of shipping in a coastal area of the North Sea, north of The Netherlands. Cargo shipping has the largest impact on the carrying capacity, where there is spatial overlap of prey and an intensively used shipping lane. In other prey distribution situations shrimp vessels may cause most disturbance. Spisula-Ensis fisheries did not limit the potential carrying capacity due to the limited catches and number of fishing trips. Scoter protection should be aimed at flexible spatial management and on only those vessel types above a Spisula bed with a large number of trips, and above all should work from a precautionary principle given the critical thresholds for scoter presence.

In 2022 is in het kader van het monitoringsprogramma “Contaminanten in vis uit Nederlandse binnenwateren” aal op 21 locaties bemonsterd. Hiervan lagen 7 locaties binnen het voor aalvisserij gesloten gebied en waren er 14 locaties waar de aalvisserij is toegestaan. Voor de bemonstering van grote alen wordt sinds 2016 rekening gehouden met het zwaartepunt van de beroepsmatige vangst, reden waarom iets grotere aal is bemonsterd (53-76 cm) dan vóór 2016 (>45 cm). Van de gevangen rode alen zijn mengmonsters samengesteld voor de lengteklassen 30-40 cm (drie trendlocaties elk jaar) en 53-76 cm. Deze monsters zijn geanalyseerd op de aanwezigheid van dioxines, dioxineachtige-PCB’s (dl-PCB’s) en niet-dioxineachtige PCB’s (ndl-PCB’s). Voor de som dioxines, som dioxines & dl-PCB’s (totaal-TEQ) en som ndl-PCB’s zijn Europese maximumgehalten (zogenaamde maximum levels, ML’s) vastgesteld (EC 1881/2006).-- De onderzochte kleine aal (30-40 cm) overschrijdt op geen enkele locatie de maximum- of beleidsregellimieten. Van de 21 onderzochte monsters grote aal overschrijden 11 monsters één of meerdere ML’s voor dioxine-TEQ, totaal-TEQ of ndl-PCB’s. Aanvullend overschrijden mengmonsters aal van de locaties Kuil en Poel meertjes verbonden met de Zaan, Braassemermeer en IJsselmeer (tussen Lelystad en Ketelbrug) één of meerdere beleidsregellimieten.-- Het gehalte totaal-TEQ in grote rode aal gevangen in het IJsselmeer (tussen Lelystad en de Ketelbrug), ten Oosten van de energiecentrale, overschrijdt de ML voor totaal-TEQ en de beleidsregellimiet voor totaal ndl-PCBs. Deze locatie ligt in het gebied dat opengesteld is voor aalvisserij. In het Spaarne gebied overschrijdt de aal vanuit het Spaarne, ten noorden van de Mooie Nel de ML’s voor totaal-TEQ en totaal ndl-PCBs. Dit gebied is opengesteld voor visserij. Ook aal uit het Binnen Liede, wat verbonden is met het Spaarne via de Mooie Nel, overschrijdt de ML’s voor totaal-TEQ en totaal ndl-PCBs. Aal gevangen in de Kuil en Poelmeertjes in de Zaan overschrijdt de beleidsregellimiet voor totaal-TEQ en totaal ndl-PCBs. Verder Noordwaarts in de Zaan nemen de gehalten af en worden de ML’s of beleidsregellimieten niet meer overschreden. -Het huidige rapport geeft de gegevens van voorgaande jaren weer voor de trendlocaties, aangevuld met de resultaten van 2022. Op een enkele locatie is een verhoogd gehalte gevonden t.o.v. voorgaande jaren, over het algemeen passen de gevonden gehalten in het beeld van de voorgaande jaren. -De mengmonsters aal zijn ook geanalyseerd op zware metalen (cadmium, lood, kwik, nikkel en arseen) en perfluoralkylstoffen (PFAS’s). Alle gehalten van cadmium, lood en kwik voldeden aan de geldende ML’s (EC 1881/2006). Arseen werd ook aangetroffen, maar voor arseen is geen ML vastgesteld, en kunnen daarom niet aan een ML getoetst worden. Door de toegenomen aandacht voor PFAS’s zijn sinds enkele jaren de resultaten van deze metingen ook in dit rapport opgenomen, en zijn dit jaar wederom de gegevens van voorgaande jaren weergegeven voor de trendlocaties, aangevuld met de resultaten van 2022. De gesommeerde gehalten van de meest voorkomende PFAS’s variëren van ongeveer 2 tot 35 μg/kg product, waarbij PFOS de belangrijkste bijdrage leverde (35-100%). Hoogste gehalten werden aangetroffen in aal uit het IJsselmeer bij Medemblik en het Ramsdiep (ten oosten van de Ramspolbrug). Voor PFAS’s zijn MLs vastgesteld die op 1 januari 2023 in werking gaan, deze zijn dus nog niet van toepassing op de gemeten gehalten in dit rapport.

Fish assemblages of different types of artificial reefs can differ greatly in abundance, biomass and composition, with some reef types harboring over five times more herbivores than others. It is assumed that higher herbivorous fish abundance results in a higher grazing intensity, affecting the benthic community by means of enhanced coral recruitment, survival and growth. Territorial fish species might affect this process by chasing away other fish, especially herbivores. In this study we compared the fish assemblage, territorial behavior and grazing intensity by fish on two artificial reef types: reef balls and layered cakes, differing greatly in their fish assemblage during early colonization. In addition, the effect of artificial reef type on benthic development and coral recruitment, survival and growth, was investigated. Although layered cakes initially harbored higher herbivorous fish biomass, this effect was lost during consecutive monitoring events. This seems to be the result of the higher territorial fish abundance around the layered cakes where almost four times more chasing behavior was recorded compared to the reef balls. This resulted in a more than five times lower fish grazing intensity compared to the reef-ball plots. Although macroalgae were effectively controlled at both reefs, the grazing intensity did not differ enough to cause large enough structural changes in benthic cover for higher coral recruitment, survival or growth. The high turf algae cover, combined with increasing crustose coralline algae and sponge cover likely explained reduced coral development. We recommend further research on how to achieve higher grazing rates for improved coral development on artificial reefs, for example by facilitating invertebrate herbivores.

