The Environment Agency has successfully culled more than 100,000 top mouth topmouth gudgeon from a pond in Surrey.

The fish, which are one of the most threatening invasive species in the country, were culled at Juniper Pond, Shalford, on behalf of Guildford Borough Council, on Wednesday.

Despite being only 3-4 centimetres long, topmouth gudgeon are particularly threatening to native fish in Britain, and are number seven on the Environment Agency’s list of most wanted invasive species. As well as eating the eggs of other fish, topmouths, which were introduced to the wild in Britain in the 1960s from their native Asia, breed at four-times the rate of native fish and carry a parasite, similar to, or possibly the same as, a disease called “rosette agent” which is found in some US fish stocks

Topmouth Gudgeon also eat vast quantities of plant life and invertebrates, which upsets the natural balance of a pond, lake or river. As a result, algal blooms can occur turning the water into a green soup. Had they been left to thrive in the pond, it was feared that they would have seriously affected the ecology of the pond.

The pond is also in the River Wey floodplain and it was feared that a larger flooding event could have washed them into local rivers, where they would have spread to the Thames and threatened a much wider fish population.

The operation involved the use of a fish-orientated pesticide, Rotanone, which killed all the fish in the pond, including several hundred native species such as tench and carp. The fish were then removed from the pond by fisheries officers using nets.

John Sutton, fisheries and biodiversity team leader, who led the operation, said: ”It is very sad for us to have to kill native fish along with the topmouth gudgeon, but we have to protect the long term future of our native species.

“Topmouth gudgeon are extremely difficult to get rid of by other means. Using nets to catch them, or transferring other fish out of the pond before the culling takes place is too risky as topmouth fry, which can be the size of an eyelash, can be hidden away in the mouths and gills of other fish and get accidentally transferred to other waters.

“Culling all the fish is the only real guarantee that the topmouth gudgeon can be removed completely and, while it was a tough decision to make, we believe this was the right thing to do.”

The pesticide used in the operation will not have any impact on other wildlife in or around the pond, such as birds or other predators feeding on the fish. It naturally breaks down in the water, and water quality will return to normal after a couple of days.

Mr Sutton added: “It is not clear how the fish first came to be in the pond, but they were originally imported into this country for ornamental aquariums. I suspect an owner may have had trouble keeping them and illegally released them into the pond, an offence which could result in a fine of up to 2,500 and serious damage to the environment.

“We also found goldfish, a common pet in many homes and gardens, but again, non-native to this country and a threat to native fish due to the parasites it often carries and its ability to cross breed with native species such as crucian carp.

“We want to stress to people who own fish that the apparently harmless action of tipping an unwanted fish into a local pond such as this can have disastrous long-term effects on the environment and other animals within it. Hopefully, however, we have eradicated the topmouth gudgeon in this pond and ensured that when it is re-stocked with native fish they will have a much brighter future.”

• Topmouth Gudgeon is listed as a category 5 risk species under the Importation of Live Fish Act (ILFA) which is the highest level of risk.
• The parasite they carry is believed to interfere with the breeding cycle of salmon and trout.
• It is an offence to introduce any fish or eggs of fish into the wild without the prior written consent of the Environment Agency.

Rotanone is not harmful to humans or other animals, but fisheries officers carrying out the work wore protective suits to avoid any potential skin irritations.

Picture 1 shows fisheries officer Dan Hatherley-Hurford with top mouth gudgeon from the pond.

Picture 2 shows Environment Agency fisheries officers spraying rotanone at the lake.

Other pictures are available by calling the press office on 0118 953 5555.