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Risk assessing pathways of invasion
Signal crayfish (Pacifastacus leniusculus) are recognised as a widely established high impact Invasive Non-Native Species (INNS), in the UK, causing extensive environmental, social, and economic impacts.
Burrator Reservoir is an asset of South West Water and located within Dartmoor National Park. It is an essential water resource for around 280,000 people in Plymouth, Tavistock, Princetown and south Devon.
The South West Lakes Trust manages the reservoir on behalf of South West Water and aims to protect and preserve the natural environment for the public for sustainable recreation.
The impacts of signal crayfish are well documented and include the reduction in abundance and diversity of fish, invertebrate and plant species, as well as increasing erosion and siltation through their burrowing activity. Complete eradication is a long-term objective, but containment and population suppression are the current priority and therefore APEM recommended a dynamic pilot crayfish removal programme, followed by a wider programme of works investigating the continued management of crayfish at the reservoir.
The low water levels of the reservoir in the Autumn of 2022 (around 40%) provided an ideal situation to facilitate the pilot programme of the removal of large numbers of crayfish from the water body because the population was concentrated into a smaller area, increasing the efficiency of trapping.
Trapping (under licence from the Environment Agency) was the recommended course of action and the APEM team planned a three week course as follows:
Throughout the three week exercise the APEM team gathered data including number per trap, size category, gender and by-catch. This ensured that APEM could facilitate a robust statistical analysis to examine the impact of the trapping on population size/density and make evidence-based recommendations on a course of action to further manage the population.
Some of the signal crayfish (Pacifastacus leniusculus) successfully trapped at Burrator Reservoir
In total, approximately 6000 American signal crayfish were captured, removed, humanely dispatched by freezing and then responsibly disposed of by incineration.
Over the course of the trapping period, there was a noticeable decline in the number of large male crayfish (the most competitive type of crayfish), which normally dominate the catch at the time of year (see graph below). There was also a striking increase in the number of berried females (females carrying eggs) in the catch which would normally be hiding away after breeding in order to protect their offspring.
The decline of large males within the population (due to trapping) could explain why less dominant crayfish, such as berried females, increased in the catch towards the end of the trapping period. Some benefits of the observed patterns include:
An analysis of the population caught during the trapping exercise
Detailed data on size, sex, reproduction and condition were collected for 617 individuals throughout the project, which helped APEM to further understand the structure of the population and inform future management considerations. Pre-control invertebrate samples were also collected to build a picture of the impacts that the management is having on the biodiversity of the reservoir in the future.
The reservoir is a popular location with anglers and throughout the exercise the APEM team and South West Lakes Trust were able to engage with them, helping to raise awareness of the issue and the work that was planned to help resolve it.
APEM’s team of invasive and non-native experts are internationally renowned and cover aquatic, terrestrial and marine environments.
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