In 2021 is een start gemaakt met een nieuwe vismonitoringsreeks voor het IJsselmeer en Markermeer op basis van gestandaardiseerde jaarlijkse bemonsteringen met een A-toomkuil en stortkuil. Deze monitoring wordt uitgevoerd in opdracht van het ministerie van Landbouw, Natuur en voedselkwaliteit en is in 2022, het tweede jaar van de monitoringsreeks, voortgezet. De opzet van de monitoring is gebaseerd op een pilot die binnen een Europees Fonds voor Maritieme Zaken en Visserij project in samenwerking met stichting Transitie IJsselmeer, ATKB en beroepsvissers is uitgevoerd in 2018 en 2019. Het doel van de nieuwe monitoringsreeks is het ontwikkelen van een integraal en breed gedragen bestandsoverzicht van de jaarlijkse ontwikkelingen in de visstand op beide meren, ten behoeve van het visstand- en visserijbeheer. Het integrale karakter houdt in dat zo goed mogelijk een beeld wordt verkregen van de veranderingen in soortsamenstellingen en van de lengteverdelingen in de visstand. Deze informatie is wenselijk vanwege de vraag vanuit het beheer om aan de hand van vismonitoring beter te kunnen voldoen aan verschillende zaken, zoals de eisen uit de Europese Kaderrichtlijn Water, de behoefte vanuit het visserijbeheer om ontwikkelingen in zowel het jonge als het oudere visbestand zo goed mogelijk in beeld te krijgen, de behoefte vanuit de Vogel- en Habitatrichtlijn om de voedselsamenstelling voor visetende vogels zo goed mogelijk in beeld te brengen, en in het algemeen de veranderende informatiebehoefte met betrekking tot de visstand in het IJsselmeergebied, waarbij steeds meer vanuit een ecosysteembenadering gewerkt wordt. De monitoring wordt in samenwerking met beroepsvissers en visexperts van ATKB opgezet en uitgevoerd, waardoor de resultaten breed gedragen worden door betrokkenen en belangenpartijen. In de komende jaren zal de methode zoveel mogelijk (gestandaardiseerd) uitgevoerd worden zoals die nu gerealiseerd is, maar waar nodig zal deze bijgeschaafd worden. Goede communicatie van de PO en vooral de meewerkende beroepsvissers met staandwant heeft ertoe geleid dat er bijna probleemloos ruimte is gemaakt voor het onderzoek in de meren, een winst ten opzichte van het jaar ervoor. Alle trekken konden hierdoor uitgevoerd worden, inclusief de trekken in de diepe putten op het Markermeer (Markermeerputten) die aangelegd zijn voor de bouw van de Marker Wadden. In september en oktober 2022 zijn er in totaal 73 A-toomkuiltrekken uitgevoerd, waarvan 42 op het IJsselmeer en 31 op het Markermeer. Daarnaast zijn er 19 stortkuiltrekken uitgevoerd in de ondiepste zone (0-2 m), waarvan 10 op het IJsselmeer en 9 op het Markermeer. Alle bemonsteringen zijn in het donker uitgevoerd. Verschillende Markermeerputten zijn dit jaar bevist om inzicht te krijgen in de visbestanden in dit habitat. De vangstuccessen van de Markermeerputten zijn daarnaast vergeleken met de overige diepteklassen (0-2; 2-4 en 4-6m) van het Markermeer. Specifiek kijkend naar commerciële soorten en lengtes, bleek dat baars (>22 cm), blankvoorn (>25 cm) en brasem (>30 cm) zich grotendeels in de ondiepere gedeeltes van het Markermeer bevinden (0-2m en 2-4m) en minder in de diepe putten op het Markermeer. Voor bovenmaatse snoekbaars (>42 cm) geldt dat het vangstsucces in de Markermeerputten ongeveer hetzelfde was als in de ondiepere delen van het Markermeer (0-2; 2-4 m). Vergeleken met vorig jaar is er dit jaar meer vis per hectare gevangen. Op het IJsselmeer ging het voornamelijk om meer spiering en baars, op het Markermeer om een toename van de oudere jaarklassen snoekbaars. Van een trend kan nog niet gesproken worden, aangezien er nog te weinig jaren achtereenvolgend gestandaardiseerd gemonitord is. Blankvoorn en brasem werden het meest gevangen bij de oevers (< 2 m), waarbij het vooral ging om oudere jaarklassen. Baars werd voornamelijk gevangen op het IJsselmeer, waarbij het grootste gedeelte 0+ baars was. Van snoekbaars is voornamelijk de 0+ jaarklasse gevangen op het IJsselmeer. Deze 0+ snoekbaars bestond dit jaar uit twee verschillende lengtegroepen: snoekbaars tussen de 5-10 cm en tussen de 20-25 cm. Op het Markermeer werden voornamelijk oudere jaarklassen snoekbaars gevangen. 2022 was een relatief goed jaar voor de spiering, waarvan het grootste gedeelte op het IJsselmeer werd gevangen, maar ook op het Markermeer was een toename van spiering te zien. De spiering op het IJsselmeer was dit jaar gemiddeld kleiner dan voorgaande jaren, vermoedelijk door intraspecifieke competitie. Pos is de enige soort die aanzienlijk minder gevangen werd, mogelijk door interspecifieke competitie met zwartbekgrondel, die in 2022 meer gevangen werd dan in 2021.

Voor het visserijbeheer van snoekbaars, baars, blankvoorn en brasem op het IJssel-/Markermeer zal in 2023 een zogenaamde beheerstrategieëvaluatie uitgevoerd worden. In een beheerstrategieëvaluatie wordt een virtuele wereld gecreëerd, waarbij de geschiedenis van een visbestand wordt nagebootst. In deze virtuele wereld worden vervolgens verschillende beheerplannen van de overheid getest. In de simulatie worden de vissen per cohort gevolgd; een cohort wordt geboren, groeit, en sterft via visserij, vogelpredatie of andere vormen van mortaliteit. De daadwerkelijke geschiedenis van het bestand wordt zo nauwkeurig mogelijk nagebootst, op basis van historische gegevens uit visserij- en surveyvangsten, aangevuld met biologische kennis van het systeem en de soort. Vooruitlopend op deze evaluatie is geprobeerd om de beschikbare modellen, gegevens en kennis te verbreden en verdiepen. Dit betreft drie onderdelen van doorontwikkeling; van de modellen zelf (Mosqueira et al. 2022), en van de gegevens die de modellen voeden. Deze gegevens betreffen ecologische informatie (de Leeuw et al. 2022) en informatie over de biologie en visserij (het‘aanvullend onderzoek’ in dit rapport).De belangrijkste gegevens in dit type modellen zijn visserijgegevens: hoeveel vis wordt door de visserij onttrokken? Een groot gemis momenteel is een betrouwbare tijdserie van de historische aanlandingen van brasem en blankvoorn. Er is uitvoerig met vissers overlegd en ondanks ruime medewerking is het niet gelukt om de tijdserie van de vangst op deze manier te verbeteren. Wel is door deze gesprekken het beeld van de historische zegenvisserij op blankvoorn en brasem verbeterd en is de onzekerheid rondom de historische vangsten iets verkleind. Ook gesprekken met SportvisserijNederland en PO IJsselmeer hebben niet tot verbeterde tijdreeksen geleid. Een andere vorm vanvisserijonttrekking die meer duidelijkheid heeft gekregen is de discarding in de fuikenvisserij. De schattingen zijn met grote onzekerheid omgeven, maar de inschatting is dat met name discarding van baars (maar ook andere bestanden) een grote bron van visserijsterfte kan zijn. Deze vorm van visserijsterfte wordt tot nu toe niet meegenomen in de tijdserie van visserijvangsten. Er wordt sterk aangeraden meer onderzoek te doen naar de vangstsamenstelling en discardsterfte van de fuikenvisserij. Een derde vorm van visserijonttrekking die verder is onderzocht is de recreatieve visserij. De hoeveelheid onttrekking door de traditionele recreatieve visserij vanaf de oevers isonderzocht, als ook de nieuw opgekomen hengelsportvisserij met geavanceerde sonarapparatuur vanaf boten. De voorzichtige conclusie hierbij blijft dat de recreatieve visserij tot een verwaarloosbare visserijonttrekking leidt, in vergelijking met de beroepsvisserij. Ook is gewerkt aan een meer representatieve schatting van de lengtesamenstelling van de commerciële vangsten en van de selectiviteitcurves van de surveytuigen.Van de vier soorten zijn er sterke aanwijzingen dat de groeisnelheid van baars, blankvoorn en brasem is toegenomen sinds de jaren 90. Alleen voor snoekbaars is geen indicatie voor een toename in groeisnelheid gevonden door de decennia heen. Snoekbaars vertoonde ook in een andere analyse afwijkende resultaten van de andere drie bestanden: in de surveyvangsten zijn nieuwe cohorten van baars, blankvoorn en brasem goed terug te vinden in het jaar erna, maar van snoekbaars niet. Dit gebrek aan volgbaarheid was gerelateerd aan afwijkende processen in de twee meren: in het Markermeer zijn cohorten wel te volgen in de survey, maar in het IJsselmeer niet. Dit gebrek aan volgbaarheid in het IJsselmeer hangt waarschijnlijk samen met een sterk afgenomen overleving vannieuwe jaarklassen in het IJsselmeer. Deze afgenomen overleving valt weer samen met een sterke toename in het aantal aalscholvers dat in het IJsselmeer jaagt. Mogelijke verklaringen zijn dat toegenomen vogelpredatie heeft bijgedragen aan de afgenomen overleving in het IJsselmeer, en/of dat beide trends veroorzaakt zijn door de toegenomen waterhelderheid op het IJsselmeer.Alle resultaten zullen in 2023 op gepaste wijze worden meegenomen in de beheerstrategieëvaluatie. Deze wijze wordt ook in dit rapport zover als mogelijk uitgelegd.

