The aggressive signal crayfish which spreads a deadly fungus-like plague has almost entirely wiped out its smaller and altogether more genteel British cousins.
Nowhere is more badly affected than the South West which is why ecologists led by the Bristol Conservation and Science Foundation have now come up with a daring plan to try to save the precious few that remain.
Some 95% of our native, white-clawed crayfish have been starved, infected or eaten by this alien species since it escaped captivity in the 1970s.
Now, volunteers armed with little more than waders, buckets and green nets have started trapping our native species in streams and rivers.
It is all part of this country's biggest ever strategic movement of the species.
Beating a retreat
The lobster-like creatures are then transferred to so-called "ark sites" - freshwater pools where neither plague nor peckish American crayfish can bother them.
It is a dignified withdrawal in an attempt to foil predictions that British crayfish will be extinct within 30 years.
Two British crayfish movements have already been carried out in the South West but the exact locations are being kept secret.
It is hoped this will prevent people looking for them and unwittingly spreading the deadly crayfish plague.
Organisations have teamed up to save the crayfish
My wellies are immediately sprayed with disinfectant on arriving for the first crayfish movement by a brook in South Gloucestershire.
Crayfish movements are also planned for Somerset, Wiltshire, Devon and the former Avon area but the conservation team won't be any more specific than that.
Ecologist Lydia Robbins from the Avon Wildlife Trust is part of the re-homing programme.
She told me a co-ordinated retreat was the only option.
"Currently there aren't any recognised methods to fully eradicate non-native crayfish which is why we're finding safe sites for British crayfish - hopefully until such a time as effective forms of control are found."
She also took umbrage with television chefs who had promoted catching the American crayfish for the dinner table, as tasty as they might look.
"It's a myth that catching North American crayfish for food is of conservation benefit.
"Casual trapping of crayfish can in fact increase the risk of spreading crayfish plague to other sites if people don't disinfect their equipment."
The £210,000 project runs for another two years and will be accompanied by a captive breeding programme at Bristol Zoo where 30 young British crayfish are currently being held in tanks.
UK conservation officer Jen Nightingale says such a scheme has never been successful in the past and that the old-fashioned British reserve may be to blame.
"They're a shy, discreet species they definitely get stressed so you have to have a hands-off approach let them get on with it really - they're not nearly as brash as these American signal crayfish!"
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