Alien species

Posted: 20 March 2008

Another form of pollution is the introduction of exotic, or non-native species into marine environments. These marine invaders are on the increase in all the world's seas, probably because of the increase in international shipping.

Though global figures of the number of exotics that have taken up residence in new waters is not known, scientists have determined that the number of invasive marine plant and invertebrates in European and North American waters has increased from around 25 in 1900 to over 175 by 2000.


Invasive species pathways
Invasive species pathways
The major pathways and origins of invasive or exotic species infestations in the marine environment. These patterns are strikingly concurrent with major shipping routes. Click on image for full-size map.

Most exotics, from micro-organisms to fish, hitch-hike in the ballast waters and cargoes of ships. Their patterns of dispersal closely follow the world's major shipping lanes, while areas of bottom trawling, pollution and other stress factors also play an important role in providing space for exotics to move in. Some 60 introduced species have been found in the Baltic Sea. In some parts of the Black Sea, up to 90 per cent of the marine biomass consists of the voracious comb-jelly, Mnemiopsis Leidyi, which feed on the eggs and larvae of many commercially valuable species of fish.