Elasmobranchs (i.e. sharks, skates and rays) play an important role as predators in marine ecosystems. In the last century the abundance of several skates and rays in the North Sea declined. Their specific life-history traits, i.e. being long-lived, showing slow growth, late sexual maturity and producing a small number of young per year, make them vulnerable to fishing, pollution and changes in essential habitats, especially spawning and nursery areas. Since 2010 a recovery in the abundances of some species is observed. Landings of skates and rays in European waters are generally managed by a group-TAC, meaning several species are managed under a single Total Allowable Catch (TAC). In the North Sea this group-TAC applies to thornback (Raja clavata), blonde (Raja brachyura), spotted (Raja montagui) and cuckoo ray (Leucoraja naevus). Landings of starry ray (Amblyraja radiata) and common skate (Dipturus sp.) are prohibited. In Dutch fisheries, skates and rays are mainly caught as by-catch in the mixed demersal fishery for flatfish, with landings dominated by thornback ray. For these species the rays below minimum landing size and catches that exceed the vessel’s weekly trip limits are discarded. Since 2019 this practise of discarding has been restricted for all quota regulated species, including rays, by the implementation of a landing obligation under the Common Fisheries Policy (European Union, 2013). The objective of the landing obligation is to create an incentive for fishers to avoid unwanted bycatches and thereby reduce discards rates. The European commission granted a temporary ‘high survival’ exemption for rays and indicated that for this exemption to be renewed or extended beyond 2023, knowledge gaps regarding discards survival probabilities of rays, as laid out in the “Roadmap skates and rays”, need to be filled. The current study therefore aims to contribute to this roadmap by filling main gaps in our knowledge on the survival probability of rays when discarded by the most important Dutch demersal fisheries. Beam trawl and flyshoot fisheries are quantitatively the main contributors to ray catches by Dutch demersal fisheries with thornback and spotted ray as main species. These two métiers and species were therefore selected for the discards survival experiments. We conducted five trips with a beam trawler during which 184 thornback and 140 spotted rays were sampled and four trips with a flyshooter during which 134 thornback and 28 spotted rays were sampled. Survival probabilities were measured by captive observation of these rays up to 25 days post-capture. Control rays were used to separate potential effects of the experimental procedures on mortality from fisheries-induced mortality. All fishery operations were conducted in the southern North Sea (ICES division 27.4.c) and in the English Channel (ICES division 27.7.d) according to the regular commercial practices of the fishing vessels. Trips were allocated to different quarters over the year to account for the effect of variable environmental and fishing conditions on discards survival. Operational and environmental conditions during sea trips were recorded. In this study we established the following discards survival probabilities (95% CI): • 45.5% (37.9-54.5) for spotted ray discarded by tickler chain beam trawl fisheries with 80 mm cod-end meshes; • 49.6% (42.9-57.4) for thornback ray discarded by tickler chain beam trawl fisheries with 80 mm cod-end meshes; • 77.6% (63.3-95.2) for spotted ray discarded by flyshoot fisheries with 80 mm cod-end meshes; • 81.0% (74.4-88.2) for thornback ray discarded by flyshoot fisheries with 80 mm cod-end meshes. The experimental procedures did not cause any mortality since all control rays survived the experiments. All mortality among sampled rays can thus be considered to be fisheries induced. Survival probability was higher for rays discarded by flyshoot fisheries compared to beam trawl fisheries. Survival probability of discarded rays decreased with increasing seawater temperature and some evidence for decreasing survival with increasing catch processing time was found. Species and body length did not affect survival probability. Survival probability was highest for rays that were landed on deck in good condition. To further increase survival probability it is recommended to keep catch processing time as short as possible and to focus on gear modifications that reduce stressors inflicted upon fish during the catch and hauling process to increase the proportion of rays that is landed on deck in good condition. In addition, we recommend to pursue reduction of fishing mortality among rays by reducing the amount of caught rays by improved selectivity of fishing gear and by actively avoiding fishing grounds where rays occur in high densities.

De Europese aal (Anguilla anguilla) trekt als juveniel (glasaal) van de zee het zoete water op en trekt als volwassen schieraal weer terug naar zee. Bij deze migratie wordt zowel de intrek als de uittrek belemmerd door kunstwerken, zoals gemalen, waterkrachtcentrales, stuwen en sluizen. In 2013 is er een overzicht van de belangrijkste migratieknelpunten voor schieraal in Nederland opgesteld (Winter et al 2013a). Het Ministerie van Landbouw, Natuur en Voedselkwaliteit (LNV) heeft Wageningen Marine Research gevraagd om deze lijst te updaten met de nieuwste informatie. Als basis voor deze rapportage is gebruik gemaakt van zogenaamde Nederlandse ‘aalmodel’ (Bierman et al, 2012; van der Hammen et al, 2021). Dit model wordt gebruikt om elke drie jaar de totale schieraal biomassa in Nederland te berekenen en daarmee te voldoen aan de verplichting van de EU doormiddel van de zogenaamde ‘3- jaarlijkse EU-evaluatie’ (van der Hammen et al. 2021). In het aalmodel wordt een schatting gemaakt hoeveel schieraal er vanuit elk gebied start met de migratie naar zee. Daarnaast wordt de beschikbare kennis uit Winter et al (2013a) gebruikt, die op basis van watersysteemkennis en informatie van waterbeheerders een lijst met potentiële knelpunten door kunstwerken hadden opgesteld die een relatief groot achterland voor aal ontsluiten. Per potentieel knelpunt is een inschatting gemaakt van de hoeveelheid schieraal die bij het kunstwerk zou kunnen aankomen, gebruikmakend van de resultaten uit het aalmodel. Vervolgens zijn de inschattingen van de sterftekans tijdens passage van gemalen en waterkrachtcentrales, de uittrek mogelijkheden via alternatieve routes in het complex en de mate van barrièrewerking bij een kunstwerk uit Winter et al (2013a) geëvalueerd. Hiervoor zijn de waterschappen voor elke 3-jaarlijkse EU-evaluatie benaderd met het verzoek om aanpassingen en onderzoek bij de knelpunten door te geven. Ook zijn de gemiddelde schattingen voor de gemalen aangepast op basis van beschikbare literatuur. Door de hoeveelheid schieraal per potentieel knelpunt te combineren met de sterftekans wordt in dit rapport de potentiële sterfte aan schieraal per kunstwerk ingeschat. De schattingen in dit rapport zijn afhankelijk van de beschikbaarheid van de onderliggende gegevens in het aalmodel en de kennis die er is van het gedrag van schieraal bij complexen met kunstwerken. Net als in 2013 zijn er nog steeds veel ontbrekende gegevens. Wanneer er geen gegeven beschikbaar zijn voor een kunstwerk wordt er uitgegaan van gemiddelde waarden voor andere vergelijkbare type kunstwerken. Met name over het gedrag van schieraal bij kunstwerken die uit een complex van kunstwerken bestaan (bijvoorbeeld een waterkrachtcentrale met stuw, vistrap en scheepsluizen, of een gemaal met scheepsluis) is vaak weinig bekend. Op basis van de geschatte verliesposten schieraal is de lijst van belangrijkste knelpunten in Nederland uit 2013 geüpdatet. Het sluizencomplex in IJmuiden is als grootste knelpunt aangemerkt. Dit complex heeft een geschatte sterfte van 12.5% en een potentieel aan doortrekkende schieraal van bijna 77 ton, waardoor het verlies aan schieraal geschat is op 9.6 ton. Daarnaast zijn een groot aantal gemalen aangemerkt als knelpunt, maar de verliezen van schieraal bij deze en andere knelpunten zijn aanmerkelijk lager in vergelijking met IJmuiden. In de rapportage van Winter et al (2013) stonden de grote Waterkrachtcentrales (WKC’s) in de Maas nog op plaats één en twee. Echter, vanwege de vele maatregelen waaronder turbinebeheer, nachtelijke stillegging en het overzetten van schieralen over het complex (Paling over de Dijk, PODD) is het sterftepercentage bij deze WKC’s omlaag gegaan. Daarnaast tonen additionele onderzoeken in deze rivieren aan dat de hoeveelheid aal die aankomt bij de WKC’s lager is dan geschat door het aalmodel. Hierdoor staan deze nu minder hoog in de lijst. Er zijn ook enkele scheepvaartsluizen-complexen opgenomen in de lijst. De grote spuisluis-complexen zoals Den Oever, Kornwerderzand en Haringvliet veroorzaken voor schieraal waarschijnlijk wel vertraging, maar zijn niet als knelpunt met extra schieraalsterfte aangemerkt.

