This research presents the invasion history as well as an up-to-date distribution of the Pacific oyster Magallana gigas in European coastal waters, concluding that this invasive species has spread to large parts of all coastal biotopes. Moreover, the measures and management of the Pacific oyster invasions of coastal and marine eco-systems is discussed including restoration and resilience of invaded ecosystems. When Magallana gigas is well established and has reached adjustment phase characterized by firm biogenic reefs, eradication or even deci-mation is difficult and costly. During the establishment phase and maybe also the expansion phase, where the oyster appear as single individuals spread out on the sediment and yet not forming larger clumps and reefs, it is more realistic to implement mitigation tools. These initiatives cannot be implemented everywhere. They should be reserved especially for protected areas under national or international legislation e.g., Natura2000 areas, where smaller areas can be restored. The present contribution discusses and evaluates known mitigation tools like physical damaging of individual oysters, dredging, small-scale hand picking as well as various fishery concepts. Additionally, this research proposes some novel strategies where triploidity is suggested for imple-mentation to avoid or decimate further spread of the species. As well as some initiatives based on choking and/or starving out the oysters by deploying a thick layer of sand/gravel or food for the competing blue mussels over oyster reefs. The efficiency of the different strategies, as well as the required frequency of follow up initiatives, are evaluated. The major conclusion from this research suggests that decisions to combat the alien oyster are mainly based on political/socioeconomic arguments and less on ecological arguments. This is because reefs of Pacific oysters most likely lead to equal or higher biodiversity as compared to native (e.g., blue mussel) beds and the oysters serve a redundant role in a trophic sense living from same resources as native bivalves. Moreover, oyster reefs serve ecosystem services as protecting shores against erosion and flooding. The most negative impact reported is on some mussel feeding birds that may have a reduced access to food. If eradication/decimation is decided, several mitigation instruments should simultaneously come into play and often repeating treatment strategy is necessary.

Denne rapporten utreder sentrale sporsmal og fagomrader knyttet til hosting av stillehavsosters, som fremmed art og ny ressurs. Arten har etablert seg i nordiske kystfarvann. Den er regnet som inva ...

The Pacific oyster has established populations in Scandinavian coastal waters. The species is considered alien and invasive and dense populations may cause changes in benthic habitats. It is, howev ...

Stillahavsostronet har sedan 2007 etablerat sig i de nordiska kustvattnen. Arten anses som frammande, invasiv och darmed oonskad. Tata bestand kan leda till andringar i bottenhabitat. Samtidigt ar ...

Siden 2007 har stillehavsosters etableret stabile bestande i skandinaviske farvande. Arten anses som fremmed og invasiv, og er saledes uonsket. Kompakte bestande kan forarsage aendringer i bundforho ...

Foraging for oysters in the protected Danish Wadden Sea National Park is an increasingly popular tourism experience. The abundance of the Pacific oyster is high and rising. As it is an invasive species, visitors' oyster picking is not only attractive for culinary reasons, but will also—when organized adequately—benefit nature, particularly in terms of living conditions for migrating birds. Creating a symbiosis between touristic and environmental objectives requires coordinated governance systems. This study demonstrates that, seen from the point of tourism, there are significant incompatibilities in the rules, regulations, and institutional set-ups that govern nature protection, food safety, and access to recreational foraging and commercial fisheries. Governance innovations are essential to ensure an environmentally sustainable use of the oyster resource and, simultaneously, a positive touristic development and a commercial oyster fishery in the local communities. Three interacting trajectories of interventions and policies are outlined: the tour guide-dominated trajectory, a coastal fisheries cluster trajectory, and the visitor empowerment trajectory. It is concluded that the traditional tourism sector cannot dissolve the paradoxes, but needs dedicated power contributions from a wider range of governance institutions and local actors.

The habitat-modifying suspension-feeding mussel Mytilus edulis may have facilitating or inhibiting effects on seagrass meadows depending on the environmental conditions. We investigated the effects of M. edulis on sediment biogeochemistry in Zostera marina meadows under eutrophic conditions in Flensborg fjord, Denmark. Sediment and plant samples were collected at 5 stations with Z. marina (Eelgrass), 5 with Z. marina and M. edulis (Mixed), and at 2 unvegetated ones, 1 with mussels (Mussel) and 1 with sand (Sand). The Mixed sediment was en riched in fine particles (2 to 3 times), nutrients and sulphides compared to Eelgrass stations. In creased sediment nutrient availability at the Mixed stations was reflected in increased N and P content in eelgrass. However, the plant biomass did not differ significantly between stations, while shoot features (number of leaves and leaf area) were significantly reduced at Mixed stations, suggesting an inhibiting effect of M. edulis on Z. marina. Negative correlations between eelgrass measures and sediment sulphide at Mixed stations indicate that the presence of mussels increases sulphide invasion in the plants. A survey of 318 stations in Danish fjords suggests a threshold of 1.6 kg M. edulis m−2 beyond which no coexistence between Z. marina and M. edulis was found.

