IberiaNature A guide to the natural history of Spain
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Invasive fish species in Spain

Invasion of alien fish

Over the last 100 years alien fish have been released in the rivers, lakes and reservoirs and Spain , sometimes accidentally, normally on purpose. The result has totally altered the ecosystems of Spanish rivers. An article in today's La Vanguardia looks at the problem in Catalonia . The trend is thought to be irreversible.

In many Catalan river basins the number of invasive species is greater than autochthonous ones. In total Catalan rivers are home to 30 native species and 22 foreign species of fish. The rate of propagation is estimated at one new species per year, and is thought to be increasing. Some of these new fish are extremely well adapted to the conditions in Catalan rivers, and in situations of contamination or eutrophication and are progressively pushing out local species.

Species to have colonised the rivers and lakes here in the last 15 years include bream (brema), bleak (alburno), river perch (perca de río) and pikeperch (lucioperca). Older introductions include the wells catfish (siluro), pike (lucio), sun fish (pez sol) and the mosquitofish (gambusia). The latter is one of the few examples of a positive introduction. The gambusia was released in 1921 to combat malaria.as it is a voracious devourer of mosquito larva. It took rapidly took to Iberian waters and played no small part in the erradication of the disease over the next 40 years. Incidentally it is now probably the most widespread freshwater fish in the world.

See History of Malaria in Spain

The appearance of many of these fish has been due to the arrival of fishing tourism. In 1974, a German biologist and fishing aficionado, one Roland Lorkowsky, released a few thousand fry of Wels Catfish ( Siluro - Silurus glanis ) into the River Ebro at Flix. The fish adapted frighteningly well to the warm murky waters and has now extended its range right up the Ebro basin, and into the River Segre (Lleida) and the River Cinca (Huesca). It has also colonized Lago Banyoles and Lago de Sau (Girona). The results have been catastrophic. In many stretches, the fish has almost completely wiped out several autochthonous species with its voracious appetite, hence its local nickname, 'The Ebro Monster'. The world record weight for Wels Catfish is 306.0 kg and individuals of up to 2.3m have been caught. Here they don't get quite so big, but their maximum size of 100kg and their fighting spirit attracts those who like that sort of thing. It is a bottom feeder and feeds at night on fish, ducks, voles and crayfish.