Coastal areas are subjected to several anthropogenic stressors with much of the world's intertidal areas receding due to human activities, coastal erosion and sea level rise. The Dutch Eastern Scheldt (ES) has been predicted to lose around 35% of intertidal areas by 2060. This study investigates differences between biogeochemical fluxes of intertidal and subtidal sediments of the ES and assesses how ongoing erosion may modify the sedimentary ecosystem functioning of this coastal bay in the coming decades. Monthly fluxes and porewater concentrations of dissolved inorganicnitrogen (DIN), phosphorous (DIP), silica (DSi), carbon (DIC) and oxygen (O 2) as well as organic matter (OM) characteristics were measured from intertidal and subtidal sediments from June 2016–December 2017. Compared to subtidal stations, OM was significantly more reactive in intertidal samples and exhibited 37% higher O2 fluxes, suggesting a strong influence from microphytobenthos. Subtidal sedimentsexhibited an average efflux of nitrates (0.28 mmol m−2 d −1) and phosphates (0.09 mmol m−2 d−1) into the water column, while intertidal areas displayed average influxes (nitrates = −1.2 mmol m−2 d−1, phosphates = −0.03 mmol m−2 d−1) directed into the sediment. The calculated removal of total DIN and DIP from the water column was 34–38% higher in intertidal compared to subtidal samples suggesting stronger denitrification and phosphorus adsorption to solid particles from intertidal sediments. As an upscaling exercise, we estimate potential erosion induced changes if the ES stations are representative for the system. With this assumption, we estimate 11% and 8% reductions for respective nitrogen and phosphorus removal in the entire ES by 2060. Given the global observations of eroding intertidal areas and rising sea levels, we suggest that the predicted habitat loss may cause significant changes for coastal biogeochemistry and should be investigated further to understand its potential consequences.

Mussel farming in the Netherlands is traditionally based on bottom culture. Mussel seed is collected in Autumn and Spring from natural stocks and translocated to on-bottom culture plots. Fishery starts in Autumn on beds which are most vulnerable for starfish predation or for being lost during winter storms. In Spring a second fishery takes place. We investigated the impact of seed fishery on sediment characteristics and the macrobenthic communities. A BACI designed experiment was run for 4 years in the subtidal western part of Wadden Sea. Sediment and benthos samples were taken directly before and after the seed fishery. Sampling was repeated for a longer period after fisheries. In this study the data before, directly after and 1–1.5 years after the fishery were analysed. The number of locations studied was less than initially planned. As a result the effect sizes that could be detected were larger than chosen at the start of the study. Yet, our study has shown a potential direct effect on sediment characteristics and the benthic communities. Although the effect was not significant, sediment became coarser and the percentage of clay lower in the fished areas compared to the control areas, and the effects were most pronounced after Spring fisheries. The effect on the number of species was significant, and although the interaction term with season was not significant, we did find a larger decrease in the Impact parts after Autumn fisheries, while after Spring fisheries there was a smaller increase. The short term effects on total density were most pronounced after Spring fisheries: a significant decline in total density in the Impact parts while density increased in the Control parts. Species composition was less similar between Impact and Control parts and was lower after fisheries, for both fishery seasons. After 1–1.5 years, the dissimilarity between the Impact and Control part gradually disappeared. Differences between Impact and Control parts in changes in median grain size and the number of species were no longer visible. For the Spring plots, however, also after 1.5 year, the percentage clay and the total densities were lower in the Impact parts then in the Control parts. The observed differences are mainly caused by the changes in species associated to mussel beds, and as a consequence of the changes in mussel densities themselves. In conclusion, changes appears to follow the development of the mussel densities, including the effect of fishery on the mussel densities.

In dit rapport staan de resultaten en aanbevelingen van een pilotstudie waarin in opdracht van Rijkswaterstaat Water, Verkeer en Leefomgeving (namens het Ministerie van Infrastructuur en Waterstaatstaat) bepaald is of en hoe de door Wageningen University & Research ontwikkelde Litter-ID-methodiek kan bijdragen aan de landelijke monitoringstrategie van rivierafval, die ontwikkeld wordt vanuit Rijkswaterstaat.---This report contains the results and recommendations of a pilot project commissioned by Rijkswaterstaat Water, Traffic and the Environment (on behalf of the Ministry of Infrastructure and Water) to determine if and how the Litter-ID methodology, developed by Wageningen University & Research, can contribute to the national monitoring strategy for riverine litter currently being developed by Rijkswaterstaat.

The study describes the socio-cultural value of fishing in the Netherlands in terms of: identity, social well-being, cultural heritage and social cohesion. It looked at the effects of policy changes for the fishing community or communities. The Netherlands has 44 fishing communities with an active fleet and 84 historical fishing communities. The fisheries ‘community of practice’ has a shared identity in which in the Netherlands the family business plays an important role. For their (subjective) social well-being, fishing is very important. These (historical) family businesses are anchored in fishing communities, often have locals on board and contribute to social cohesion, (im)material cultural heritage and the social well-being of fishing communities. At the same time, social structures seem to be changing due to consolidation, fishers with a non-Dutch nationality and continuous fishing. The fishing community is unanimously very concerned about the future of fisheries as a result of all policy changes. In particular, they see the redesign of the North Sea with less space for fisheries as a threat. The policy process has led to much distrust. This study describes how the reduction of the fleet has social implications and pleads for monitoring social factors, developing a shared vision for future fisheries and restoring trust

Will the oceans feed the world? Optimistic views on a ‘Blue Revolution’ have recently been challenged and in particular the future role of marine fish culture is questioned. Important steps in raising global production of protein-rich animal seafood can only be expected from aquaculture of low-trophic organisms such as shellfish. Additional to the provisioning of healthy marine protein products, shellfish culture has many benefits as it causes little environmental impact, and provides the opportunity to contribute to ecosystem services.

