American Signal Crayfish wrecks UK waters

The formidable American Signal Crayfish poses a massive threat to native species in rivers, lakes and ponds.

	American Signal Crayfish will eat plants, snails, small fish, fish eggs, invertebrates and its own young

Although on the surface everything might appear normal, beneath the waterline the crayfish is waging war on anything that stands in its path.

The six-inch-long killing machine has already annihilated the smaller native White Claw crayfish from most of the waterways in the south of England.

A voracious predator it will eat almost anything it finds including plants, invertebrates, snails, small fish and fish eggs. It is also a cannibal that makes a meal of its own young.

The Signal also digs burrows up to three feet long in river banks where each year it lays more than 250 eggs at a time. At a time of increased flooding risk the numbers and size of the burrows is increasingly causing river banks to collapse.

Introduced in the 1970s and bred on farms for the restaurant trade a handful of escapers have now grown to an aquatic army numbering millions which has infiltrated river systems from Cornwall to Scotland.

The crayfish is extremely aggressive, encased in a tough shell and armed with two large pincers. They are equally at home on land and can walk for several miles across country in search of new territory. When the crayfish move into a stretch of river it is virtually a death warrant for other species.

The loss of plants means there are fewer places for insect larvae and for fish to lay their eggs which in some rivers has reduced trout and salmon stocks.

The invasive crayfish has now reached plague proportions and marine biologists have been desperately seeking a way of halting its relentless spread. Suggestions have included breeding huge numbers of sterile males - as happens with mosquito control - to wreck breeding success rates.

Trapping and taking the Signal out of the water would have to be done on a massive scale to have any significant impact and attempts to introduce a specific crayfish fatal disease has also failed.

Professor of Freshwater Biology at the University of Derby, David Rogers, appeared on a River Cottage Spring TV programme this week helping trap 140 Signal crayfish in a small stretch of water on the River Kennet, a chalk stream and tributary of the Thames which flows through Berkshire and Wiltshire.

He said: "They are a major threat to the ecosystems of our rivers, there is no doubt about that. On the Kennet we estimate there are millions of them and you would have to trap thousands of them every day not to eradicate them but simply to reduce their impact."

"They will eat plants, insects, fish, snails, detritus and even their own young. When they are in dense colonies you can actually see the river bank retreating as they burrow into it."

Now an eco engineering company, Willowbank of Somerset, have developed a river banking system which they say deprives the crayfish of the habitat it needs for shelter and breeding. They have developed a way of incorporating a steel mesh into traditional willow used to reinforce river banks which stops the crayfish burrowing and forces it to move on.

Partner James Hector said: "This creature is beginning to dominate life in our rivers and streams and we have to find a way of getting rid of it. We are not saying this is the complete answer but it is a start. Where we have installed the mesh we have seen crayfish numbers fall.

"If we can install it in tributaries as river banks are repaired we might be able to stop it getting into the rivers in large numbers."

But Prof Rogers remains sceptical that it will work. "It may stop bigger crayfish getting through but it might even help smaller crayfish, who can get through, to survive predation by adults.

"Small sections of bank might be helped but the mesh would have to be buried deep and across the breadth of the river to stop them burrowing. Even then crayfish might be able to get into pools and burrow into the banks from the other side," he said.

Signal crayfish facts