This article has been prompted by the recent announcement of an outbreak of crayfish plague in the River Ure at Askrigg. The purpose of this article is to provide some background and practical advice.

Last autumn I wrote an article about an invasive non-native species (an INNS), the fungal disease causing Chalara Ash Dieback. This article is about two more INNS, but whose affect is far less obvious to the casual observer than a countryside littered with dead and dying trees. One INNS is yet another fungal disease, Crayfish plague (Aphanomyces astaci), the other INNS is its chief vector, the North American Signal Crayfish (Pacifastacus leniusculus). In this case the species suffering is the only native crayfish, the once widespread White-clawed crayfish (Austropotamobius pallipes).


Signal Crayfish

Signal Crayfish by White Knight, CC BY-SA 3.0 , via Wikimedia Commons

The Signal Crayfish has a European history starting in the middle of the 19th century; however, it is only relatively recently that it was first recorded in to the UK, in 1975. It is one of several species introduced for a variety of reasons such as keeping as pets, or in the case of the Signal, as an adjunct to fish-farming. The charity Buglife has an informative webpage outlining the various other species that have been introduced as well as providing a central hub of related information. Escapes from such introduction sites have been shown to lead to the rapid demise of the native crayfish with one local example being in the River Wharfe. Native crayfish disappeared from approximately 20 km of the river either side of Grassington over a period of approximately a decade in the 1990s (Lee, pp 242-244)[1]. The Signal crayfish not only carries the disease to which the native crayfish is highly susceptible, but is also an aggressive competitor with more widespread deleterious effects on the riverine ecosystem. Crayfish plague has also recently spread to the island of Ireland where it is also being treated with great seriousness, additionally due to the potential impact on salmon and trout fisheries. This must also be a concern for rivers in the Dales.

To illustrate the seriousness of this relatively local reported event, it is worth quoting some of an email from Tom Pagett of the Environment Agency shared with the Yorkshire Dales Biodiversity Forum, on which I represent the Field Society, via Tony Serjeant, Senior Wildlife Conservation Officer of the YDNPA:

‘White-clawed crayfish are our largest native, freshwater invertebrate but are now rare and protected in Britain. The River Ure is considered to have the third most important population remaining in England, so an outbreak of plague here has very serious implications. The disease is transmitted through spores in the water, and by the crayfish themselves as they interact. Plague is fatal to white-clawed crayfish, and we anticipate that over the course of the winter, it will decimate the entire population on the Ure.’

In this context, I think ‘decimate’ can be taken to mean ‘wipe out’. It is hoped that this brief introduction to the crayfish plague, its host, and its consequences, is sufficiently strongly convincing of the seriousness with which this hidden problem must be taken.

Means of Infection

There are two ways in which the plague can be introduced in to a waterbody. One is directly through the fungal spores being carried from a previously infected waterbody, the other is by the movement of infected signal crayfish.

Deliberate introduction of non-native crayfish is now illegal and any activity involving them must be appropriately licenced by the Environment Agency. Unfortunately, Signal crayfish are quite able to transfer themselves between waterbodies and courses, travelling considerable distances over land from existing populations. Furthermore, it is difficult to know that they have arrived in a watercourse for some years until they start to have obvious impacts on their new home. This, and a variety of other factors means that their spread is exceedingly difficult to control. Once established, they are difficult, if not impossible, to remove.

It is also possible to transfer the plague fungal spores directly between waterbodies and courses. Even such seemingly innocent activities as paddling or using a rubber dinghy in a river one day, and another river the next, can transfer viable spores. Angling organizations are particularly aware of the possibility that anglers may inadvertently contribute to this spread and encourage the practice of good biosecurity.

In connection with this outbreak, Tony Serjeant writes:

Plague-causing fungal spores can survive in water that has been in contact with crayfish hosts and, at this time, it is thought that this is how it has been transmitted to native crayfish in the upper Ure (rather than through invasion or colonisation by infected Signal Crayfish). Unfortunately, the spores can swim and will actively seek out crayfish hosts. The fungus has been shown to be able to survive for several months in water in temperatures down to 2°C and still be capable of producing active spores. It should be noted that this is a species specific disease and there are no implications for human or other animal health.’

In order to retain some populations of native crayfish, remnant populations are being kept isolated in ‘Ark’ sites, such as was established in Threshfield Quarry. The choice, establishment and maintenance of such sites is complex as shown by the Buglife guidance. Where possible, suitably isolated water courses with populations of native crayfish are also being managed as ark sites.


Having provided the background to the local resurgence of this national problem, it is also possible to provide some guidance intended to minimise risk of transmission by quoting the Environment Agency response:

‘We are moving quickly to protect the remaining white-clawed crayfish in the catchment.

Surveys have just been completed to understand the current extent of the infection

We are actively searching for Ark sites. These are areas away from the main river, where we could move healthy crayfish to in order to safeguard the population

We are investigating the use of our facilities to quarantine crayfish before we move them to Ark sites

We have set up a dedicated incident team who will lead our response.


How can you help

This incident is likely to continue until next spring, and the priority is to stop this disease spreading to other catchments. Key to this is ensuring that we all follow good biosecurity principles. If you are visiting the area, please follow our biosecurity guidance.

Do not use the same equipment or footwear in another stream without treatment

Following your visit clean off any mud or vegetation and remove any standing water from your equipment, preferably with hot water

Either thoroughly dry the equipment, preferably in sunlight making sure all nooks and crannies are dry for at least 48 hours; or

Treat with an iodine based disinfectant capable of killing fungal spores, such as Virkon Aquatic S’

My concluding remark in the context of Chalara Ash Dieback was that it is my view that nationally we need to take biosecurity a good deal more seriously, follow a precautionary approach, and act much more swiftly in the event of suspected problems. That conclusion is reinforced by the impact of thoughtless importation in the case of a completely different taxon.

Chris Alder

All websites accessed in October 2020

[1] Lee, J. (2015) Yorkshire Dales. London: William Collins.