High risk exotic species with respect to shellfish transports from the Oosterschelde to the Wadden Sea

This study presents the results of a literature review on 11 exotic marine species that are present in the Oosterschelde and could potentially be introduced into the Wadden Sea with shellfish transfers. Ten of the species result from a previous risk study, where they were identified as the species posing the highest risk. The Japanese oyster drill, that was recently observed in the Oosterschelde was also included in this study. The species discussed in the present study are: colonial tunicates Didemnum sp and Botrylloides violaceus the American oyster drill (Urosalpinx cinerea) and the Japanese oysterdrill (Ocinebrellus inornatus), the copepods Myticola orientalis and M. ostreae, the colonial bryozoan Smittoidea prolifica, Marteilia refringens and the macroalgae Gracilaria vermiculophylla, Polysiphonia senticulosa and Undaria pinnatifida. Four of these species (Gracilaria vermiculophylla, Undaria pinnatifida, Didemnum vexillum and Botrylloides violaceus) have recently been observed in the Wadden Sea. Some of these species are known pest species and could have an impact on the Wadden Sea ecosystem and/or the shellfish culture. The information that is collected in this study can be used to get a more realistic estimation of the risks compared to the risk assessment study of 2008 which was based on a worst-case approach. Moreover, the results can be used in the development of mitigating measures to reduce the risks of introducing these exotic species with the shellfish transfer from the Oosterschelde to the Wadden Sea.