The Environment Agency has successfully culled more than 100,000 topmouth gudgeon from a lake and nearby ditch at Eversley in Hampshire. The fish,which are one of the most threatening invasive species in the country, were culled in a two day operation on Wednesday and Thursday this week. 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 hit list of most wanted invasive species.

Topmouths were introduced to the wild in Britain in the 1960s from their native Asia. As well as eating the eggs of other fish they breed at four-times that rate of native fish and carry parasites, similar to 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 lake and ditch, it was feared that they would have seriously affected the ecology of the waterbodies.

The lake and ditch are in the Loddon floodplain and it was feared that they could spread to local rivers and threaten a much wider fish population.The operation involved the use of a fish-orientated pesticide, Rotanone, which kills all fish in the treated waterbody, including native species.
Environment Agency fisheries officers reduced the water level in the lake before treating it and were able to rescue a number of large  carp, which are being kept in isolation tanks to ensure no live topmouth gudgeon are accidentally transferred.
Unfortunately several hundred roach and carp that were too small to safely isolate were left in the lake and were killed by the treatment. The dead fish were then removed from the pond by fisheries officers using nets.

Dominic Martyn, the fisheries officer 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. That’s why we were only able to save the larger carp. They can be closely inspected and monitored to ensure they are free of topmouth gudgeon before restocking.

“Culling all the remaining 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 lake and ditch, 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 Martyn added: “It is not clear how the fish first came to be in the lake, but they were originally imported into this country for ornamental aquariums. “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 case and ensured that when the lake is re-stocked with native fish they will have a much brighter future.”