In order to quantify the amount of carbonate, precipitated as calcium-carbonate in the shells of blue mussel (Mytilus edulis) in a temperate climate, an existing Dynamic Energy Budget (DEB) model for the blue mussel was adapted by separating shell growth from soft tissue growth. Hereby, two parameters were added to the original DEB-model, a calcification cost [J/mgCaCO3] and an energy allocation fraction [-], which resulted in the energy allocated for structural growth being divided between shell and meat growth. As values for these new parameters were lacking, they were calibrated by fitting the model to field data. Calibration results showed that an Energy allocation fraction of 0.5 and a calcification cost of 0.9 J/mgCaCO3, resulted in the best fit when fitted on 2017 and 2018 field data separately. These values however, show the best fit for data obtained within the first couple of years of the shellfish life, and do not take later years into account. Also it could be discussed that some parameters vary throughout the lifespan of the species. The results were compared to a regular DEB model, where the shell output was calculated through a simple allometric relationship. It is sometimes assumed that the carbon storage in shell material as calcium carbonate could be regarded as a form of carbon sequestration, with a positive impact on the atmospheric CO2 concentrations. However, studies on the physical-chemical processes related to shell formation have shown that from an oceanographic perspective, shell formation should be regarded as a source of atmospheric CO2 rather than a sink. The removal of carbonates, through the biocalcification process, reduces the buffer capacity (alkalinity) of the water to store CO2. As a result CO2 is released from the water to the atmosphere when shell material is formed. The actual amount of CO2 that escapes from the water to the atmosphere as a result of biocalcification depends strongly on local water characteristics. In this study, the effect of calcification by mussels on the CO2 flux to the atmosphere is studied using an adapted DEB model where energy costs of calcification are modelled explicitly. The model was subsequently run under two future climate scenarios, (RCP 4.5 and RCP 8.3) with elevated temperature and decreased pH, and the total released CO2 as a result of shell formation was calculated with the SeaCarb model. This showed growth of mussels, under future climate conditions to be slower, and with that the cumulative shell mass and carbonate precipitated to CaCO3 to decrease. Yet the amount of CO2 released, due to biocalcification, increased. This is due to the fact that the amount of CO2 released/gr of CaCO3 precipitated will be higher, as a result of the decreased buffering capacity of seawater under future climatic environmental conditions. In summary the conclusions of the project were: • Biocalcification (shell formation) of marine organisms, such as bivalves, cannot be regarded as a process resulting in negative CO2 emission to the atmosphere; • The actual amount of CO2 that, due to biocalcification, is released from the water to the atmosphere depends on the physicochemical characteristics of the water, which are influenced by (future) climate conditions; • Our first model calculations suggest that at future climate conditions mussel’s grow rate will be somewhat reduced. While the amount of CO2 that due to biocalcification, escapes to the atmosphere during its life-time will slightly increase. Making the ratio of g CO2 release/g CaCO3 precipitated slightly higher; • Our model calculations should be considered an exercise rather than a definite prediction of how mussels will respond to future climate scenarios. Additional information/experimentation is strongly needed to validate the model settings, and to test the validity of the above mentioned outcome of the model.

Migrating birds flexibly adjust their individual migratory decisions, i.e. departing, routing and landing, based on intrinsic (e.g. energy stores) and extrinsic (e.g. landscape features and weather) factors modulating the endogenous stimuli. So far, these decisions have mostly been studied separately. Notably, we lack information on which factors landing decisions during active flight are based on. Therefore, we simultaneously recorded all three decisions in free-flying long-distance migratory songbirds in a coastal stopover area via regional-scale radio-telemetry and related them to the prevailing weather. Birds departed under favourable weather conditions resulting in specific nights with increased departure probability. Once departed, birds could either fly offshore or take a route along the coast, which was predicted by wind support. Radio-tracking revealed that departed individuals more likely interrupted their migratory endurance flight under overcast or headwind conditions. Studying departure, routing and landing decisions in concert, we highlight the importance of weather as a common driver across all migratory decisions. By radio-tracking individuals between stopovers, we provide evidence that avoidance of adverse weather conditions is an important function of stopover. Understanding how birds adjust migratory decisions and how they affect the timing of migration and survival is key to link migration performance to individual fitness.

We reflect on the innovation process that led to the development of the pulse trawl that was successfully trialed at a commercial scale, but eventually ended with the European Parliament passing legislation to ban its use. The ban was imposed despite published and emerging evidence that suggested that the environmental performance and catch efficiency of the pulse trawl was superior to the conventional beam trawl design. We used a stakeholder questionnaire to understand which factors undermined wider acceptance of the pulse trawl. The main factors where a lack of involvement of certain key stakeholders earlier in the process that would have ensured better co-development of innovation and a shared vision of the environmental or governance questions that needed to be addressed. Although the stakeholder process itself was seen to be positive, it was implemented too late in the innovation process, as was the implementation of an independent peer review process. We conclude by identifying a pathway for future fishing gear innovation processes that integrate the lessons learnt from the pulse trawl innovation process.

Migrating birds flexibly adjust their individual migratory decisions, i.e. departing, routing and landing, based on intrinsic (e.g. energy stores) and extrinsic (e.g. landscape features and weather) factors modulating the endogenous stimuli. So far, these decisions have mostly been studied separately. Notably, we lack information on which factors landing decisions during active flight are based on. Therefore, we simultaneously recorded all three decisions in free-flying long-distance migratory songbirds in a coastal stopover area via regional-scale radio-telemetry and related them to the prevailing weather. Birds departed under favourable weather conditions resulting in specific nights with increased departure probability. Once departed, birds could either fly offshore or take a route along the coast, which was predicted by wind support. Radio-tracking revealed that departed individuals more likely interrupted their migratory endurance flight under overcast or headwind conditions. Studying departure, routing and landing decisions in concert, we highlight the importance of weather as a common driver across all migratory decisions. By radio-tracking individuals between stopovers, we provide evidence that avoidance of adverse weather conditions is an important function of stopover. Understanding how birds adjust migratory decisions and how they affect the timing of migration and survival is key to link migration performance to individual fitness.

During the summercampaign of 2022, Heerema supported Wageningen University & Research (WUR) and Dr. Joop Coolen in collecting data for the Artificial Structures and the functioning of the North Sea EcoSyStem(ASSESS) project.

Elasmobranchs (i.e. sharks, skates and rays) play an important role as predators in marine ecosystems. In the last century the abundance of several skates and rays in the North Sea declined. Their specific life-history traits, i.e. being long-lived, showing slow growth, late sexual maturity and producing a small number of young per year, make them vulnerable to fishing, pollution and changes in essential habitats, especially spawning and nursery areas. Since 2010 a recovery in the abundances of some species is observed. Landings of skates and rays in European waters are generally managed by a group-TAC, meaning several species are managed under a single Total Allowable Catch (TAC). In the North Sea this group-TAC applies to thornback (Raja clavata), blonde (Raja brachyura), spotted (Raja montagui) and cuckoo ray (Leucoraja naevus). Landings of starry ray (Amblyraja radiata) and common skate (Dipturus sp.) are prohibited. In Dutch fisheries, skates and rays are mainly caught as by-catch in the mixed demersal fishery for flatfish, with landings dominated by thornback ray. For these species the rays below minimum landing size and catches that exceed the vessel’s weekly trip limits are discarded. Since 2019 this practise of discarding has been restricted for all quota regulated species, including rays, by the implementation of a landing obligation under the Common Fisheries Policy (European Union, 2013). The objective of the landing obligation is to create an incentive for fishers to avoid unwanted bycatches and thereby reduce discards rates. The European commission granted a temporary ‘high survival’ exemption for rays and indicated that for this exemption to be renewed or extended beyond 2023, knowledge gaps regarding discards survival probabilities of rays, as laid out in the “Roadmap skates and rays”, need to be filled. The current study therefore aims to contribute to this roadmap by filling main gaps in our knowledge on the survival probability of rays when discarded by the most important Dutch demersal fisheries. Beam trawl and flyshoot fisheries are quantitatively the main contributors to ray catches by Dutch demersal fisheries with thornback and spotted ray as main species. These two métiers and species were therefore selected for the discards survival experiments. We conducted five trips with a beam trawler during which 184 thornback and 140 spotted rays were sampled and four trips with a flyshooter during which 134 thornback and 28 spotted rays were sampled. Survival probabilities were measured by captive observation of these rays up to 25 days post-capture. Control rays were used to separate potential effects of the experimental procedures on mortality from fisheries-induced mortality. All fishery operations were conducted in the southern North Sea (ICES division 27.4.c) and in the English Channel (ICES division 27.7.d) according to the regular commercial practices of the fishing vessels. Trips were allocated to different quarters over the year to account for the effect of variable environmental and fishing conditions on discards survival. Operational and environmental conditions during sea trips were recorded. In this study we established the following discards survival probabilities (95% CI): • 45.5% (37.9-54.5) for spotted ray discarded by tickler chain beam trawl fisheries with 80 mm cod-end meshes; • 49.6% (42.9-57.4) for thornback ray discarded by tickler chain beam trawl fisheries with 80 mm cod-end meshes; • 77.6% (63.3-95.2) for spotted ray discarded by flyshoot fisheries with 80 mm cod-end meshes; • 81.0% (74.4-88.2) for thornback ray discarded by flyshoot fisheries with 80 mm cod-end meshes. The experimental procedures did not cause any mortality since all control rays survived the experiments. All mortality among sampled rays can thus be considered to be fisheries induced. Survival probability was higher for rays discarded by flyshoot fisheries compared to beam trawl fisheries. Survival probability of discarded rays decreased with increasing seawater temperature and some evidence for decreasing survival with increasing catch processing time was found. Species and body length did not affect survival probability. Survival probability was highest for rays that were landed on deck in good condition. To further increase survival probability it is recommended to keep catch processing time as short as possible and to focus on gear modifications that reduce stressors inflicted upon fish during the catch and hauling process to increase the proportion of rays that is landed on deck in good condition. In addition, we recommend to pursue reduction of fishing mortality among rays by reducing the amount of caught rays by improved selectivity of fishing gear and by actively avoiding fishing grounds where rays occur in high densities.

