River Barle Signal Crayfish

This volunteer led project trapped and sterilised signal crayfish on the River Barle Site of Special Scientific Interest between 2015 and 2023. We used both baited funnel and Artificial Refuge Traps originally but found ARTs more effective. Over 20,000 crayfish were removed and 4,000 males sterilised.

This project was the focus of my PhD, which investigated the potential of sterilising male signal crayfish as a control tool using field studies on the River Barle and controlled experiments at Bournemouth University. The hypothesis was that large males dominate breeding behaviour and cannibalise smaller animals, therefore keeping them in the population but rendering them infertile would reduce reproduction but maintain cannibalism. Behavioural studies found that sterilised males are equally capable of becoming dominant and competing for food, mates and shelter as non-sterilised males.

Females are as likely to choose to mate with a sterilised males as a non-sterilised male (read more here). We also found that sterilised males produced fewer spermatophores (packets of sperm) but this did not translate to reduced female brood sizes in either in the lab or the field. There are several potential reasons for this, some of which I am investigating. My full thesis is available here.

After initial fluctuations catch rates have decreased by 50% – there is no evidence that this is due to male sterilisation so we think it’s due to the removal of females and immature animals using ARTs. From 2019 to 2021 the project continued as part of the Water Environment Grant funded ENNIS project and included studies of other signal crayfish sightings on Exmoor.

Exmoor Non-Native Invasive Species (ENNIS) Project

Since 2019 the ENNIS project and has included surveys of signal crayfish across Exmoor with the help of skilled and enthusiastic volunteers. Current works are focusing on the ‘upstream edges’ of two crayfish populations with a view to creating ‘containment zones’ to limit their spread into vital salmon spawning habitat. In 2022 and 2023 we trialled several trap types, which we are going to use to create ‘crayfish reefs’ aimed at providing underwater habitat barriers to upstream migration.

One method we trialled was the brash bundle, made of birch twigs. Most conventional crayfish traps are biased towards age, sex or both and it is extremely difficult to catch small crayfish, which comprise 80% of populations, by these means. We found that brash bundles captured large numbers of small and juvenile crayfish. They were also colonised by large numbers of elvers. European eel Anguilla anguilla are a critically endangered species but also an excellent predator of signal crayfish. Learning that brash bundles are great habitat for this species provides lots of opportunities for us to help them survive and ideally grow up to eat crayfish!

Prior Park Landscape Gardens, Bath

Since 2019 I have been helping a group of National Trust volunteers with advice and training on a voluntary basis. Prior Park contains three ornamental lakes of architectural and historic importance, all of which are infested with signal crayfish which have burrowed into the 18th century dams, instigating a multimillion-pound restoration project.

The works have included removal of fish from bottom to top lake prior to dewatering and rebuilding of the dams over a two year period. In spring 2023 the lakes were refilled and unsurprisingly, crayfish were still present though catch rates in the top lake reduced dramatically after the introduction of perch – read all about it here. We are now working to a five year management plan to include adaptive management of crayfish and monitoring of fish populations.

Saving Devon’s Crayfish

I have been advising on two white-clawed crayfish conservation projects since 2022. The Saving Devon’s Crayfish project successfully received funding to set up a crayfish hatchery at the Wildwood Trust, Escot, using animals sourced from the Culm and Creedy/Yeo rivers. The first baby crayfish hatched in June 2023!

The Creedy Catchment Crayfish Conservation project is an ambitious river restoration project focussed on the Creedy/Yeo river, one of only two rivers supporting white-clawed crayfish in Devon. Crayfish numbers have decreased by 80% since 2012 and the catchment suffers from major water quality issues. A steering group of dedicated stakeholders is guiding the development of this multi faceted project which including crayfish surveys, ark site creation and land management advice.  

Culm Community Crayfish Project

The Culm Community Crayfish Project in 2018 was funded by the Heritage Lottery Fund and managed by the Blackdown Hills Area of Outstanding Natural Beauty. It aimed to raise awareness of river health using crayfish as a flagship species via crayfish surveys, environmental education, practical works and community engagement. I trained 48 volunteers and helped them survey 27km of the river Culm, confirming the extent of white-clawed crayfish and FIVE signal crayfish populations in the catchment. The project also engaged over 1700 children and 45 teachers!

The plight of native crayfish uncovered in this project inspired the Devon Crayfish Ark Site Project which seeks out and evaluates ark sites for native crayfish from this and the river Creedy/Yeo, which I surveyed in 2021 with more lovely volunteers, providing the evidence and impetus to set up the Creedy Catchment Crayfish Conservation Project.

Reservoir Surveys and Control Options

Since 2021 I have been developing a technique to deploy and test large ARTs in water up to 30m deep and study the distribution of crayfish in relation to water depth and the presence of predatory fish. I have also been advising South West Water, in collaboration with APEM Ltd, on options to control signal crayfish at their sites, and investigating the potential for signal crayfish to be transferred between waterbodies via pipework (raw water transfers).

In large waterbodies crayfish are difficult to locate and impossible to quantify from shore-based surveys alone. In 2018 and 2019 I worked with expert colleagues Tim Clements and Phil Short to develop a dive survey technique to determine crayfish signal abundance in two reservoirs for SWW.