Research and analysis

White-clawed crayfish mortality investigation: summary

Published 23 October 2026

Applies to England

1. Chief Scientist’s Group report summary

This project investigated unexplained mortalities of the protected white-clawed crayfish (Austropotamobius pallipes) in England’s rivers in 2024. Understanding the cause of such wildlife mortalities can help the Environment Agency and others better conserve protected species and ensure the health of ecosystems and the services they provide. 

1.1 Background

White-clawed crayfish are Britain’s only native freshwater crayfish and are classified as endangered. Reports of dead crayfish and shell abnormalities were received from several rivers in the North East area of the Environment Agency during July and August 2024.

Initial investigations found no evidence of pollution, impacts on fish or other aquatic wildlife, or known notifiable crayfish diseases. Similar abnormalities were subsequently identified in two white-clawed crayfish populations in the Midlands area. The project aimed to determine the cause of the mortality events and assess implications for white-clawed crayfish conservation.

1.2 Approach

Investigators carried out extensive field surveys across affected catchments in the North East and Midlands area to assess the extent of mortalities and shell abnormalities. Water quality assessments and disease screening investigated potential environmental and disease-related causes.

Live white-clawed crayfish from affected populations were monitored under laboratory conditions to observe the progression of the shell abnormalities and whether individuals recovered.

Researchers from Teesside University and Cefas conducted visual (pathology) and microscopic examinations to look for evidence of disease or tissue damage. Molecular (genomic) techniques were used to compare healthy and affected white-clawed crayfish and identify viruses, bacteria, fungi and other microorganisms that might be associated with the symptoms.

1.3 Results

The investigation ruled out pollution, crayfish plague, white spot syndrome virus and other known diseases as the direct cause of the mortality events. Although a wide range of viruses, fungi, protozoans and other microorganisms were detected on both healthy and affected white-clawed crayfish, none could be conclusively linked to the shell abnormalities or mortalities.

Laboratory observations showed that many white-clawed crayfish survived and recovered over time. Shell abnormalities often reduced after moulting, where crayfish shed their hard outer shell, particularly in juvenile crayfish, which moult more frequently than adults. Of the 23 individuals with shell abnormalities monitored, 43% showed visual signs of abnormalities by the end of the study and a further 20% showed visible improvement.

Follow-up surveys in 2025 found a substantial reduction in shell abnormality occurrence. The proportion of white-clawed crayfish displaying abnormalities fell from 26% in 2024 to 4% in 2025, abnormalities decreased, and no further widespread mortality events were reported. Incidence of shell abnormalities were even lower during 2026.

The study also highlighted the limited understanding of the normal microorganisms associated with wild white-clawed crayfish populations, making it difficult to distinguish between harmless organisms and potential disease agents.

1.4 Conclusions

Although no single disease, pathogen, pollution event or environmental factor could be conclusively linked to the mortalities, the project ruled out several important causes, including pollution and known notifiable diseases, and provided evidence that affected populations have persisted and are recovering. It is possible that multiple factors, potentially including naturally occurring pathogens and environmental stressors, may have contributed to the mortalities.

The project has improved understanding of white-clawed crayfish health and demonstrated the value of combining pathology and genomic techniques to support future disease surveillance, mortality investigations and conservation management of white-clawed crayfish.

1.5 Publication details

This summary relates to information from project SC240055, reported in detail in the following output:

• Report: SC240055/R
• Title: White-clawed crayfish mortality investigation
• Science managers: Ian Marshall, Fisheries and Biodiversity, North East Area, and Judy England, Research, Chief Scientist’s Group
• Research Contractor: Jamie Bojko, National Horizons Centre, Teesside University
• Research Collaborator: Centre for Environment, Fisheries and Aquaculture Science (Cefas)

This project was commissioned by the Environment Agency’s Chief Scientist’s Group, which provides scientific knowledge, tools and techniques to enable us to protect and manage the environment as effectively as possible.

Enquiries: research@environment-agency.gov.uk.

© Environment Agency