Officials from the Environment Agency have culled more than 100,000 alien fish from a pond in Surrey.
The non-native fish, which are members of the carp family, were culled last week at Juniper Pond in Shalford on behalf of Guildford Borough Council.
The tiny Topmouth gudgeon or Clicker barbs, Pseudorasbora parva, which are less than a couple of inches in length, are one of the UK's most invasive non-native fish species.
The Environment Agency said: "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."
FloodplainJuniper Pond is on the River Wey floodplain and experts feared that flooding could result in the fish being washed into local rivers, where they could have spread to the Thames river system and threatened a wider fish population.
The fish were killed with the poison Rotenone, which is non-specific, so also resulted in the death of other fishes in the pond, including hundreds of native carp and tench. The dead fish were removing by fisheries officers using nets.
Fisheries and Biodiversity team leader John Sutton 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 Environment Agency claims that the effects of Rotenone are short lived and that it biodegrades in days, leaving an environment that is safe for other organisms. They also claim that it will not have an effect on birds or other predators which feed on the dead fish.
Fishkeepers blamedSutton believes that the fish originally entered the pond after being dumped by a fishkeeper: "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 have been illegal to keep in the UK for several years and are now no longer sold in the aquarium trade.