Signal Crayfish invading North Wales streams

A signal crayfish with eggs.

North Wales Wildlife Trust have been working on raising awareness of invasive species in North Wales Rivers.

The environmental threats posed by American Signal Crayfish are such that walkers and river users are being urged to take special precautions. North Wales Wildlife Trust (NWWT) wants everyone to wash their shoes, dry their dogs and clean their boats and fishing tackle before leaving riverbanks and heading home.

Introduced from the US in the late 1970s, to be reared in farms for restaurants and food shops, the species quickly became established in the wild. Since then they have decimated populations of the UK’s native White-clawed crayfish, a much smaller crustacean that’s regarded as a keystone species of British aquatic habitats.

The undersides of the claws of White-clawed crayfish are pale-coloured, unlike US Signals, whose claws are coloured red underneath (Image: National Trust/Annapurna Mellor)

Some scientists estimate 90% of White-clawed crayfish have been wiped out by their larger US rivals in parts of the UK. As well as predating on native crayfish, Signals are immune to the crayfish plague they carry, but which causes 100% mortality in White-claws.

Establishing exactly where Signals are present in North Wales is an ongoing exercise . Focusing on the Dee catchment in north east Wales, eDNA testing found one river completely devoid of White-claws. And even in rivers where no Signals were found, crayfish plaque was detected in some, indicating wider prevalence.

Earlier this month, the team descended on a Flintshire river when Signals had been reported. It was astonished by the sampling results. “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.”

Mature Signals can measure up to 12 inches (30cm), excluding the claws – more than twice the size of White-claws. Like natives, they burrow into riverbanks, but their greater size means the impact is so much greater, causing bankside failures.

They are avaricious consumers of fish eggs, eat White-claws and will even dine on smaller Signals, a problem for eradication campaigns: as trapping tends to snare larger Signals, a source of predation is lost.

In the past, trapping was advocated by some chefs and environmentalists. The idea was to eat our way out of the problem using baited traps, similar to lobster pots.

Signals are regarded as a delicacy, not too far removed from the taste of lobster – hence its Welsh name, Cimwch dir Croyw. In principle the idea sounds great, notwithstanding the fact that it was our appetite for Signals that caused the problem in the first place. In practice, catching and eating Signals doesn’t work.

Just how pointless it is, was illustrated by a remarkable project reported in the Journal of Applied Ecology in 2020. Researchers completely drained short sections of a stream in the Yorkshire Dales, then removed all stones from the river bed to expose any Signals present. Any found were removed.

Using pumps, the stream was refilled and drained three times, each time looking for Signals. Incredibly, they found up to 110 per square metre. Many were small, with the team estimating that less than 2.5% were big enough to be caught in conventional traps.

For this reason, Helen from NWWT acknowledges that the 140 Signals caught in Flintshire would “hardly have caused a dent” in the local population. The idea instead was to train up biosecurity volunteers to be on the crawfish frontline.

Differences between Signals and White-claws

• Killing Signal crayfish is mandatory but not without risk for the species that conservationist want to preserve.

• There is always a danger that, when reaching for a boulder, people might mistakenly bludgeon to death a White-clawed crayfish.

• For this reason, NWWT’s Helen Carter-Emsell is asking people to learn to tell the difference.

• Differences in size can be obvious but this is not always a reliable indicator. 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 less aggressive if caught.

Signal numbers will always increase faster than attempts to trap them. Moreover, using traps between rivers can spread crayfish plague, and illegal traps have drowned many otters.

As with many invasive species, the inevitable conclusion is that containment is the only option. Which is why NWWT wants people to help prevent further spread by practicing good biosecurity.

“Cleaning footwear and equipment is something people should do whenever they visit wild areas, as other invasive species such as Himalayan Balsam can also be carried around,” said Helen. “Prevention is by far the most effective way of tackling the threat.

“We’d also encourage people not to develop a taste for crayfish and start creating their own food supplies in rivers, as this risks spreading Signal crayfish to new areas.”