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Stuff that matters

Unusually, Brits are being asked to kill a small animal because of the deadly threat it poses to a native species.

The tiny foreign invader is an American signal crayfish. The ten-legged miniature lobster has become common in many UK waterways but fishermen and walkers are now being advised to kill it, albeit humanely.

The best method, although it is not always practical, is to stick a knife into its brain.

Crayfish expert Helen Carter-Emsell told North Wales Live : “We realise not everyone will have a knife with them.

“The alternative is to squash them under a rock, though we recognise some people might find this unpleasant.”

If you see one of the signal crayfish you’re asked to kill it straight away – after making sure that it’s not one of the now-endangered white-claw native ones.

A key difference is claw colour – signal claws are red underneath with a white or bluish blotch below the fingers – the “signal” patch.

In contrast, native crayfish have claws that are a pale colour, hence their name. They are also less aggressive if caught.

The American signal crayfish, which can grow to lengths of up to 12 inches, has menacing claws, and a voracious appetite. The female of the species can carry up to 250 eggs.

The species was introduced to the UK from the US in the late 1970s when crayfish plague had decimated native white-claw crayfish populations.

Signals are immune to the crayfish plague, which causes certain mortality in white-claw crayfish, but they remain carriers of the disease.

While they were originally reared in farms for restaurants and food shops, the species quickly became established in the wild, causing further disease and competition for native populations of crayfish.

Scientists have estimated that 90% of white-claw crayfish have been wiped out by their larger US rivals in parts of the UK.

Helen is wellbeing officer for the North Wales Resilient Ecosystems pilot project, an initiative that’s been set up to tackle invasive non-native species such as the US Signal Crayfish.

Helen and her colleagues are tasked with the ongoing exercise of establishing exactly where signals are present in North Wales.

Recent eDNA testing conducted near Dee in north east Wales found one river completely devoid of white-claw crayfish.

Even in areas where no signals were found, crayfish plague was detected in some, indicating wider prevalence.

Earlier this month, the team was astonished by the sampling results from a Flintshire river where large numbers of signals had been reported.

“In four visits, we found 140 of them in a quarter-mile stretch,” said Helen. “That’s about one caught every five minutes.

“Along this section, the riverbanks were badly eroded and were collapsing where people walk along a footpath.

“We didn’t find a single fish along the whole stretch, and the riverbed was covered in sediment, disrupting spawning grounds.”