Their populations are facing the twin threats of direct competition and predation by non-native crayfish (the North American signal crayfish Pacifastacus leniusculus being the biggest culprit) and ‘crayfish plague’, a disease carried by non-native  crayfish species from North America  to which they are largely immune, but causes mass mortality of our native species. It’s estimated that populations of our native species have undergone a 70% decline in the last 50 years.

Working with landowners and  a range of partners (including  the Vitacress Conservation Trust, Environment Agency, Natural England, Sparsholt College and the Bristol Zoological Society) , the Wildlife Trust’s Southern Chalkstreams Project is making sure that we can secure robust and viable populations of these engaging little creatures where they are still safe.

One of the most exciting and novel ways we are doing this is through the use of captive-rearing / breeding techniques, guided by the information gathered through our long-term survey and monitoring programmes for native and signal crayfish.

I’ve been lucky enough to be able to help out with a few aspects of this work over the last few months.

Release of captive bred juveniles

In April, we released over 100 juvenile native crayfish into to a tributary of the River Itchen, immediately upstream of the upper limit of their historic distribution. They were reared at Bristol Zoo Gardens from the eggs of wild ‘berried’ females (laden with eggs), which had been collected further downstream in previous years. Captive rearing ensures a much better survival rate for the young crayfish so wild populations can be boosted more rapidly.  We also checked Artificial Refuge Traps (ARTs) and hand-searched to collect more ‘berried’ females  which would be taken to Bristol Zoo Gardens for another round of captive rearing.

Monitoring of wild populations

Since July, we have been carrying out regular surveys to monitor the population at the release site. This year, for the first time, the crayfish are being tagged which will allow us to build a better picture of the movements of individuals and of the population size in the long-term.

When a crayfish is found, we record its size, sex and any signs of damage and disease. For example, white-clawed crayfish can suffer from the naturally occurring porcelain disease, which weakens them and can be an issue if it becomes prevalent within a population.

If it is already tagged or big enough to be newly tagged we also record the tag number. So far we have recaptured 11 tagged individuals which has been very exciting – especially as it is showing that some of the crayfish released this year have moved quite a way downstream since April, and a couple have even moved upstream from their release site!

Surveying for invasive signal crayfish

Although there are some parts of our rivers that are currently free of signal crayfish, there are many sites where they are established. Knowing where these are is vitally important as it affects how these sites are managed and informs the conservation strategy for our native crayfish.

Signal crayfish are bigger, hungrier and more mobile than our native crayfish and can rapidly out-compete them, even when they are not carriers of crayfish plague. Studies have shown that it can take less than 10 years for a white-clawed crayfish population to be completely eliminated by signal crayfish, which can expand their range at up to 1 km per year. They also cause significant damage to river banks through their burrowing and impact the populations of fish and other invertebrates which make up their diet.

Moving between sites supporting native crayfish and invasive crayfish species can risk spreading crayfish plague between these sites. Biosecurity is therefore a hugely important part of responsible survey work - we ensure that all equipment that has been in contact with the water is thoroughly cleaned, disinfected and dried between sites and surveys. To make sure our conservation efforts pay off in the long-term, it is important that other river users such as anglers and land managers do the same.

In addition to signal crayfish, there are a number of other invasive non-native species currently causing our waterways serious problems, and the GB Non-Native Species Secretariat website is an excellent source of information, highlighting the key threats and providing resources on how to identify them. Three of these species - quagga mussel, killer shrimp and water primrose – are being particularly monitored as they are in the early stages of invasion and their spread can hopefully be managed.

Sadly for our native crayfish in many areas, signal crayfish are too well established now to be able to eliminate them entirely.  The best outcome we can achieve is to ensure that some rivers and ponds remain free of their grasp to allow our little white-claws to flourish there in safety.

White-clawed crayfish are a protected species and can only be surveyed under licence from Natural England. All crayfish can only be trapped under licence from the Environment Agency.