Crayfish 'trapping' doesn't control invasive species


The American signal crayfish is bigger and more aggressive than native crayfish species in Britain, in addition to carrying a disease called crayfish plague. Photo by Eleri Pritchard/UCL

Oct. 13 (UPI) -- In Britain, a handful of celebrity chefs have encouraged the practice of crayfish "trapping" to control the invasion of American signal crayfish.

Unfortunately, new research -- published Tuesday in the Journal of Applied Ecology -- suggests the practice doesn't work. In fact, crayfish trapping can have a host of unintended consequences.

"Trapping has been linked to a range of risks to our waterbodies, including the spread of invasive species on wet or unclean equipment, as well as the direct capture and release of invasive crayfish to seed new harvestable populations," study co-author Eleri Pritchard told UPI in an email.

"Sadly, trapping also risks protected native wildlife, and has been responsible for the deaths of otters and water voles," said Pritchard, a postdoctoral researcher at University College London.

American signal crayfish have led to significant declines of native crayfish in Britain and Europe. The invasive species is a carrier of what's called crayfish plague, a disease that is lethal to the native white-clawed crayfish. American signal crayfish are also bigger and more aggressive than native species, outcompeting them for available resources.

Beyond threatening native species, American signal crayfish also burrow into stream banks, undermining natural stream structures and increasing flood risks. Researchers suggest the invasive species also poses a threat to fish, invertebrates and aquatic plants.

Taking a cue from efforts to curb the spread of invasive fish species, chefs in Britain have encouraged people to trap and eat American signal crayfish.

"Crayfish trapping involves the use of funnel traps, very similar to lobster pots," Pritchard said. "The traps are submerged in the water and baited with something to attract the crayfish, like fresh oily fish or cat-food. This entices the crayfish through the funnel entrances of the trap and once inside, it is difficult for them to escape. The traps are then retrieved from the water with crayfish trapped inside."

For the new study, researchers compared the effectiveness of three survey methods -- baited funnel trapping, hand-searching and a novel "triple drawdown technique" -- deployed to analyze local crayfish populations. The triple drawdown method involves the draining of a short section of stream in order to tally the number of crayfish present, including infants.

The triple drawdown technique proved most effective at providing scientists a comprehensive and precise picture of the size and makeup of crayfish population within a stream. The method showed crayfish population densities have likely been underestimated in many British streams.

The novel survey method also showed that only a small percentage of any given stream's American signal crayfish is trappable. Most of the invasive crayfish are too small.

"In our research, [the triple drawdown technique] helped us understand how trapping would not be effective at controlling populations, which is really important for management and conservation," Pritchard said.

Researchers are currently working to develop effective American signal crayfish control methods.

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In the meantime, Pritchard and her colleagues suggest prevention of the spread of both crayfish and the crayfish plague is essential for the protection of native crayfish.

"Please disinfect all gear when traveling between waterbodies," Pritchard said.

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