American Signal Crayfish have invaded the city’s waterways and are decimating native species.
The alien crustaceans are also burrowing into riverbanks, eroding precious farmland and putting landowners out of pocket.
City folk who come across the whopping pincered beasts have been ordered to destroy the creatures on sight.
According to Ursula Juta, education and catchment manager at the Norfolk Rivers Trust, these creepy crayfish are "highly invasive, breed prolifically, decimate everything in the aquatic ecosystem - including all plants and all fish. They are very big, very aggressive, and high in number compared to the endangered native, white-clawed crayfish.
"When they burrow into the riverbanks, this causes mud and silt to go into the water, polluting it. This creates an environment fine for crayfish, but where fish really struggle to survive."
The alien species were imported from North America back in the '60s as a food source to satisfy larger seafood appetites in Britain. But they quickly spread to British waterways and began wreaking havoc.
In Norfolk, the signal crayfish are now "absolutely rife" in the Wensum and Bure.
However we can't eat our way out of the problem now, according to Jutra.
"Trapping them for food is counter productive and more likely to spread the crayfish plague, which signal crayfish are immune to, but which kills native ones. "
Despite other parts of the country advising traps, trapping crayfish in Norfolk is illegal.
The current government advice is to humanely kill signal crayfish by stabbing the brain with a knife. However, Juta insisted, "We don't want to encourage lots of people to carry knives around with them on country walks.
"Use a large rock or a very big stick and crush the head. Don't crush them under your foot! If you do this will spread crayfish plague spores to other waterways when you walk and kill the native crayfish.
"When you do go to kill crayfish - do it confidently and with force. If this is not done properly, this may leave the crayfish maimed and struggling alive on the side of the riverbank. This is not ethical as they are a living species and as such should be treated with respect."
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