De Europese aal (Anguilla anguilla) trekt als juveniel (glasaal) van de zee het zoete water op en trekt als volwassen schieraal weer terug naar zee. Bij deze migratie wordt zowel de intrek als de uittrek belemmerd door kunstwerken, zoals gemalen, waterkrachtcentrales, stuwen en sluizen. In 2013 is er een overzicht van de belangrijkste migratieknelpunten voor schieraal in Nederland opgesteld (Winter et al 2013a). Het Ministerie van Landbouw, Natuur en Voedselkwaliteit (LNV) heeft Wageningen Marine Research gevraagd om deze lijst te updaten met de nieuwste informatie. Als basis voor deze rapportage is gebruik gemaakt van zogenaamde Nederlandse ‘aalmodel’ (Bierman et al, 2012; van der Hammen et al, 2021). Dit model wordt gebruikt om elke drie jaar de totale schieraal biomassa in Nederland te berekenen en daarmee te voldoen aan de verplichting van de EU doormiddel van de zogenaamde ‘3- jaarlijkse EU-evaluatie’ (van der Hammen et al. 2021). In het aalmodel wordt een schatting gemaakt hoeveel schieraal er vanuit elk gebied start met de migratie naar zee. Daarnaast wordt de beschikbare kennis uit Winter et al (2013a) gebruikt, die op basis van watersysteemkennis en informatie van waterbeheerders een lijst met potentiële knelpunten door kunstwerken hadden opgesteld die een relatief groot achterland voor aal ontsluiten. Per potentieel knelpunt is een inschatting gemaakt van de hoeveelheid schieraal die bij het kunstwerk zou kunnen aankomen, gebruikmakend van de resultaten uit het aalmodel. Vervolgens zijn de inschattingen van de sterftekans tijdens passage van gemalen en waterkrachtcentrales, de uittrek mogelijkheden via alternatieve routes in het complex en de mate van barrièrewerking bij een kunstwerk uit Winter et al (2013a) geëvalueerd. Hiervoor zijn de waterschappen voor elke 3-jaarlijkse EU-evaluatie benaderd met het verzoek om aanpassingen en onderzoek bij de knelpunten door te geven. Ook zijn de gemiddelde schattingen voor de gemalen aangepast op basis van beschikbare literatuur. Door de hoeveelheid schieraal per potentieel knelpunt te combineren met de sterftekans wordt in dit rapport de potentiële sterfte aan schieraal per kunstwerk ingeschat. De schattingen in dit rapport zijn afhankelijk van de beschikbaarheid van de onderliggende gegevens in het aalmodel en de kennis die er is van het gedrag van schieraal bij complexen met kunstwerken. Net als in 2013 zijn er nog steeds veel ontbrekende gegevens. Wanneer er geen gegeven beschikbaar zijn voor een kunstwerk wordt er uitgegaan van gemiddelde waarden voor andere vergelijkbare type kunstwerken. Met name over het gedrag van schieraal bij kunstwerken die uit een complex van kunstwerken bestaan (bijvoorbeeld een waterkrachtcentrale met stuw, vistrap en scheepsluizen, of een gemaal met scheepsluis) is vaak weinig bekend. Op basis van de geschatte verliesposten schieraal is de lijst van belangrijkste knelpunten in Nederland uit 2013 geüpdatet. Het sluizencomplex in IJmuiden is als grootste knelpunt aangemerkt. Dit complex heeft een geschatte sterfte van 12.5% en een potentieel aan doortrekkende schieraal van bijna 77 ton, waardoor het verlies aan schieraal geschat is op 9.6 ton. Daarnaast zijn een groot aantal gemalen aangemerkt als knelpunt, maar de verliezen van schieraal bij deze en andere knelpunten zijn aanmerkelijk lager in vergelijking met IJmuiden. In de rapportage van Winter et al (2013) stonden de grote Waterkrachtcentrales (WKC’s) in de Maas nog op plaats één en twee. Echter, vanwege de vele maatregelen waaronder turbinebeheer, nachtelijke stillegging en het overzetten van schieralen over het complex (Paling over de Dijk, PODD) is het sterftepercentage bij deze WKC’s omlaag gegaan. Daarnaast tonen additionele onderzoeken in deze rivieren aan dat de hoeveelheid aal die aankomt bij de WKC’s lager is dan geschat door het aalmodel. Hierdoor staan deze nu minder hoog in de lijst. Er zijn ook enkele scheepvaartsluizen-complexen opgenomen in de lijst. De grote spuisluis-complexen zoals Den Oever, Kornwerderzand en Haringvliet veroorzaken voor schieraal waarschijnlijk wel vertraging, maar zijn niet als knelpunt met extra schieraalsterfte aangemerkt.

According to the Kunming-Montreal Global Biodiversity Framework, a “Whole-of-Society” approach is needed to initiate transitions to a nature-positive society. Many look at civil society to initiate and accelerate such transitions. In this article, we investigate strategies from Civil Society Actors (CSAs) to contribute to transformative change, with specific focus on Tiny Forests and Beach Clean-Ups in the Netherlands. Results show that CSAs have a clear Theory of Change to achieve their goals, and act upon that vision through assembling power and resources, developing policy-relevant environmental knowledge, mobilising public support and media coverage and initiating innovative sustainable practices. Adopting mosaic governance approaches, CSAs strategically position themselves in social and institutional networks, connecting professionals and citizens for political leverage. However, our findings show that the step from local impact towards transition remains a large one and the contribution of CSAs should be valued as emergent, co-produced and part of a broader transition movement.

