Crayfish in Britain

Signal crayfish are a non-native species which cause considerable harm to the ecosystem and potentially the economy. They are carriers of the crayfish plague, (a water mould that infects crayfish), to which they are resistant but other crayfish species are not, and it is pushing the White Clawed Crayfish closer to extinction. The White Clawed Crayfish is already classed as endangered due to this.

Signal crayfish have a faster growth rate than the native White Clawed Crayfish and this means that resources get taken by the non-native species before the White Clawed Crayfish has chance to mature. There are numerous ways to spot the differences between the two species, as the White clawed crayfish has a white underside to its claws compared to the singular white spot on the claw hinge of the Signal Crayfish. Both crayfish have different colours, the White Clawed Crayfish is an olive-brown colour while the Signal Crayfish is a blueish-brown to reddish-brown colour. The general sizes of the crayfish can be very similar as the native species can grow up to 12cm, but sizes under 10cm are far more common, while the Signal Crayfish can grow from anywhere between 6cm and 18cm. The habitats for both species are very similar, as they both live in freshwater rivers, lakes or streams where there is little to none salt content.

Signal Crayfish can cause economic damage as they burrow up to 2 metres into the riverbank which weakens it, but it only gets worsened as they can have multiple burrows intersecting with each other which causes the river bank to be weakened significantly more.  This can cause riverbanks to collapse which then makes it a flooding risk to buildings on the riverbank and can be a risk to livestock safety.

Both of these species of crayfish are present within the River Don and this is problematic due to the endangerment of the White Clawed Crayfish.

White clawed cray fish (Austropotamobius pallipes):

crayfishWhite-clawed Crayfish. Photo by Natural England/Jenny Wheeldon / CC BY

  • Appearance – olive/brown colour, pale undersides to the claw (pallipes translates to pale footed), can grow up to 12cm/5 inches but sizes below 10cm are common.
  • Habitat – Lives in lakes, rivers or streams usually about 1m deep where it hides amongst rocks and submerged logs, comes out to forage for food.
  • Conservation status – Falls under threatened as signal crayfish kill them off as it is a carrier for crayfish plague. Insecticides are also a major factor in white claw crayfish population declines.


Signal crayfish (Pacifastacus leniusculus):

crasyfish 2A Signal Crayfish. Photo by Andrew Gray  / CC BY

  • Appearance – 6-9cm (2.4-3.5 inch long) can grow up to 16-18cm (6..3 – 7.1 inch), blueish-brown to reddish-brown colour with large claws. They have a white to pale blue-green patch near the claw hinge.
  • Habitat – Rivers, streams or lakes, places that White Clawed Crayfish live and this cause them to die due to crayfish plague.
  • Conservation status – Least concern
  • Effects as an alien species – Signal crayfish are driving White Claw Crayfish and other native crayfish species towards extinction as they spread the crayfish plague and compete for resources. They grow faster and are far more aggressive than WCC, they can tolerate a much larger variety of conditions as well. Diet consists of fish and amphibian eggs, tadpoles, detritus, aquatic invertebrates, juvenile fish and aquatic vegetation which can decline populations of other species. Signal crayfish also have an economic impact as their burrows can cause river banks to collapse which then need replacing before flooding occurs which can damage livestock safety and stability of structures that are built upon the banks. Crayfish take refuges from salmonid fish and predate their eggs, which can reduce the value of fisheries.

Blog by
Josh Laidlow