Delving into the Deep: on the hunt for white-clawed crayfish!

Delving into the Deep: on the hunt for white-clawed crayfish!

The white-clawed crayfish is the UK's only native freshwater crayfish, but sadly it is under threat from a non-native competitor. Hampshire and Isle of Wight Wildlife Trust has been working with Bristol Zoological Society to protect this incredible animal, and to look after the unique Hampshire chalk streams that the crayfish calls home.

Wandering through nature gives us the chance to encounter a huge variety of the wildlife that calls our wild forests, vast wetlands and luscious grasslands home. It’s easy to spot a butterfly overhead, or a bee buzzing past us – however, there are some animals that hide just out our view. In fact, they are so well-hidden, that most people don’t even know they exist.  

Freshwater habitats are one of the most important ecosystems on the planet, providing a home to over 100,000 species worldwide. By taking a peek under the surface of the water, we can shed a light on these mysterious creatures and discover an entirely new world living on the other side.  

I’ve been working as an environmental biologist for a few years, but as my speciality has mostly been terrestrial animals I have rarely come into contact with freshwater species. One of the things that attracted me to the role of trainee ecologist with Hampshire & Isle of Wight Wildlife Trust was the chance to encounter some of these animals: in particular, the white-clawed crayfish. 

Two white-clawed crayfish in a bucket

Two white-clawed crayfish found during a survey. Credit: Carla Broom

White-clawed crayfish (Austropotamobius pallipes) are the UK’s only native freshwater crayfish. They are lobster-like creatures with large claws that have a pale underside, hence their name, and are actually one of our largest freshwater invertebrates. They inhabit shallow streams, hiding underneath rocks and within small crevices where they forage for invertebrates, plants, and dead matter. They grow up to 12cm and can live for up to 12 years! 

Unfortunately, the white-clawed crayfish is under severe threat from the invasive signal crayfish (Pacifastacus leniusculus), which were brought from America to the UK for food but quickly took over the waterways. Signal crayfish carry with them a crayfish plague, to which our native white-claws have no natural resistance. The plague has caused a huge decline in populations over the last 30 years, and they are now classified as a protected species under the Wildlife and Countryside Act 1981 and are listed as ‘Endangered’ by the IUCN Red List. 

A signal crayfish being held to see the underside and characteristic red claws

An invasive signal crayfish, showing the characteristic red underside to the claws. Credit: Carla Broom

Historically, Hampshire’s beautifully clear chalk streams provide the perfect habitat for white-clawed crayfish, and as such, the Wildlife Trust has been working very hard to protect their few remaining strongholds on these rivers. While habitat protection is a vital part of any conservation work, the Trust has also been working closely with Bristol Zoological Society for many years to breed and rear white-clawed crayfish in captivity for release back into the wild. 

So far, the Trust have released crayfish into a tributary of the river Itchen, in an effort to extend its natural range, though in the future we hope to create new ‘ark’ sites for our white-clawed crayfish too. Once released, we hope that the crayfish will survive and go on to breed naturally, further boosting the populations. Of course, we need to monitor their populations to check whether the releases were successful, but due to the shy nature of the crayfish and their tricky habitat, we have to get a little bit creative.  

In order to find the crayfish again, we deploy artificial refuge traps, or ARTs, which recreate the cosy shelters that crayfish seek out. They hide within the ARTs, making it easy for us to check the traps and record anything that might be inside. When we find a crayfish, we record the sex, size, and signs of damage or disease, and use these records to estimate the strength of the population. 

Artificial Refuge Trap

An artificial refuge trap, designed to provide shelter for crayfish. Credit: Ben Rushbrook

This presents us with a problem: how do we know we aren’t just counting the same crayfish over and over? If this were the case, it would skew that data and trick us into thinking the population is bigger than it actually is. To solve this problem, we have started employing a neat little bit of technology known as a PIT-tag. 

‘Passive integrated transponder’ (PIT) devices have been in use since the 1980’s to investigate wildlife populations. A tiny marker with a unique code, like a dog’s microchip, is inserted under the skin of the animal and allows us to identify individuals. With crayfish, we insert them into the abdomen, just under the shell, where it sits securely without causing the animal any pain or discomfort.  

Using a PIT scanner, we can read the unique code from the tag, like a barcode, and keep track of whether we have seen this particular individual before. Once we have ‘recaptured’ enough tagged crayfish, we can estimate the size of the crayfish population in that area, potentially keep track of how fast they are growing, and record any developments in disease or injury. 

A crayfish on top of a PIT tag scanner

The device used to scan the PIT tag within the crayfish to read the unique code. Credit: Ben Rushbrook

This tiny technology allows us to gain a much clearer insight into the health and size of the population, as well as telling us whether the releases were successful. This information helps us decide where to take the project in the future – do we need to release more individuals? Do we need to create more suitable habitat? Is it time to start releasing crayfish at a new site? It’s incredible that a tag the size of a grain of rice is assisting in the survival of this incredible species.