Warning to river users as crayfish plague detected in Blackwater River

Photo by Sidney Pearce.

River users have been urged to take precautions after confirmation of an outbreak of plague affecting crayfish on the River Blackwater.

The crayfish plague causative agent was also detected in the River Awbeg and Spa Glen Stream, which are within the extensive Blackwater River catchment.

Described as ‘worrying’, the situation is being monitored by the National Parks and Wildlife Service (NPWS), Inland Fisheries Ireland (IFI), the Marine Institute and independent ecologists.

Announced on Wednesday, July 5, this is the first recorded outbreak of the deadly crayfish plague in County Cork since it was first discovered in Ireland in 2015 in County Cavan, having since spread to several other rivers across the country.

“This outbreak is of great concern as it is within the Blackwater River (Cork/Waterford) Special Area of Conservation (SAC) which contains an internationally important population of white-clawed crayfish,” the statement read.

Crayfish plague was detected in samples taken from the Blackwater River at Longfields Bridge and Mallow and in the Spa Glen Stream at Castlelands, Mallow.

In response to the outbreaks, a National Crayfish Surveillance Programme was established in 2018 as a Memorandum of Understanding between NPWS and the Marine Institute. This programme uses environmental DNA (eDNA) – a novel, non-invasive method of detection of the DNA of crayfish and the disease from water samples. It monitors the spread and persistence of crayfish plague throughout Ireland and the distribution of the white-clawed crayfish. 

The white-clawed crayfish is a globally threatened species and Ireland holds one of the largest surviving populations. Crayfish plague is devastating, causing 100% mortality to the species.


Given the experience of outbreaks elsewhere, a total kill of the crayfish population is expected which will have major consequences for the ecology of the Blackwater, Awbeg and the whole of the Munster Blackwater catchment.

There is no indication of how crayfish plague reached the catchment, but the disease is easily transmitted in water or via contaminated equipment (e.g. kayaks, waders or nets). As the Blackwater catchment is popular with anglers, kayakers and other recreational users, the NPWS and IFI are urging all users of any river to implement the ‘Check, Clean and Dry’ protocol which involves routine checking, cleaning and drying of equipment after leaving a river and before entering another waterbody.

Cleaning everything that has been in contact with the water using hot water (above 45oC) or a high-pressure spray if possible followed by a drying period where all equipment and wet gear is dry for at least 48 hours, should be adopted as standard practice in all freshwaters. Disinfect everything if complete drying is not possible.

“Containment of the outbreak is essential to prevent spread to other as yet unaffected populations in Ireland. If the crayfish plague continues to spread, there is a high probability that the White-clawed Crayfish will become extinct from most rivers in Ireland,” the statement added.

It is completely harmless to people, pets, livestock and all other freshwater organisms.


White-clawed crayfish is the only native freshwater crayfish species found in Ireland and is present in lakes, rivers and streams over much of the island. Throughout its European range, this species has been decimated by the impact of crayfish plague which spread to Europe with the introduction of North American species of crayfish, especially Signal Crayfish.

Many American crayfish species are resistant to crayfish plague but can act as carriers of the disease which is rapidly fatal when passed to the white-clawed crayfish.

To date there has been no record of American crayfish species in Ireland, suggesting that the spread of the crayfish plague is solely due to human activities and lack of biosecurity. 

White-clawed crayfish are protected under Irish Law and the EU Habitats Directive and it is illegal to introduce any non-native species of crayfish to Ireland.

The combined impact of the introduced crayfish species (which may out-compete the smaller native crayfish) and crayfish plague have completely eliminated the white-clawed crayfish from much of its European range, now leaving Ireland as the last stronghold of the species.