The Northern European waters are amongst the world's most important fishing grounds. The regular recordings of the American ctenophore Mnemiopsis leidyi since 2005 in this area alarmed the scientific community, as severe ecological and economic impacts were registered in the 1980s and late 1990s after this non-indigenous species settled in the Black and Caspian Sea, respectively. In order to improve our knowledge of M. leidyi and its impact in the Eastern English Channel and Southern North Sea region (EEC-SNS), a detailed survey was carried out along the French, Belgian, Dutch and English coasts during the blooming period of M. leidyi in October 2012 within the InterReg-2Seas MEMO project (2010-2013). High densities of M. leidyi were observed in the Scheldt estuary and along the Dutch coastal waters, correlated with very low mesozooplankton densities, particularly copepods. This was in contrast with the absence of M. leidyi along the English coast and in the English Channel, where high mesozooplankton densities were recorded. Bloom densities in the Dutch waters were comparable to those recorded in the Black Sea and the native (American) regions. We argue that M. leidyi can be a significant competitor of native zooplanktivores, and can seriously impact larval densities of economically important bivalve and fish species, such as mussels, oysters, herring and sprat.
Keywords: Mnemiopsis leidyi, spatial distribution, plankton typology, English Channel, Southern North Sea
