American signal crayfish - Invading a water body near you...

Wed, 04/06/2016 - 19:07 -- Chloe Robinson

What are they?

An invasive (non-native) crayfish species originally from North America; dark brown in appearance with distinctive red underside to claws and blue-white ‘hinge’ on top of claws. They are often found in lakes, ponds and slow-flowing stream and river systems with soft substrate.

Signal Crayfish

How did they get here?

Signal crayfish were extensively farmed in the UK throughout the 1970s to supply the Scandinavian market. Inadequate facilities and lack of escape prevention protocols led to many individuals escaping into the local environment, along with purposeful stocking and release of ‘pet’ signal crayfish. This has resulted in many introduction points across the UK of signal crayfish, which has been one of the factors which led to their success establishing as an invasive species.

What impact are they having?

Signal crayfish are much larger, more aggressive and more fertile than our native white-clawed crayfish (Austropotamobius pallipes) and therefore outcompete them for food and space. Signal crayfish are also carriers of the crayfish plague (Aphanomyces astaci) which does not affect the signals but causes 100% mortality in native crayfish populations. They also trip river banks of aquatic vegetation, destroy river banks through extensive burrowing and prey on fish eggs and juveniles. The UK government spent on average £1 million/year trying to control the spread of this invasive species.

What is being done?

Environmental companies have been using a variety of methods to try and control the spread/eradicate signal crayfish, these include:


-Manual removal of crayfish

-Removal of habitat

-Installing physical barriers to dispersal (i.e. in-channel weirs, dams, screens, catch-pit outfalls, fencing)

-Introduction of predators (e.g. eels, chub)

Despite all the efforts above, signal crayfish populations continue to spread. However, new scientific methods are being developed which will allow scientists to detect signal crayfish presence in the local water body from the DNA they leave behind in the water.  This will allow potentially newly colonised sites to be identified and targeted as areas to focus eradication efforts to prevent further dispersal.

How can you help?

Part of the problem with trying to eradicate signal crayfish is that we do not know exactly where they are – this is where you can help. The AquaInvaders project allows you to record any sightings of signal crayfish (and a large number of other aquatic invasive species) on your smartphone, tablet or via the website. The app is free, very easy to use and is a great way of getting involved with helping your local conservation groups.

VOLUNTEERS REQUIRED: If you would like to get involved or more information on a nationwide citizen science project which involves use of the app to monitor signal crayfish then please send an email to

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