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Invasive crayfish cannot be controlled through trapping

In Europe, concerns are growing that American signal crayfish will wipe out other populations, including Britain’s endangered white-clawed crayfish. A new study from University College London reveals that trapping the invasive animals is not helping to control them. 

The idea that “trapping to eat” American signal crayfish may control their numbers has caught on in recent decades. However, scientists are warning that this will inadvertently lead to the further spread of the species – not to mention the trapping of protected native crayfish.

“Invasive signal crayfish from the US were introduced to England in the 1970s. Since then they have spread rapidly, displacing native crayfish, impacting fish and damaging ecosystems,” said study co-author Eleri Pritchard.

“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. We are also concerned that trapping risks spreading the fungal pathogen, called crayfish plague, which is lethal to native European crayfish.”

The researchers found that trapping is also not effective in determining signal crayfish numbers, considering that most are too small to catch using standard baited traps.

The research was focused on surveying populations in an upland stream in North Yorkshire, UK. The experts set out to compare the effectiveness of three methods: baited funnel trapping, handsearching, and the triple drawdown technique, which involves draining a section of stream to calculate crayfish numbers. 

The study showed that the triple drawdown method was the best for determining the size of crayfish populations. Using this technique, the scientists estimated signal crayfish densities of up to 110 per square meter. This estimate far exceeds the numbers calculated in similar streams using traps.

The researchers discovered that only two percent of the individuals were big enough to be caught in standard traps. They explained that signal crayfish can become sexually mature before reaching a size large enough to be trapped.

“We now have strong data to show that trapping does not help to control invasive crayfish. On the contrary, there is growing evidence that trapping furthers the spread,” said study co-author Paul Bradley.

“In the short-term, conservation efforts should re-focus on promotion of aquatic biosecurity, and in the longer-term we need further research to better understand the invasion biology of American signal crayfish. This will help to inform more effective and sustainable approaches to the management and control of this problematic invasive species.”

The scientists strongly recommend that recreational trapping is stopped. They want to shift the focus to initiatives like the UK’s national Check Clean Dry campaign, which is aimed at educating the public. 

“Signal crayfish have a devastating impact on our waters, harming fish populations, increasing economic costs for fisheries and causing a nuisance for anglers,” said Dr. Emily Smith. 

“These findings present a new chapter in management practices, providing invaluable intelligence for fisheries and angling clubs, supporting them in refining practical solutions to controlling this highly aggressive species.”

The study is published in the Journal of Applied Ecology.

By Chrissy Sexton, Staff Writer

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