Other Invasive Species Control
It’s not all about plants. The River Annan Trust works to prevent the spread of a number of other invasive species in the catchment. The two biggest threats are American mink and American Signal crayfish. American mink have been wide spread for some time however in recent years their numbers have declined dramatically, particularly at the top of the catchment. Signal crayfish are not present in the River Annan and we have been working hard to ensure that this remains the case given the damage they can cause to rivers and water bodies.
The American mink was first brought to Britain through fur farming in the 1920’s. Since then a number of escapes and deliberate releases have allowed it to establish in the wild. An efficient, generalist predator, the mink has a huge impact on populations of native fish, birds and mammals. It has had a particularly devastating impact on the water vole, with populations declining as much as 90% in some parts of the UK.
Control work for this species has been on going for some time if in a somewhat piecemeal fashion. A more strategic approach has been taken over the last few years with a reasonable degree of success. Currently there are 24 rafts within the catchment, spaced roughly 1 kilometre apart. Most of these are concentrated on the upper reaches of the catchment, however the number of prints recorded on rafts has dropped dramatically as have the number of sightings by anglers and river users.
To increase our presence on the river we have begun building our own mink rafts while recruiting volunteers to monitor these rafts for prints. The new rafts are lightweight and ideal for volunteers to check and this has added eight more rafts to our monitoring fleet. This is something we hope to expand further over the coming year. The recent county mammal report indicated that although still critically endangered water voles had been reported in new sites and had begun to make a come back in some areas of the catchment.
American Signal Cryfish
Signal crayfish are voracious predators introduced to Britain from North America in the 1970s for aquaculture although the first record of them in Scotland was not until 1995. They impact on a river system by predating on native insects, amphibians, fish and their eggs while excluding fish from their preferred habitat making them more vulnerable to predation. Large numbers of crayfish can burrow into river banks resulting in destabilisation and erosion which has significant implications for flood control.
Signal crayfish have not been found in the Annan but are abundant in the headwaters of the Clyde. To prevent migration from one catchment to another early 2011 saw the construction of a landmark crayfish barrier in the headwaters of the Clyde. The Annan and Clyde can be hydraulically linked under certain conditions by the network of land drains in the area and there is a very real threat of invasion from these animals. The barrier is the first of its kind in the UK and a unique approach to stopping the crayfish was needed as there are currently no techniques available for removing crayfish from a river system once they are in.
The construction of the crayfish barrier began in May 2011 and was completed one month later in early June. Two barriers were put in place to create an eradication pool in case crayfish had somehow managed bypass the first barrier. Once completed, the bank was seeded and vegetation quickly began to take hold.
Survey work was carried out on the Annan side just before the barrier was constructed and will continue annually to ensure the barrier is working as hoped. Surveys for Signal crayfish are also carried out in areas that have been identified as vulnerable to introduction by identifying potential pathways into the catchment. This has become even more important recently with the discovery of crayfish in another neighbouring catchment.