From the team at BBC Wildlife Magazine

American crayfish trapping fails to control invasive species, finds study

A new study has found that the trapping of invasive American crayfish to prevent it from overrunning the native British species, could be more harmful than beneficial.

Signal crayfish, Pacifastacus leniusculus on the edge of a river/Credit: Getty Images
Published: October 27, 2020 at 10:00 am
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In a new study, published in the Journal of Applied Ecology, researchers from UCL and King’s College London found evidence that suggests this approach appears not to work.


The report concluded that trapping is ineffective in determining and controlling signal crayfish numbers.

For many years the British public has been encouraged to trap and eat the American signal crayfish by celebrity chefs and some environmentalists. The rationale has been that this could serve as a control measure and help reduce the prevalence in UK waters of the introduced species and thereby improve the fortunes of the endangered white-clawed crayfish.

However, it has now been found that it may even exacerbate the problem.

American crayfish in cooking pot/Credit: Getty Images
American crayfish caught from a freshwater river in the UK/Credit: Getty

This is because the vast majority of individuals are too small to catch using standard baited traps. The scientists found that the policy may also inadvertently incentivise members of the public to spread the species to new habitats and greatly increases the risk of accidental catches of the strictly protected native species.

In addition, the research suggested that trapping risked spreading a fungal pathogen, called crayfish plague, which is lethal to native European crayfish.

“While celebrity chefs and conservation charities have, with good intentions, promoted trapping and foraging as a way to control American signal crayfish, our research shows trapping to be ineffective,” says co-author and PhD researcher, Eleri Pritchard.

The white-clawed crayfish is the only native crayfish species found in UK waters. There are six invasive species, of which the greatest concern focuses on the US signal crayfish, which was introduced from the US in the 190s.

The American crayfish is 16cm in length, compared with the 12cm white-clawed crayfish and the former has spread rapidly, displacing native crayfish, impacting fish and damaging ecosystems.

American signal crayfish babies/ Credit: Getty Images
American signal crayfish babies on shell tail covered in freshly hatched baby crayfish and juveniles, legs and sharp claws with shellfish caught from freshwater river/Credit: Getty

Trials in an upland North Yorkshire stream found that only 2.3% of crayfish identified were large enough to be caught in standard traps. Most were smaller than a one pence piece and too tiny to be trapped. This renders much of the policy of trapping useless as signal crayfish can become sexually mature before reaching a ‘trappable’ size. Populations therefore reproduce despite the best efforts to control them through trapping.

The scientists strongly recommended that recreational crayfish trapping be curtailed to prevent the further spread of these invasive species. More emphasis should also be given to initiatives such as the UK’s national ‘Check Clean Dry’ campaign, aimed at educating regular water users on the way that non-native invasive aquatic plants and animals can unwittingly be transported between waterbodies on contaminated clothes and equipment, as well as by fish stocking and water transfer schemes.

The scientists also recommend that instead, short-term conservation efforts should focus on promotion of aquatic biosecurity. In the longer term, they suggested more work was required to better understand the invasion biology of American signal crayfish and devise more effective and sustainable approaches to the management and control of the species.

“Signal crayfish have a devastating impact on our waters, harming fish populations, increasing economic costs for fisheries and causing a nuisance for anglers,” says Dr Emily Smith, environment manager at Angling Trust. “These findings present a new chapter in management practices, providing invaluable intelligence for fisheries and angling clubs.”


Main image: Signal crayfish on the edge of a freshwater river in the Midlands. © Getty



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