Following the discovery of Signal Crayfish in the Upper Ettrick two years ago and, more recently, this summer, two specimens in the Upper Tweed and one in the River Till, the Tweed Foundation has developed a strategy to control their spread in the Tweed catchment.
Why don't we want them?
If Signal Crayfish spread through the Tweed and Scotland, it will mean a fundamental change to our freshwater ecology. Scotland has never had a crayfish, not even the native British one, so our freshwater systems are not adapted to them. Indeed, the absence of crayfish made Scottish freshwaters different from virtually all of the rest of Europe, so the colonisation of Scotland by this American species would represent a loss of European biodiversity. It is also known that these crayfish compete with young salmon and trout for cover under stones which means the salmonids being left out in the open when predators are around and that some insect species are reduced in numbers where there are Signal Crayfish. Their burrows, up to 1m in length, can cause serious damage to rivers with soft banks, making it of particular importance within the Tweed catchment that they be kept out of the Till area.
How do they spread?
While the Signal Crayfish can leave the water and move over damp ground, the main method by which it has been spread across England has been by people. Children have, apparently, been prime agents, finding these animals, keeping them for a while and then, when bored with them, putting them into the nearest available water. Other transfers are made by people thinking that they can make money out of them, despite the failures elsewhere. It is, however, now illegal to release Crayfish into the wild and a licence is needed to trap or be in possession of them.
The Crayfish Strategy
As there is, as yet, no effective method such as poison or disease, for eliminating Signal Crayfish from running waters, the main elements of the strategy have to be :-
a) Restriction: Making the general public aware of the problem and so preventing further introductions to new sites within the catchment. Action to generate publicity so that the public are aware that spreading Signal Crayfish around is a bad thing to do and that there are penalties for doing so.
b) Containment: Keeping the present known sites under control by trapping and other methods and so limiting their spread by reducing their population pressure. Actions are to continue trapping / electric-fishing work on known sites in running water;
elimination of populations from ponds, either by draining and drying these out, poisoning or by stocking with eels.
c) Refugia: The information from America is that Signal Crayfish, though able to travel out of the water on damp ground, can be restricted in their spread upstream by obstacles. The caulds near the foot of the Leithen, Gala, Jed, Whiteadder and Till therefore offer protection against their spread upstream into those tributaries from the main channel. Caulds further upstream on channels elsewhere (on the Rule, Wooler, etc.) will also offer protection to the areas above them). Actions are: to write to all farmers and landowners in the catchments that are protected by caulds calling attention to the special status of these areas as refuges from Signal Crayfish and asking them to inform us of any attempts to bring crayfish into these areas; to put up notices on local signboards in these areas; in the letters to owners in the Till catchment, special attention should be drawn to the particularly vulnerable state of the middle Till, where the riverbanks are of high, soft, sediments and would be seriously damaged by crayfish tunnelling. Whilst we have now found one specimen in the Till, it is vitally important that all landowners are made aware of this threat.
d) Research: The only long-term answer to the problem of Signal Crayfish is for some specific poison or disease to be found that can destroy them, which requires basic research. Action is to put pressure on government agencies to fund this type of research.
e) Eradication: While this is not yet possible for running waters, there are techniques available for still waters such as ponds. Even where such ponds are not connected to the wider catchment, their crayfish are still sources of further infestation and eradication should be attempted. Methods are to be trialled by the Tweed Foundation at the pond location where crayfish were found and will start with drainage and / or Eel stocking.