Crayfish Project

A signal crayfish from the lake at Hatfield Forest

The lake and water courses at Hatfield Forest are being invaded by a visitor from North America, the signal crayfish. We are undertaking an extensive study to assess the ecological impact of this unwelcome guest.


Hatfield Forest, including the lakes and feeder streams, has been a Site of Special Scientific Interest (SSSI) since 1956.  It is also a National Nature Reserve.

The natural environment is always changing, but some of the changes are less welcome and not due to natural causes.

Signal Crayfish

The lake now has a population of  a non-native, invasive, crayfish, the signal crayfish Pacifastacus Leniusculus.  This is a native of North America.  It was first introduced into Europe in the 1960's, to try and supplement stocks of the native European species Astacus astacus which were being depleted by disease.  Unfortunately, the imported species was also found to be a carrier of the disease.

Signal crayfish are typically 60 to 90mm long, although can grow as large as 160 to 180mm.  They are blueish-brown to reddish-brown in colour with robust, large, smooth claws. They have a white to pale blue-green patch near the claw hinge and can live for up to 20 years.  The signal crayfish is much larger than the native species.

The negative impacts of the signal crayfish invasion include the almost complete loss of the native crayfish through the spread of disease and direct competition. They also undermine riverbanks through burrowing and can predate on native fish eggs and aquatic invertebrates.

The Forest project

Our project is centered on the main lake and its feeder stream. We are also conducting a wide-ranging ecological study around all the lakes and watercourses within the forest. The project includes an extended program of trapping, removal and study of crayfish. This will assist future plans regarding the ecological impact on the lake and its immediate surroundings. The research will support the trapping project.

This project is part of a larger project being run by CEFAS/DEFRA (Centre for Environment, Fisheries, and Aquaculture Science) together with the Angling Trust to investigate crayfish trapping and control methods.


01 Oct 18

Update Autumn 2018

The crayfish volunteers have been regularly setting traps in the lake, the preferred bait being 'boilies', a carp bait. Over a period ot time, the catches have been getting noticeably smaller, suggesting that the population is decreasing.  Netting sizes have been reduced to catch more juveniles as these seem to be forming a greater proportion of the population.

25 Feb 19

Update February 2019

The Crayfish Volunteers have been reporting very small catches recently. Over the last 12 weeks they have caught only 12 crayfish, from 200 traps. Only one female was caught this week, marking a really important break in their breeding cycle. They think that the combination of consistent weekly trapping over the last six years and the conservation work on the land to remove their food source - removal of alders along the dam and coppicing around the main lake - has made a significant impact on reducing the numbers. The volunteers are now planning to reduce their number of visits to maybe once a month, and see what happens.