Signal Crayfish

(Pacifastacus leniusculus)

The Signal Crayfish Pacifastacus leniusculus (hereafter P. leniusculus) is an omnivorous  crustacean native to western North America. The diet of P. leniusculus is varied and can include juvenile fish, the eggs of fish and amphibians and plant matter.


In the UK and across continental Europe, signal crayfish have had and continue to have a variety of negative ecological impacts on freshwater systems. Impacts include but are not limited to the introduction of fine sediments into water course through burrowing and adjustment of native macro invertebrate communities.


The species was first introduced to the UK in 1976 to supplement the Scandinavian Asticus asticus fisheries that were suffering the effects of the crayfish plague. Unfortunately, it was soon discovered that P. leniusculus from the US were also infected but the species appeared immune to its effects. P. leniusculus escaped into Southeast rivers and streams from holding ponds and today, the species is present across England & Wales with a few isolated populations in Scotland. They are found in over 25 countries internationally and are the most widespread invasive species in Europe. The spread of P. leniusculus and indeed, crayfish plague in the UK, has led to significant declines in our native White Clawed Crayfish. This smaller species is highly vulnerable to the plague carried by the invaders. Furthermore, given their small relative size they are less able to cope for resources, including food and residence habitats, meaning populations dwindle when P. leniusculus invade the territories of white clawed crayfish.

Debate exists around the efficacy and suitability of trapping as a management method for P. leniusculus. Some studies have shown trapping to be effective but only where management occurs persistently over long time periods. Importantly, only approved traps can be used to manage P. leniusculus populations and each trap must be registered with the Environment Agency. More information on crayfish trapping and trap consenting can be found here .

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