Tackling alien invaders under Exmoor’s dark skies

  • Contributor information: CNP

We hear from Jackie Kiberd of the River Barle Signal Crayfish project, a shortlisted project in the 2017 Park Protector Award.

Exmoor’s treasured rivers are under threat from an aquatic invader – Pacifastacus leniusculus, the signal crayfish. To be frank, it’s a bit of a beast and is recognised as one of the most damaging invasive species in the UK.  It was introduced in the 1970’s to diversify farming and create a new food market. 

However, the signal crayfish was subsequently found to be a carrier for crayfish plague and is a great escapologist. Finding its way into our river systems, it has rapidly outcompeted our smaller native white clawed crayfish, pushing it towards extinction. Signal crayfish eat anything that moves, such as fish fry and invertebrates, even their own kind and much that doesn’t move too, including plants and lichens.  Their burrows destabilise riverbanks, causing erosion and sedimentation. 

In 2014 signal crayfish were found in the River Barle, one of our most pristine rivers in Exmoor National Park and a registered site of special scientific interest (SSSI). Quite when they arrived here we’re not sure and just how far they’ve spread we’ve yet to find out. So far they have been found over a five mile stretch of the river and their presence here poses a threat to the river’s biodiversity, salmon fishery, water quality and the river’s SSSI status and we need to redress the balance, which is where the River Barle Signal Crayfish Project comes in.










Signal crayfish, photo credit: River Barle crayfish project

Most attempts at controlling signal crayfish elsewhere in the UK and overseas have proved ineffective but this project, started in 2015 is hoping to turn things round and save our precious aquatic ecosystems.  The project is headed up by Nicky Green and forms the basis of her PhD, supported by Bournemouth University and the Centre for Environment, Fisheries and Aquaculture Science (CEFAS).  Eradication of signal crayfish is seen by many as impossible. 

Where previous trapping programmes have failed they have often been short-term, using traps biased towards capturing large males. You might think that removing the dominant breeding males might be a good idea but when you remove a dominant male, the smaller males have increased opportunities to reproduce and the population increases.  Removing just the females too can lead to those left behind producing more eggs. The approach currently being trialled as part of this project aims to control the signal crayfish population and through a long-term commitment weaken the population to a point where it is no longer sustainable.

The methodology involves using two different types of traps, baited and artificial refuge, plus sterilisation of dominant males, to target as wide a range of the signal crayfish population as possible. Large male crayfish are sterilised manually and returned to the river where they continue to dominate the groups but not to increase the population.  All other signal crayfish trapped are humanely despatched on site. Trapping takes place weekly from April – October with the help of up to 15 volunteers per session.

Volunteers are at the heart of the project and notwithstanding the attraction of working in a stunning location, they benefit from a comprehensive training and support from Nicky and other partner organisations (Exmoor National Park, the Environment Agency, South West Water, Natural England, the Rivers Exe and Tributaries Association and the River Barle Fishing Club).  Developing new skills, meeting new people and knowing they are taking part in valuable scientific research are just a few of the benefits of their involvement.  Several volunteers are members of the local fishing club and have a vested interest in the health of our rivers, as do local landowners and members of local communities.  We are in it for the long haul so it is vital that we continue to encourage interest and active participation in local communities. Just upstream of the main study site we are conducting a ‘Citizen Science’ trial where volunteers are managing the trapping themselves and able to go and trap any time of the day/week. This flexibility allows the volunteers to take advantage of good weather, and optimal river flows, as when it rains on Exmoor the river levels can rise rapidly in a short space of time. This self-managed approach will give us a good model for future support needs, which is one of the key aims of the project.

Lanacre Bridge, Exmoor National Park. Photo credit: Exmoor National Park Authority.

So how do we know whether what we are doing is working? Throughout the 3 years there is a comprehensive recording of various data including monitoring the river’s invertebrates, fish, juvenile and berried (egg-bearing) female crayfish.  Nicky is analysing this as part of her research. Over 1000 males were sterilised in 2015/16 and the following year numbers of egg-bearing females were showing a marked reduction. It might not be a quick win and requires sustained effort but as Albert Einstein once said, ‘It’s not that I’m so smart, it’s just that I stay with problems longer’. Analysing our results all adds to our collective scientific knowledge about how best to deal with this invasive species and preserve the quality, peace and tranquillity of our rivers. 

Jackie Kiberd is the Get Involved Project Coordinator at Exmoor National Park Authority. The River Barle Signal Crayfish Project was shortlisted for a 2017 Park Protector Award.