Sterilisation of Male Crayfish
My PhD 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.
River Barle Signal Crayfish & Exmoor Non-Native Invasive Species (ENNIS) Project
This volunteer led project has been trapping and sterilising signal crayfish on the River Barle Site of Special Scientific Interest since 2015. We used both baited funnel and Artificial Refuge Traps originally but found ARTs more effective. To date over 20,000 crayfish have been removed and 4,000 males sterilised.
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.
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. This stalwart group of volunteers have been trapping crayfish with three types of trap, sterilising males and introducing predatory perch in the top lake and will be continuing their efforts in the restored lower two lakes in 2022. Unfortunately draining these lakes during the restoration period is unlikely to have completely eradicated the signal crayfish but we hope to keep them in check once the lakes are re-filled.
The Culm Community 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, discovering that the population has decreased by 80% since the last survey in 2012.
Artificial Refuge Trap Research
With the help of colleagues and volunteer groups I continue to research and develop new designs of ARTs. Different models of trap are being tested at stillwater and river sites in Devon, Somerset, Hampshire, Bath and Surrey. The results will be published and feed into the design process.
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.