This region has 168 non-indigenous and cryptogenic (obscure or of unknown origin) species. The majority (148 species) were found between 1950 and 1999. Since 2000, a total of 37 new species have been recorded, of which approximately a third (14 species) are recorded in Europe for the first time. The annual rate of discovery of non-indigenous species was ≈ 3.0 species in 1950–1999 and ≈ 2.5 species in 2000–2014.
The principal vectors/pathways transmitting species into this region are thought to be vessels (biofouling and ballast water), regional oyster and mussel stock movements, and commercial activities as well as water currents (secondary spread from neighbouring areas).
The most impacting species are oyster disease agents and predators which result in summer mortalities of Pacific oyster stocks and oyster diseases that have led to the demise of the native flat oyster production. Other species cause trophic competition with native species, changes in communities, habitats and ecosystem functioning, and fouling problems.
Annual rate of new non-indigenous and cryptogenic species discoveries in the Celtic Seas during 1950–1999 and 2000–2014. Click image to enlarge.