A DEADLY parasite spread by a foreign species of fish may be threatening Britain's salmon and fresh water fish, researchers have found.  Biologists say they have found the rosette agent, a disease that has caused widespread damage in the United States, in a UK waterway for the first time.

They have shown the disease, which is carried by an invasive fish called the topmouth gudgeon, can cause 90% mortality in salmon species found in British rivers. It can also cause high numbers of deaths in other fish including the common carp, beam and roach. They are now calling for widespread testing of Britain's waterways for the disease so it can be controlled.

Professor Rudy Gozlan, said: "We have shown that British salmon are not just vulnerable to this parasite but suffer 90% mortality.

"We knew from the experience in the United States, where the parasite has been a problem for some time, that it can cause high mortality, but this shows just how susceptible British species are."

Topmouth gudgeon, which are originally from Asia, have spread rapidly through out Europe since they were first introduced in the 1960s and came to Britain in the 1980s to be put in ornamental ponds, but have since entered the wild.              –TG

They breed four times faster than native British fish and spread rapidly.

In research published in the journal Parasitology, Dr Demetra Andreou and Professor Gozlan show that sunbleak, a fish in the same family as carp introduced from elsewhere in Europe, from a pond in North Stoneham, Hampshire, are infected with the parasite.

Rosette agent, also known as Sphaerothecum destruens, is a microorganism that infects the cells of fish, destroying their internal organs.

Researchers claim there are at least 26 isolated lakes, ponds and watercourses in England and Wales that are infested with topmouth gudgeon.

The fish, which grow to be about an inch to two inches long, also eat the eggs and food of Britain's native freshwater species.

The disease has caused widespread damage in the United States with more than 80% of salmon on fish farms being killed by the parasite.

Until now, the parasite had never been detected in the UK. It is feared rivers most at risk of becoming infested with topmouth gudgeon include the Test in Hampshire, Tadburn Lake in Hampshire, the river Lee, near Luton, and stretches of the Trent and Severn.

Testing of British rivers is currently carried as part of a five year rolling programme and the Fish Health Inspectorate says it has yet to identify the rosette agent in any native fish.

The Environment Agency also screens fish for diseases following reports of disease-related deaths. The Environment Agency has been leading a major eradication programme against the fish in an attempt to control their numbers and has removed hundreds of thousands them from eight waterways.

A spokesman said: "Measures are already in place to monitor and investigate fish diseases in the wild and we will continue to evaluate the risks posed by non-native parasites as and when new evidence becomes available."