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Asian killer fish found in stream

An invasive species of aggressive Asian fish has been found in an Aarhus river

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It may be pretty and small, but it’s an angry little devil and it certainly isn’t welcome in the Danish waterways where it has now been found.

A mere 12 centimetres long when fully grown and with the attractive looking name of pseudorasbora parva, a shoal of the savage topmouth gudgeon has now been found by experts from the National Museum of Natural History in a tributary to Aarhus River.

“By attacking again and again it bites right down to the spine of its prey. In 2008 we had a six centimetre long topmouth in an aquarium. In less than a day it killed both the other fish in the tank – two 12 cm tenches,” says Henrik Carl of the Fish Atlas Project at the National Museum.

A member of the carp family, the topmouth gudgeon matures after one year and females spawn up to 14 times in a season – multiplying quickly and becoming a danger to the natural balance in waterways and lakes.

“In Greece there are examples of gudgeons almost driving other species to extinction and in Belgium and Poland the species is seen as an invasive one. There is every reason to be worried,” Carl says.

Originally from East Asia, gudgeons have previously been seen in North Jutland, but the Aarhus population is believed to have escaped somehow from a garden pond.

“It’s more likely they have come from a garden pond as gudgeons are often sold at garden centres as an ornamental variety of fish. It’s worrying when you know how big a problem they can be. It may be a pretty little fish, but its temperament makes it unfit as a pond fish and it is certainly not welcome out in nature,” Carl says.

In January this year, British environmental authorities in Wales continued a programme to eradicate the topmouth gudgeon from lakes in and around Llanelli, and from some 25 other sites in the country. Apart from its aggressive nature, gudgeons are also said to carry disease that spread to other native species.

In some cases, park lakes have been drained in order to remove native fish before a piscicide is sprayed into the lake to kill off the gudgeons..

“Primarily found here in still waters, the topmouth gudgeon can reproduce rapidly with numbers in excess of 65 individuals per square metre. At these levels, competition with the native fish species for food and spawning habitat is a major concern,” Environment Agency Wales says.

It adds that eradication of the species is both costly and extremely difficult.

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