Successful Eradication of Signal Crayfish (Pacifastacus leniusculus) Using a Non-Specific Biocide in a Small Isolated Water Body in Scotland

The North American signal crayfish (Pacifastacus leniusculus) has been present in Scotland since at least 1995 and the species is now known to be present in a number of catchments. Once established, few opportunities for containment exist and eradication can often be impossible to achieve. However, in small, isolated water bodies, the application of a non-crayfish-specific biocide has provided the opportunity to remove this species permanently. In July 2011, signal crayfish were discovered in a flooded quarry pond at Ballachulish in the Scottish Highlands. This is an isolated site located ~100 km from the nearest known population and it is likely that the population was established as the result of a deliberate release of these animals 10 years previously. Experience gained from using the eradication technique at other sites in the UK led to the site being treated with a natural pyrethrum biocide (Pyblast®) in June 2012. Post treatment monitoring from 2012–2017 indicates that eradication has been successful. Monitoring of native species affected by the biocide suggests that both invertebrates and amphibians quickly recolonised the quarry pond. Eradication of crayfish using biocide is only feasible in water bodies where the entire population of crayfish can be exposed to a lethal dose and the impact on non-target species can be accepted. The technique is not appropriate for large, connected water bodies, although it may be possible to treat short stretches of canals where biocide exposure can be controlled and isolated populations of crayfish can be effectively treated.