The American mink is in every river in Wales - pushing some native species to the brink of extinction, some conservationists are warning.
The non-indigenous predator was introduced into Britain in the 1920s for fur farming - but many escaped.
The British Association for Shooting and Conservation (Basc) claims the animals have decimated populations of lapwing birds and water voles.
It is trying to promote tracking, capture and eradication of them.
"There's been a tremendous hit on the water vole population. Records show there were nine million water voles in the UK - that's now down to a million," said Meurig Rees, from Basc.
"Mainly because of the mink - it's a killing, eating machine which doesn't belong in the UK - it's a non-native invasive species from America."
Mr Rees has been placing "mink rafts" and traps on land up and down Wales, in a bid to control the animal.
"A mink raft acts as a monitor, which encourages mink to leave evidence they've paid a visit in the form of footprints left in a clay and sand mix," he explained.
"Once a mink is detected, the raft on which it left its tracks also becomes the best place to set a trap."
He bluntly adds: "The animal is then dispatched, in other words, shot."
According to Basc, the extermination of the mink has the support of many animal welfare bodies - backed by strong scientific grounds supporting this method of control.
Fur farming was made illegal in the UK in 2000 and it is claimed the mink population increased in the wild even further.
It is against the law to release non-native animals such as grey squirrel or mink under the Wildlife and Countryside Act.
The RSPCA has previously said any culling of invasive species should be carried out by "trained operators using humane, established methods".
Landowner Charles Grisedale, who runs a lapwing reserve near Pennant in Ceredigion, said he is considering placing a floating mink raft and traps on his lakes to protect the ground-nesting bird, which has also seen a 90% decline in numbers.
"It's a constant fight to save the lapwing, we will lose them in Wales and this is a Welsh bird which has to battle many factors to stay alive including the non-native mink," said Mr Grisedale.
"It's a natural born killer, if it's gets in with your chickens, they'll all be dead. If you're walking a little Chihuahua or other small dog along a river bank it'll go for that and try and kill it.
"It's a mass killer with very few predators in the UK."
While there is no easy way to estimate the population of American mink, Basc said it is now in every river.
Mr Grisedale added: "They now rule our river beds, with a reign of terror over the other animals in the water and the surrounding land."