Non-native species invasions

The introduction of harmful aquatic organisms to new marine environments is believed to be one of the four greatest threats to the world's oceans. An alien or non-native species is one that has been intentionally or accidentally transported and released into an environment outside of its historic or resident geographical range or habitat. Such species are described as 'invasive' if they are ecologically and/or economically harmful. Most accidental marine species invasions are transported in ships' ballast water, and associated sediments, from fouling on ships' hulls and through aquaculture. Ballast water is the water taken in by ships for stability and is usually released on arrival at the destination port. Invasive species can dramatically change the structure and function of marine ecosystems by changing biodiversity and eliminating vital components of the food chain.

Over 100 non-native species have been recorded in the NE Atlantic, mainly in the North Sea, the Celtic Sea, the Bay of Biscay and along the Iberian coast. Over the past twenty years, the number of alien species transported into the Baltic has increased and poses a significant threat to the region given its naturally low species diversity. At least 50 non-native species have entered the Black Sea in the last century. Some have been invasive e.g. the comb jelly Mnemiopsis leidyi was the primary cause of collapse of the fisheries in the area in the early 1990s.