We love the ‘All About’ series, published on the Severn Rivers Trust website. These blogs have been written by Unlocking the Severn volunteers and apprentices, so we figured we’d share them here too! See the original blog on the Severn Rivers website HERE.

Written by Jodie Rochford, a volunteer for Unlocking the Severn and Severn Rivers Trust. 

What are Signal Crayfish and why are they such a problem?  

Signal crayfish are a non-native, invasive species that were introduced from North America by the British Government in the 1970s [1]. They were farmed for food and exported to the Scandinavian market, but soon escaped the fishing farms and entered our waterways, causing havoc with our native ecosystems [1].   

The decimation of our native White-Clawed crayfish population is possibly the most devastating impact Signal crayfish have caused. Our White-Clawed crayfish have had a population decline of 50-80% across Europe in the last 10 years, classifying them now as endangered [1]. Signal crayfish carry a crayfish plague which is fatal to the White-Clawed crayfish. The disease spreads by waterborne spores which can last up to 2 weeks in our waterways [2]. The spores can be killed by disinfecting or drying equipment that has come in contact with the waterway [2]. 

Signal crayfish also outcompete the White-Clawed crayfish for habitat and food, as they feed on just about anything – fish, tadpoles, juvenile fish, invertebrates, amphibian eggs and even their own species [2]! This impacts other native species and can disturb the natural food web of our aquatic ecosystems [1]. Signal crayfish are also more tolerant to varied conditions and reproduce more quickly [1], so have become widely distributed across our waterways [3].   

Signal Crayfish are not only detrimental to our aquatic ecology but also to river stabilisation. They burrow interconnecting tunnels up to 2m into the riverbanks, which destabilises the sediment and erodes the riverbank [1]. This causes sediment pollution by increasing the sediment load of the river [4]. Bank destabilisation can also increase the flood risk [5], so it is important to try and control the spread of Signal crayfish.  


An invasive Signal crayfish (left), and a native White-Clawed crayfish (right). 

How to spot the difference between Signal Crayfish and our native White Clawed Crayfish 

Crayfish are most active in summer. Signs of their presence can be shown by burrows in the riverbanks, and from claws and body shells found on the river edges due to predation from birds, rodents and otters [3]. The species are prevalent in still and slow-flowing waterbodies [2]. You may be able to see Signal crayfish in the day but native White-Clawed crayfish tend to be nocturnal, coming out at night [3]. Identification features to help tell the difference between Signal crayfish and White-Clawed crayfish are shown in the photo below. 

Comparing features of Signal and White-Clawed crayfish. Images courtesy of Malvern Hills Crayfish Group. 

What methods can be used to reduce the spread of Signal Crayfish? 

Signal crayfish are unfortunately very widespread across the UK [3]. It is vital that we try to reduce their spread as much as possible. The most effective method to follow is the ‘Check, Clean, Dry’ campaign to make sure that we are not moving the crayfish plague from one waterbody to the next [7,8]. 

Trapping is another method, which involves surveying to assess whether Signal crayfish are present in the waterway. This can only be conducted if you have a license from the Environmental Agency or Natural Resources Wales [5]. In addition, there are strict regulations on trapping crayfish: licenses have to be obtained and the traps have to be a certain size to avoid harming other species like otters and water voles [9]. Signal crayfish are not allowed to be removed from the site of capture, due to the risk of spread, so they must be killed on site [9]. Due to the complexities of ensuring best practice, trapping is not a widely accessible method for reducing the Signal crayfish population, so it is vital that we all follow the ‘Check, Clean, Dry’ campaign. 

Check, Clean, Dry 

The Check Clean, Dry campaign is a UK wide initiative that was started in 2011 By DEFRA [7] to prevent spread of harmful species like Signal crayfish. It is important that when you leave an area, especially if there are any waterbodies (rivers, streams, lakes and canals), to follow ‘Check, Clean, Dry’ [8].   

  • Check – Check your footwear, clothes and any recreational equipment for any plant material, mud or aquatic animals.  
  • Clean – Thoroughly wash your footwear, clothes and any recreational equipment, use hot water if possible.  
  • Dry – Allow footwear and equipment to dry, as this kills any crayfish plague spores, making sure that these are not spread into other waterbodies. 

We need you! 

The Unlocking the Severn project and the Severn Rivers Trust are working with Malvern Hills Crayfish Group to support them in their efforts to control the spread of Signal crayfish locally. They are currently recruiting new members to help out in a voluntary capacity. More information can be found on their Facebook page, along with their calendar of events. 



[1] Inland Waterways. 2021. Signal Crayfish | Inland Waterways. [online] Available at: <https://www.waterways.org.uk/about-us/news/signal-crayfish> [Accessed 23 May 2021].  

[2] Inside Ecology. 2021. Invasive non-native species (UK) – Signal crayfish – Inside Ecology. [online] Available at: <https://insideecology.com/2017/09/27/invasive-non-native-species-uk-signal-crayfish/> [Accessed 23 May 2021].  

[3] Booy, O., Wade, M. and White, V., 2021. Signal Crayfish. [online] Non-Native Species Secretariat. Available at: <https://thecrayfishcompany.com/Identifying-Signal-Crayfish.pdf> [Accessed 23 May 2021].  

[4] Johnson, M., Rice, S. and Reid, I., 2010. Topographic disturbance of subaqueous gravel substrates by signal crayfish (Pacifastacus leniusculus). Geomorphology, 123(3-4), pp.269-278.  

[5] Alvis, H., 2021. American signal crayfish – the law – Bristol Avon Rivers Trust. [online] Bristol Avon Rivers Trust. Available at: <https://bristolavonriverstrust.org/signal-crayfish-law/> [Accessed 23 May 2021].  

[6] Malvern Hills Crayfish Group. 2021. Crayfish. [online] Available at: <https://www.facebook.com/groups/1066656443838283> [Accessed 23 May 2021].  

[7] GOV. 2021. Stop the spread for Invasive Species Week. [online] Available at: <https://www.gov.uk/government/news/stop-the-spread-for-invasive-species-week> [Accessed 22 May 2021].  

[8] Non Native Species Secretariat. 2021. Check, Clean, Dry – GB non-native species secretariat. [online] Available at: <http://www.nonnativespecies.org/checkcleandry/> [Accessed 22 May 2021].  

[9] GOV. 2021. Permission to trap crayfish, eels, elvers, salmon and sea trout. [online] Available at: <https://www.gov.uk/guidance/permission-to-trap-crayfish-eels-elvers-salmon-and-sea-trout> [Accessed 23 May 2021].  


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