In our series of blogs for 2024, we’re going back to basics. This is a guide for budding ecologists who are keen to get started in the industry, or anyone interested in learning a little more about professional ecology practices in the UK.

This month, we're looking at one of our under-appreciated aquatic invertebrates; the crayfish.

About Crayfish

Among the seven species of crayfish that inhabit the UK, the white-clawed crayfish (Austropotamobius pallipes) is the only one that is native. All crayfish have the appearance of small lobsters, and the white-clawed is identified by its small size, bronze-coloured body, with pale undersides to its claws. Despite its widespread presence, the white-clawed crayfish is considered rare and is increasingly difficult to spot in the wild.

White-clawed crayfish are typically found in shallow, freshwater streams, preferring clean, well-oxygenated water where they can be found hiding under rocks, seeking shelter from predators. Their preference for such specific habitats makes them particularly vulnerable to environmental changes and pollution.

As omnivores, white-clawed crayfish have a varied diet, including plants, small invertebrates, and detritus. Their role as predators and scavengers makes them a crucial part of the food web in their freshwater ecosystems. They help maintain the balance by controlling populations of smaller organisms and contributing to the decomposition process.

Crayfish Conservation

Due to its declining population and numerous threats, the white-clawed crayfish has been granted protection under both the Wildlife and Countryside Act (1981) and the Conservation of Habitats and Species Regulations (2017). It is illegal to intentionally kill, injure, or take white-clawed crayfish from the wild without a proper license.

The white-clawed crayfish is classified as "Endangered" on the International Union for Conservation of Nature (IUCN) Red List of Threatened Species. This is a global biodiversity indicator with records of over 150,000 species based on assessments by experts and organisations. Being listed as endangered means that the white-clawed crayfish is at a very high risk of extinction in the wild.

The population of white-clawed crayfish has been in rapid decline since the 1970s. Several factors have contributed to this sharp decrease, including pollution, habitat destruction, and the introduction of invasive species. The loss of clean, unpolluted freshwater habitats has significantly impacted their numbers. In addition, the fragmentation of their habitats has made it difficult for populations to recover and expand.

Their most severe threat comes from the invasive signal crayfish (Pacifastacus leniusculus). The signal crayfish was initially brought from America for aquaculture, however, it quickly spread into the wild, where it became a formidable competitor to the native white-clawed crayfish. Not only is the signal crayfish larger and more aggressive, but it also carries a deadly fungal disease known as crayfish plague. White-clawed crayfish have no natural immunity to this disease, and as a result, entire populations can be wiped out rapidly once infected. The signal crayfish can easily be distinguished from the white-clawed due to it's robust size, and the bright-red underside to it's large claws which it waves to deter predators.

Signal Crayfish by Vaclav Matous via shutterstock

Survey Methods

Due to their protected status, handling or disturbing white-clawed crayfish requires a special license. Licenses are typically issued for scientific research, conservation work, or education, and strict guidelines must be followed to prevent any harm to the crayfish or their habitats. Surveys and handling are typically avoided during key periods such as the breeding season (May to July) and wintering months (November to February). During the breeding season, female crayfish carry eggs under their tails, and any disturbance could jeopardise the next generation of crayfish. Similarly, in the winter months, crayfish are less active and may be more vulnerable to stress and injury.

  • One of the most common methods for surveying crayfish populations is through the use of nets or by hand. This method is often employed in shallow water habitats where white-clawed crayfish are likely to be found hiding under rocks or in crevices. Surveyors carefully lift rocks and other debris to reveal the crayfish, which can then be gently captured using nets or by hand.
  • White-clawed crayfish are primarily nocturnal, meaning they are most active during the night. As a result, night surveys are an effective way to observe and monitor their populations. During these surveys, surveyors use torches to illuminate the water and search for crayfish as they emerge from their hiding places to feed.
  • In addition to hand and net surveys, traps are also used to capture crayfish for monitoring purposes. However, trapping requires very specific requirements to ensure that it does not harm other wildlife, particularly water voles, which are also protected species. Traps must be designed to allow the safe release of non-target species and to prevent drowning. You can find more information on trap licenses here.

When surveying areas that might contain non-native crayfish species, such as the invasive signal crayfish, special care must be taken. It is illegal to return non-native crayfish to the water once they have been captured. This is because non-native species pose a significant threat to the native white-clawed crayfish and the overall ecosystem. Captured non-native crayfish must be humanely dispatched or removed from the environment according to legal requirements, helping to control their spread and reduce their impact on native species.

Take a look at our equipment for surveying crayfish here!

Mitigation Methods

One of the immediate mitigation methods used is the removal and exclusion of crayfish from areas where habitat management or construction work is planned. This process involves carefully capturing the crayfish and relocating them to a safer area, away from the immediate area of disturbance. By doing this, the risk of injury or death due to machinery, construction activities, or habitat alterations is significantly reduced.

To aid the long-term survival of crayfish-populations, effective habitat management and maintenance plans must be put in place. This involves:

  • Ensuring that streams and rivers remain unpolluted and well-oxygenated with regular monitoring.
  • Preserving shelter like rocks, submerged logs and vegetation.
  • Restoring habitats where they have been degraded by reintroducing natural features, controlling erosion and planting native vegetation to stabilise banks.
  • Most importantly, measures to remove invasive species to reduce competition and prevalence of crayfish plague

Further Reading

by valda butterworth via Shutterstock