For the management of the shellfish sector, shellfish are regularly imported from various European countries into the Dutch coastal waters. With the import of shellfish, there is a risk of unintentional introduction of exotic species, pests and diseases that could have harmful effects. Therefore, the transports or shellfish are watched critically by the various stakeholders. The present study focuses on the risks of the unintentional introduction of new exotic species with the shellfish transports. We define exotic species as species that are non-indiginous for the Northeast Atlantic continental shelf area and are introduced into this area by human activities. The biogeographical area Northeast Atlantic continental shelf region ranges from the Northern part of Spain to the coastal areas of Norway. Once established somewhere in the Northeast Atlantic continental shelf, shellfish transports can accelerate the introduction into the Dutch coastal waters. Far from every introduction of exotic species becomes a pest. However, once introduced it is very difficult if not impossible to remove the species from the ecosystem. In this study, the risks of unintentional introduction of exotic species with various shellfish transports are evaluated. The Dutch Ministry of Agriculture, Nature and Food Quality will use this information to develop a new national policy for the transfer of shellfish. Recently, the debate on the present policy that dates from 1997 has intensified due to judgments of the Council of State and the European Court of Justice. Moreover, the shellfish farmers are more and more interested to transfer mussels and seed mussels that have been captured with mussel seed capture devices from the Oosterschelde and the Voordelta to the Dutch part of the Wadden Sea. This transport is prohibited under the present policy. The invasiveness of a species is an important factor in a risk assessment for the introduction of exotic species. The list â100 of the Worldâs most invasive alien speciesâ gives a good overview of the species with the highest risks. As a first indication this list is very useful, however, it should not be assumed that the list is complete. Besides as assessment on species, the risks can also be assessed on the location from where the shellfish are imported. Most of the exotic species that are present in the Dutch coastal waters have their area of origin in NW Pacific (39 species) or the NW Atlantic Ocean (26 species). The majority of he exotic species are introduced the Dutch coastal areas by secondary transport. Shellfish imports from Ireland, UK and France show the highest risks on introducing exotic species due to the number of exotic species in these countries and the environmental conditions that are comparable to the conditions in the Dutch coastal waters. According to the national policy on shellfish transfer, it is obliged to transfer shellfish from the Oosterschelde and Voordelta to the Dutch part of the Wadden Sea (South â North transport). The risks of this transport on introduction of exotic species have been evaluated by means of expert judgment and literature review. A group of 11 national and international experts have been consulted for this study. A list of exotic species of the South (Oosterschelde, Voordelta, Grevelingenmeer and Veerse Meer) has been compiled and compared with the list of exotic species present in the North (Dutch part of the Wadden Sea). The target species, species that are present in the South but not in the North could potentially be introduced in the Dutch Wadden Sea with the shellfish transfer from the South. The risks of each of the 65 target species have been evaluated semi-quantitatively from the chance of successful introduction and the expected impact. The species with the highest risks are the tunicates Didemnum sp. and Botrylloides violaceus, the copepods Mytilicola orientalis and M. ostreae, the American whelk tingle Urosalpinx cinerea, the bryozoa Smittoidea prolifica, the protist Marteilia refringens and the marcoalgae Gracilaria vermiculophylla, Polysiphonia senticulosa and Undaria pinnatifida. At present it is allowed and common practice to transfer mussels from the Danish and German parts of the Wadden Sea to the Dutch part of the Wadden Sea. The risks assessment indicates that with these shellfish transfers 46 exotic species could potentially be introduced into the Dutch part of the Wadden Sea. The species with the highest risks are mainly macroalgae: Gracilaria vermiculophylla, Heterosiphonia japonica and Laminaria ochotensis. The risks of shellfish transfers within the Wadden Sea are lower than the risks of shellfish transfer from South to North. During the Summer of 2008, 30 samples have been taken from mussels in the water column (rope culture and Mussel seed capture devices) and from the bottom (culture plots and natural beds), both in the Oosterschelde and the Voordelta. The samples have been analyzed for associated flora and fauna. Special attention was paid to the exotic species. In total 12 exotic species were identified in the samples. In the samples from the Voordelta only one exotic species was found (Sargassum muticum). The exotic species accounted for more than 10 percent of the total number of species identified in samples from the Oosterschelde. There was a clear distinction in species composition between the samples from the water column and from the bottom. Characteristic species for the bottom samples were the American slipper limpet (Crepidula fornicata), the Pacific oyster (Crassostrea gigas), the brush-clawed shore crab (Hemigrapsus takanoi) and the polychate Syllis gracilis. Characteristic species for the suspended culture are the macroalgae Sargassum muticum, Codium fragile and Undaria pinnatifida and the tunicate Styela clava. The present study is primarily focused on exotic species that might be introduced with shellfish transports. However, also other harmful not-exotic organisms such as toxic algae, pests and diseases could be unintentionally (re-)introduced with the shellfish. Based on the results of this study the research questions as formulated by the Ministry of Agriculture, Nature and Food Quality can be answered as follows: 1. Shellfish import from France, Ireland and the UK constitutes the highest risk at introduction of exotic species with shellfish transfer into the Dutch coastal waters. The risks of import from Ireland and UK to the Oosterschelde have been quantified in a previous study, the PRIMUS study. 2. The list with â100 of the Worldâs most invasive alien speciesâ gives a good indication of the invasiveness, and thus potential impact of species. However, the list is not exhaustive. 3. The risks of introducing exotic species with shellfish transfer from South to North are not absent. The report gives an overview of risk species and a quantification of the risks. The number of exotic species associated with rope culture and mussel seed capture devices in the Oosterschelde corresponds to the number of exotic species associated with the bottom culture. However, the composition of the exotics differ. 4. The risks of introducing exotic species with shellfish transfer within the Wadden Sea (Danish and German Wadden Sea to the Dutch Wadden Sea) is less than the risks of shellfish tranports from the Oosterschelde and Voordelta to the Wadden Sea. The report gives an overview of the risk species and a quantification of the risks.