Innovative techniques are needed to assess oyster performance in flat oyster reef restoration projects. A valve gape monitor, a device that continuously measures opening and closing of live bivalves, can potentially be used as an effective method to determine survival and behaviour of the European flat oyster Ostrea edulis. The method has been successfully used in combination with a number of bivalve species to investigate valve gape activity in response to environmental factors. In this study, eight O. edulis were equipped with valve gape sensors in order to relate gape to environmental conditions such as food availability. Valve gape activity was monitored under controlled laboratory conditions, with and without food, in a concrete basin in the Oosterschelde and in the field (Voordelta, Dutch North Sea). Under controlled laboratory conditions, oysters clearly responded to changes in food availability. Starved oysters closed their valves significantly longer than oysters that received food, and the relative gape width in fed oysters was larger. In the concrete basin (Oosterschelde), a positive correlation between valve opening and Chlorophyll-a was found. Additionally, valve gape activity and tidal movement appeared to be linked. When exposed to a full tidal cycle (Voordelta), a negative correlation between valve opening and Chlorophyll-a was found. However, there was no correlation between valve gape and current velocity. In autumn, longer periods of inactivity were seen, but when valves opened, the valve gape was larger. These data indicate that valve gape can provide valuable information on behaviour (gape frequency and gape width), but also show that it is not necessarily a good proxy for feeding rate. Nevertheless, these results show that the gape monitor can be used to determine the natural behaviour of flat oysters under field conditions, and that gape opening provides information on behaviour and the stress response of bivalves to environmental conditions.

In 2022 is in het kader van het monitoringsprogramma “Contaminanten in vis uit Nederlandse binnenwateren” aal op 21 locaties bemonsterd. Hiervan lagen 7 locaties binnen het voor aalvisserij gesloten gebied en waren er 14 locaties waar de aalvisserij is toegestaan. Voor de bemonstering van grote alen wordt sinds 2016 rekening gehouden met het zwaartepunt van de beroepsmatige vangst, reden waarom iets grotere aal is bemonsterd (53-76 cm) dan vóór 2016 (>45 cm). Van de gevangen rode alen zijn mengmonsters samengesteld voor de lengteklassen 30-40 cm (drie trendlocaties elk jaar) en 53-76 cm. Deze monsters zijn geanalyseerd op de aanwezigheid van dioxines, dioxineachtige-PCB’s (dl-PCB’s) en niet-dioxineachtige PCB’s (ndl-PCB’s). Voor de som dioxines, som dioxines & dl-PCB’s (totaal-TEQ) en som ndl-PCB’s zijn Europese maximumgehalten (zogenaamde maximum levels, ML’s) vastgesteld (EC 1881/2006).-- De onderzochte kleine aal (30-40 cm) overschrijdt op geen enkele locatie de maximum- of beleidsregellimieten. Van de 21 onderzochte monsters grote aal overschrijden 11 monsters één of meerdere ML’s voor dioxine-TEQ, totaal-TEQ of ndl-PCB’s. Aanvullend overschrijden mengmonsters aal van de locaties Kuil en Poel meertjes verbonden met de Zaan, Braassemermeer en IJsselmeer (tussen Lelystad en Ketelbrug) één of meerdere beleidsregellimieten.-- Het gehalte totaal-TEQ in grote rode aal gevangen in het IJsselmeer (tussen Lelystad en de Ketelbrug), ten Oosten van de energiecentrale, overschrijdt de ML voor totaal-TEQ en de beleidsregellimiet voor totaal ndl-PCBs. Deze locatie ligt in het gebied dat opengesteld is voor aalvisserij. In het Spaarne gebied overschrijdt de aal vanuit het Spaarne, ten noorden van de Mooie Nel de ML’s voor totaal-TEQ en totaal ndl-PCBs. Dit gebied is opengesteld voor visserij. Ook aal uit het Binnen Liede, wat verbonden is met het Spaarne via de Mooie Nel, overschrijdt de ML’s voor totaal-TEQ en totaal ndl-PCBs. Aal gevangen in de Kuil en Poelmeertjes in de Zaan overschrijdt de beleidsregellimiet voor totaal-TEQ en totaal ndl-PCBs. Verder Noordwaarts in de Zaan nemen de gehalten af en worden de ML’s of beleidsregellimieten niet meer overschreden. -Het huidige rapport geeft de gegevens van voorgaande jaren weer voor de trendlocaties, aangevuld met de resultaten van 2022. Op een enkele locatie is een verhoogd gehalte gevonden t.o.v. voorgaande jaren, over het algemeen passen de gevonden gehalten in het beeld van de voorgaande jaren. -De mengmonsters aal zijn ook geanalyseerd op zware metalen (cadmium, lood, kwik, nikkel en arseen) en perfluoralkylstoffen (PFAS’s). Alle gehalten van cadmium, lood en kwik voldeden aan de geldende ML’s (EC 1881/2006). Arseen werd ook aangetroffen, maar voor arseen is geen ML vastgesteld, en kunnen daarom niet aan een ML getoetst worden. Door de toegenomen aandacht voor PFAS’s zijn sinds enkele jaren de resultaten van deze metingen ook in dit rapport opgenomen, en zijn dit jaar wederom de gegevens van voorgaande jaren weergegeven voor de trendlocaties, aangevuld met de resultaten van 2022. De gesommeerde gehalten van de meest voorkomende PFAS’s variëren van ongeveer 2 tot 35 μg/kg product, waarbij PFOS de belangrijkste bijdrage leverde (35-100%). Hoogste gehalten werden aangetroffen in aal uit het IJsselmeer bij Medemblik en het Ramsdiep (ten oosten van de Ramspolbrug). Voor PFAS’s zijn MLs vastgesteld die op 1 januari 2023 in werking gaan, deze zijn dus nog niet van toepassing op de gemeten gehalten in dit rapport.

Sexual size dimorphism (SSD) is caused by differences in selection pressures and life-history tradeoffs faced by males and females. Proximate causes of SSD may involve sex-specific mortality, energy acquisition, and energy expenditure for maintenance, reproductive tissues, and reproductive behavior. Using a quantitative, individual-based, eco-genetic model parameterized for North Sea plaice, we explore the importance of these mechanisms for female-biased SSD, under which males are smaller and reach sexual maturity earlier than females (common among fish, but also arising in arthropods and mammals). We consider two mechanisms potentially serving as ultimate causes: (1) male investments into male reproductive behavior might detract energy resources that would otherwise be available for somatic growth, and (2) diminishing returns on male reproductive investments might lead to reduced energy acquisition. In general, both of these can bring about smaller male body sizes. We report the following findings. First, higher investments into male reproductive behavior alone cannot explain the North Sea plaice SSD. This is because such higher reproductive investments require increased energy acquisition, which would cause a delay in maturation, leading to male-biased SSD contrary to observations. When accounting for the observed differential (lower) male mortality, maturation is postponed even further, leading to even larger males. Second, diminishing returns on male reproductive investments alone can qualitatively account for the North Sea plaice SSD, even though the quantitative match is imperfect. Third, both mechanisms can be reconciled with, and thus provide a mechanistic basis for, the previously advanced Ghiselin-Reiss hypothesis, according to which smaller males will evolve if their reproductive success is dominated by scramble competition for fertilizing females, as males would consequently invest more into reproduction than growth, potentially implying lower survival rates relaxing male-male competition. Fourth, a good quantitative fit with the North Sea plaice SSD is achieved by combining both mechanisms while accounting for sex-specific costs males incur during their spawning season. Fifth, evolution caused by fishing is likely to have modified the North Sea plaice SSD.

