Although crayfish ‘trapping’ has been promoted by celebrity chefs and conservation charities alike it is not helping to control the invasive American signal crayfish, finds new research from King’s College London and UCL.

There have been grave concerns within the science community and amongst conservationists that American signal crayfish are wiping out other species of crayfish across Europe - including Britain’s only native crayfish, the endangered white-clawed crayfish.

While ‘trapping to eat’ has been touted as a way to decimate the number of American signal crayfish in the UK’s streams, the new study published in the Journal of Applied Ecology indicates that it may actually worsen the problem. It warns that trapping inadvertently incentivises members of the public to spread the species to new habitats and greatly increases their risk of accidentally catching the strictly protected native species.

The study found that trapping was ineffective in determining and controlling signal crayfish numbers as most individuals are too small to catch in standard baited traps.

The researchers tested three methods for surveying population numbers in a stream in North Yorkshire, UK, the most effective and accurate of which was a novel ‘triple drawdown technique’ which involves draining a short section of stream in a carefully controlled way and calculating the number of crayfish present, including infants.

This method revealed that most of the crayfish caught were tiny (smaller than a 1 pence piece), with only 2.3% of the population large enough to be caught in standard traps. Crucially, signal crayfish can become sexually mature before reaching a ‘trappable’ size, allowing populations to reproduce despite best efforts to control them through trapping.

“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”, said Dr Emily Smith, Environment Manager at Angling Trust.

The research was led by London NERC DTP students Daniel Chadwick and Eleri Pritchard who were co-supervised by academics at both UCL and King’s College London. Dr Michael Chadwick, who co-authored the journal article, is a Senior Lecturer in Physical and Environmental Geography in King’s College London’s Department of Geography.

Read or download the full journal article "A novel ‘triple drawdown’ method highlights deficiencies in invasive alien crayfish survey and control techniques".