The first ever crayfish barrier' has been installed between two rivers in the south of Scotland.

A landmark in attempts to control the spread of North American signal crayfish, the barrier has been specially designed to stop them moving from one river catchment to another. It is the first time this approach has been tried anywhere in the UK or beyond.

Signal crayfish are voracious predators that have a devastating effect on native wildlife. Since they first arrived in Scotland in 1995, the invasive non-native creatures have spread to many river catchments around the country. In most cases they have been deliberately introduced by the irresponsible actions of people, even though it is illegal. Once established in rivers, they are virtually impossible to eradicate.

The animals feed on young fish and their eggs including those of salmon as well as insects and frogs. They can also burrow into and undermine river banks. The unstable banks can lead to increased erosion, damage the spawning grounds of a variety of fish and pose a safety risk for people and livestock.

Recent survey work on the River Clyde has shown that signal crayfish have spread into many of its headwaters, after they were first discovered in the river in 1996. There are now fears that they might move into the neighbouring River Annan catchment, near the Beattock summit in south Lanarkshire. The Annan, which runs from Moffat in the north to Annan on the Solway coast, is one of the best salmon and sea trout rivers in the south of Scotland, and is very important to the local economy.

To tackle the problem, a partnership of bodies including Scottish Natural Heritage (SNH), the Annan District Salmon Fisheries Board, the Clyde River Foundation and South Lanarkshire Council have installed the custom-made barrier at a site close to where the headwaters of the rivers Clyde and Annan meet.

"Unfortunately there are no techniques available that will allow us to get rid of signal crayfish from rivers and streams," explained Dr Colin Bean, a freshwater adviser with SNH, "so taking the radical step of developing and installing a physical barrier may offer us the best hope of stopping the species from moving into new catchments.

"The headwaters of the Clyde and Annan rivers are separated by a small field that occasionally floods. As signal crayfish are able to travel short distances across land, the partnership decided to put two barriers into the burn to stop them crossing from one catchment to the other."

The two dams have been installed 20 metres apart at a cost of £50,000. They were specially designed by South Lanarkshire Council in collaboration with the other project partners. Funds for the project came from SNH's Species Action Framework, which identifies the signal crayfish as a species of conservation concern. Both the River Annan and the River Clyde will now be monitored regularly to check whether the barrier has been successful in stopping the spread of crayfish.

Nick Chisholm, director of the River Annan Trust and Fisheries Board said: "The presence of crayfish on the Clyde has long been of concern to us. The Clyde and the Annan are hydraulically linked under certain conditions and there is no reason why, given time, that crayfish could not effectively walk over the hill.

"We know from bitter experience that once these animals are in an open water course, they are impossible to eradicate and can cause massive damage to the native fauna. The headwaters of the Annan are very important for both salmon and trout as they sustain a fishery that is not just important for the biodiversity of the area, but also creates a highly significant industry worth many hundreds of thousands of pounds. It also helps to provide employment for a number of people. The way in which all the organisations involved have been able to pool their respective expertise to deliver a solution to this problem is great news for all the anglers on the Annan, and an excellent example of interagency partnerships working."

A spokesperson for South Lanarkshire Council said: "We are pleased that we could help support this project to stop the crayfish population spreading.

"We have played our part with the design and build of the barrier but this has been very much a team effort - everyone involved recognises the threat crayfish pose to the long-established wildlife and habitat of the Clyde and Annan."

Dr Willie Yeomans, Catchment Manager with the Clyde River Foundation said: "Anglers from the United Clyde Angling Protective Association Ltd first reported the invasive, non-native American signal crayfish from the river more than a decade ago. Subsequent trapping work by the anglers, supplemented by surveys by scientists from the Clyde River Foundation (CRF) and the University of Glasgow, identified the threat to the Annan catchment. The CRF is monitoring the spread of the crayfish within the Clyde and identified the potential site for the barrier. The Annan District Salmon Fisheries Board showed clearly that there are no crayfish in the headwaters of its river. We know the crayfish are within 1km of the barrier and it is likely its effectiveness will be tested within the next few years.

"The barrier itself is the product of a consortium of interests and is a very fine example of an engineering solution to a potentially serious ecological problem. The CRF is grateful to SNH for funding the construction of the barrier and South Lanarkshire Council for their efforts with the design and build. This was a team effort - we didn't want these things in the Clyde and none of us want them getting to the Annan either.

"It is the culmination of years of costly research and surveying work, all of which would have been unnecessary if the crayfish had not been introduced to the Clyde system in the first place. The ecosystem of the upper Clyde had existed without crayfish for 10,000 years; they have made a measurable difference to the numbers and different kinds of animal living in the river within a decade. Crayfish have changed the ecology of the Clyde forever and this is a stark reminder to us all about the need for biosecurity and the need to avoid the introduction of invasive non-native species."

"If the barrier proves to be successful, then we may use it in other parts of the country to help prevent crayfish moving into new areas," Colin added. "We can't prevent them from becoming a problem in rivers where they currently exist, but at least this would give us some hope that we can control their spread in the future."

Spanish Crayfish Image by cristina miguel