4 van 48 | Wageningen Marine Research rapport C012/23 Samenvatting Rijkswaterstaat heeft op de Eems een opgave om de toegangsgeul naar de Eemshaven bereikbaar te houden voor schepen met een diepgang tot 14m (Panamax). Rijkswaterstaat heeft een Duitse vergunning om de baggerspecie te verspreiden op verspreidingslocatie P0 in de Eems, op Duits grondgebied. Voorwaarden vanuit deze vergunning is dat 1) monitoring van de effecten op bodemfauna dient te worden uitgevoerd en 2) de ecologische impact op het gehele natuurgebied Borkum Riffgrund dient te worden vastgesteld. Monitoring van de bodemfauna is uitgevoerd in de jaren 2020, 2021 en 2022, met bemonsteringen in het voor- en najaar. Het verspreidingsgebied is sinds 2020 ieder jaar gebruikt in de periode tussen 1 juni en 31 september, de ruiperiode van de zwarte zee-eend. Dit rapport, een tweede tussenrapportage, beschrijft de resultaten van de monitoring en veranderingen in 2020, 2021 en 2022. Er is in alle jaren zowel in het voor- als najaar, na het verspreiden van de baggerspecie, bemonsterd met een van Veen-happer (infauna) en een Kieler Kinderwagen (epifauna), binnen het verspreidingsgebied en in een referentiegebied, deels ten oosten en deels ten westen van het referentiegebied gelegen. Na het verspreiden van baggerspecie wordt het sediment in het verspreidingsgebied fijnzandiger. Nadien neemt de mediane korrelgrootte weer toe. De mediane korrelgrootte is in het najaar is steeds significant lager in het verspreidingsgebied dan in het referentiegebied. Vooralsnog zijn er weinig verschillen in de bodemfauna vastgesteld gerelateerd aan het gebruik van P0 als verspreidingslocatie, wellicht omdat het om een soortenarm gebied gaat met lage dichtheden, zowel per soort als totaal. Opgemerkt moet wel worden dat een aantal factoren het trekken van deze conclusies bemoeilijkt: 1) het sediment en de fauna in het oostelijk en westelijk deel van het referentiegebied verschillen, 2) er is niet over de hele verspreidingslocatie verspreid, en 3) het onderscheidingsvermogen laag is, door de lage hoeveelheid bemonsterpunten. Omdat er geen aanwijzingen zijn voor enig effect in de referentiegebieden, is ook geen effect te verwachten op de bodemdiergemeenschap in het natuurgebied Borkum Riff, waar P0 ligt. Het onderzoeksgebied ligt binnen het gebied “Schiermonnikoog-Rottum-Borkum” waar soms grote aantallen zwarte zee-eenden kunnen zitten, als de voedselomstandigheden daar gunstig zijn (veel Spisula of jonge Ensis). Een locatie als P0 waar weinig voedsel voorkomt, zal voor eenden als foerageergebied niet aantrekkelijk zijn maar mogelijk wel als tijdelijke rustplaats. Eenden op die locatie zijn dan eenden die verstoord werden op de plek waar ze eigenlijk liever zitten en hun aanwezigheid is hier kort durend. Zolang daar geen rijke bodemfauna aanwezig is, is hier voor eenden niets anders te halen dan rust.

In order to quantify the amount of carbonate, precipitated as calcium-carbonate in the shells of blue mussel (Mytilus edulis) in a temperate climate, an existing Dynamic Energy Budget (DEB) model for the blue mussel was adapted by separating shell growth from soft tissue growth. Hereby, two parameters were added to the original DEB-model, a calcification cost [J/mgCaCO3] and an energy allocation fraction [-], which resulted in the energy allocated for structural growth being divided between shell and meat growth. As values for these new parameters were lacking, they were calibrated by fitting the model to field data. Calibration results showed that an Energy allocation fraction of 0.5 and a calcification cost of 0.9 J/mgCaCO3, resulted in the best fit when fitted on 2017 and 2018 field data separately. These values however, show the best fit for data obtained within the first couple of years of the shellfish life, and do not take later years into account. Also it could be discussed that some parameters vary throughout the lifespan of the species. The results were compared to a regular DEB model, where the shell output was calculated through a simple allometric relationship. It is sometimes assumed that the carbon storage in shell material as calcium carbonate could be regarded as a form of carbon sequestration, with a positive impact on the atmospheric CO2 concentrations. However, studies on the physical-chemical processes related to shell formation have shown that from an oceanographic perspective, shell formation should be regarded as a source of atmospheric CO2 rather than a sink. The removal of carbonates, through the biocalcification process, reduces the buffer capacity (alkalinity) of the water to store CO2. As a result CO2 is released from the water to the atmosphere when shell material is formed. The actual amount of CO2 that escapes from the water to the atmosphere as a result of biocalcification depends strongly on local water characteristics. In this study, the effect of calcification by mussels on the CO2 flux to the atmosphere is studied using an adapted DEB model where energy costs of calcification are modelled explicitly. The model was subsequently run under two future climate scenarios, (RCP 4.5 and RCP 8.3) with elevated temperature and decreased pH, and the total released CO2 as a result of shell formation was calculated with the SeaCarb model. This showed growth of mussels, under future climate conditions to be slower, and with that the cumulative shell mass and carbonate precipitated to CaCO3 to decrease. Yet the amount of CO2 released, due to biocalcification, increased. This is due to the fact that the amount of CO2 released/gr of CaCO3 precipitated will be higher, as a result of the decreased buffering capacity of seawater under future climatic environmental conditions. In summary the conclusions of the project were: • Biocalcification (shell formation) of marine organisms, such as bivalves, cannot be regarded as a process resulting in negative CO2 emission to the atmosphere; • The actual amount of CO2 that, due to biocalcification, is released from the water to the atmosphere depends on the physicochemical characteristics of the water, which are influenced by (future) climate conditions; • Our first model calculations suggest that at future climate conditions mussel’s grow rate will be somewhat reduced. While the amount of CO2 that due to biocalcification, escapes to the atmosphere during its life-time will slightly increase. Making the ratio of g CO2 release/g CaCO3 precipitated slightly higher; • Our model calculations should be considered an exercise rather than a definite prediction of how mussels will respond to future climate scenarios. Additional information/experimentation is strongly needed to validate the model settings, and to test the validity of the above mentioned outcome of the model.

Intertidal coastal sediments are important centers for nutrient transformation, regeneration, and storage. Sediment resuspension, due to wave action or tidal currents, can induce nutrient release to the water column and fuel primary production. Storms and extreme weather events are expected to increase due to climate change in coastal areas, but little is known about their effect on nutrient release from coastal sediments. We have conducted in-situ sediment resuspension experiments, in which erosion was simulated by a stepwise increase in current velocities, while measuring nutrient uptake or release in field flumes positioned on intertidal areas of a tidal bay (Eastern Scheldt) and an estuary (Western Scheldt). In both systems, the water column concentration of ammonium (NH4+) and nitrite (NO2−) increased predictably with greater erosion as estimated from pore water dilution and erosion depth. In contrast, the phosphate (PO43−) dynamics were different between systems, and those of nitrate (NO3−) were small and variable. Notably, sediment resuspension caused a decrease in the overlying water PO43− concentration in the tidal bay, while an increase was observed in the estuarine sediments. Our observations showed that the concentration of PO43− in the water column was more intensely affected by resuspension than that of NH4+ and NO2−. The present study highlights the differential effect of sediment resuspension on nutrient exchange in two contrasting tidal coastal environments.

Heatwaves affect tidal flat ecosystems by altering the bioturbating behavior of benthic species, with potential consequences for sediment oxygenation, particle mixing, and erodibility. Although the frequency and duration of heatwaves are expected to increase under global warming scenarios, we lack insights into how heatwaves' temporal dynamics affect bioturbating behaviors. Using the widely distributed bioturbator Cerastoderma edule as model species, we quantified how heatwaves with identical heat-sum but different temporal dynamics (i.e., 3- vs. 6-d heating and normal temperature cycles) affect bioturbating behaviors and the sediment mixing processes in tidal mesocosms. Our results show that short but frequent 3-d heatwave cycles increased the magnitude of bioturbating behaviors, thereby resulting in more bio-mixed sediment than observed under infrequent prolonged 6-d heatwave cycles. This unexpected result could be ascribed to the weakening health condition indicated by a high death rate (47.37%) under 6-d heatwave cycles than in 3-d and no-heatwave control cycles. Present findings reveal that the impact of heatwaves on sediment bioturbation will strongly depend on the temporal dynamics of future heatwaves: bioturbation will be enhanced unless the heatwave duration exceeds species resistance and increases mortality.