Management of invasive species is addressed in both national and international regulations regarding the protection of marine habitats and biodiversity and in regulations of aquaculture. The geographical range of the invasive Pacific oyster, Crassostrea gigas, is expanding, both through human mediated vectors and by natural dispersal. The species is now spreading in Scandinavia. In order to optimize the management of the oyster, including targeted monitoring and mitigation activities, knowledge on the present and future distribution and impact on the ecosystem is important. The development of the population and the potential impacts on native ecosystems were analyzed, based on the present scientific knowledge on the distribution in Scandinavia, data on new settlements and existing literature. Data was first evaluated by 14 experts (including the authors) during a workshop, relating the current status of habitats where Pacific oysters are found in Scandinavia (Low energy rock, Littoral sand and mudflats, Littoral biogenic reefs, Sublittoral sand and Sublittoral biogenic reefs) to a predicted development, thereafter assessed in relation to impact on the habitats. The assessment was done as a function of climate change in a longterm IPCC climate scenario (A1B). We conclude that Littoral biogenic reefs are at risk to obtain the highest expected increase, while all other habitats are at risk of low to moderate development of the oyster populations. Accordingly, Littoral Biogenic reefs was assessed as the habitat type at risk of the largest ecosystem effects as high densities of oysters already exist in these areas, and the densities are expected to increase rapidly until reaching a threshold density. Low energy rock and Littoral sand and mud were assessed as being subjected to moderate to high ecosystem effects. Sub-littoral sand and Sub-littoral biogenic reefs were assessed as currently being at risk of moderate ecosystem effects as there are low densities of oysters in these habitats, although densities in sublittoral biogenic reefs has the potential to increase. We discuss management and mitigation strategies based on the forecasted development and effects of the Pacific oyster populations.

At an approximately 12000 m2 sheltered intertidal bivalve bed in the western part of the Limfjord, Denmark, the Pacific oyster Crassostrea gigas co-occurs with the blue mussel Mytilus edulis. The relative significance of the impact of the 2 species on phytoplankton density during a tidal cycle was estimated by combining field measurements of clearance rates and modelling of the bivalve bed (topography, biomass distribution, temporal and spatial water coverage and depth). The average density of C. gigas and M. edulis was 35 ± 36 and 1001 ± 685 ind. m-2, respectively. The water volume cleared during a tidal cycle was estimated at 45838 m3, of which C. gigas and M. edulis contributed 9169 and 36669 m3, respectively. Therefore, M. edulis contributed 4 times as much as C. gigas to the bivalve bed’s clearance, and the 2 bivalves were estimated to clear the water volume 1.9 times during each tidal cycle. However, the estimated water column cleared during low tide is overestimated due to phytoplankton depletion. Hence, it is concluded that the bivalve bed clears the water close to 1 time each tidal cycle. This, together with a low dry weight of soft parts, indicates that the bivalve bed, in general, is food-limited.

The invasive Pacific oyster, Crassostrea gigas Thunberg, 1793 was introduced in Denmark for aquaculture in the 1970s.Presently, feral populations are found in many parts of the country, with the largest populations established on existing beds of blue mussel, Mytilus edulis Linnaeus, 1758.This study was conducted in the Limfjord estuary, at Agger Tange, where C. gigas was introduced in 1972.The study site is a large cluster of raised intertidal bivalve beds inhabited by C. gigas and M. edulis in a sheltered part of the estuary.The two bivalves have some of the same living requirements, and as C. gigas have been present in the ecosystem for more than 40 years, we hypothesize that the presence of C. gigas has altered the spatial and temporal distribution of M. edulis by inducing a niche separation.The spatiotemporal development of the bivalve bed was determined using orthophotos.C. gigas and M. edulis were collected from the bivalve bed, shell lengths were converted into biomass, which were interpolated to create biomass contours and combined with modelled topography of the bivalve bed to study niche separation.The bivalve bed slowly extended northwards over a period of 11 years, where it also became more fragmented.The northern part of the bed was composed of mussel mats on top of soft sediment.This area was dominated by M. edulis, while areas in the south were dominated by C. gigas.In the southern part, the bivalve bed was composed of thick and compact sediment suggesting it represent the oldest part of the bivalve bed.There were no differences in the conditions of C. gigas and M. edulis from old or newly established areas, and there were no difference in the vertical distributions of the bivalve species.Thus, spatial and temporal separation of the two species is not pronounced at present, and thus unable to explain why they seemingly coexist.

Dredging blue mussels (Mytilus edulis) and thus removing structural elements, inducing resuspension of sediment as well as reducing filtration capacity, will inevitably affect the ecosystem. The study demonstrates that the impacts of fishing can be reduced through gear developments. A new light dredge was tested on commercial vessels using two different experimental setups. First, a twin haul experiment tested the standard gear (i.e., a Dutch dredge) against the light dredge by fishing the two gears side by side onboard the same vessel. Second, a single dredge experiment tested the absolute performance of the two gears by fishing in areas with a known blue mussel density. Results from the twin haul experiment demonstrate that the weight of sediment retained in the gear per square metre fished is 49% less in the light dredge compared with the Dutch dredge which will reduce resuspension of sediment at the surface. Also, the drag resistance of the light dredge was significantly less (177.1 vs. 202.7kgm(-1)). In the twin haul experiment no significant difference was found in the catch per unit effort (CPUE) of the two gears. The single dredge experiment, on the other hand, demonstrated a significant increase in CPUE exceeding 200% when using the light dredge. Seafloor tracks made by the two dredges could not be distinguished by use of side-scan sonar and the tracks were still detectable2months after fishing. It was concluded that replacement of the Dutch dredge with the light dredge would reduce the impact of the fishery on the ecosystem by (i) reducing resuspension of sediment, (ii) reducing fuel consumption, and (iii) potentially reducing energy transfer to the sediment through a reduced gear drag resistance. A potential increase in catch efficiency may reduce the area affected. Fishing with the light dredge is discussed in relation to management of Natura 2000 sites. Copyright (c) 2014 John Wiley & Sons, Ltd.