River Chelmer

Essex Biodiversity Project, Essex Wildlife Trust and Essex Ecology Services LTD (EECOS) have worked together with the Environment Agency, Writtle College and Essex and Suffolk Water to create the county's first safe 'Ark' site for our threatened native crayfish – the White-clawed Crayfish.

In the 1960s White-clawed Crayfish (Austropotamobius pallipes) could be found in most Essex rivers, from the Roding and Stort to the Colne and Stour. As the largest native freshwater invertebrate, White-clawed Crayfish play an important role in the aquatic food web being considered a 'keystone' species and an indicator of healthy, unpolluted rivers. They are omnivorous, although they mainly eat smaller invertebrates, and are themselves predated by species such as dragonfly larvae, Brown Trout, Grey Heron and Otter (spraints can be found packed with crayfish remains!). The ‘freshwater lobster’ is nocturnal and most active during the late summer and autumn, at which time adults search for mates; during the day they shelter under stones, in a burrow or amongst submerged tree roots and aquatic vegetation. The oldest evidence of the species in the UK comes from Clacton, where fossils have been found in Pleistocene deposits.

The fate of Britain's only native crayfish has changed dramatically for the worse since the 1970s, when an aggressively invasive American crayfish species – the Signal Crayfish (Pacifastacus leniusculus) – was first introduced into the country's rivers via Scandinavia. Signal Crayfish have now been recorded in most rivers in Essex, where they out-compete and predate the smaller native species. In addition, the American species can carry Crayfish Plague – a fungus-like disease that is lethal to White-clawed Crayfish, and which has wiped out populations across the country. Just as the Grey Squirrel has replaced the Red, this is an all-too-familiar tale of a native species losing out to an exotic species introduced through human actions.

Faced with possible extinction from our rivers, there is a sense of urgency to save remaining populations of White-clawed Crayfish, a UK and Essex Biodiversity Action Plan (BAP) species. A recent approach to crayfish conservation is to establish isolated safe sites – known as 'Ark' sites – where new populations can become established, well away from Crayfish Plague and invasive crayfish. Translocation of a species from the wild into a new site is a last resort – however, with time running out for the native crayfish, emphasis was placed on moving threatened individuals and stocking the county's first Ark site.

Locating an 'Ark' site

The Essex Biodiversity Project has been working together with Essex Wildlife Trust’s own ecological consultancy EECOS and the Environment Agency since March 2011 to locate a suitable Ark site. Over 80 standing water sites were considered following consultation with Trust staff and other organisations, and these were finally whittled down to two candidate Ark sites. Strict criteria determine the suitability of a site for White-clawed Crayfish (i.e. good water quality, suitable habitat, absence of fishing and being isolated from Signal Crayfish), and a number of surveys had to be carried out and hurdles overcome. Water chemistry tests and surveys for invertebrates, Great Crested Newts and fish surveys were all carried out before the two sites could be declared fit for crayfish. The translocation from the proposed donor site on the River Chelmer could now be organised.

Before crayfish could be moved, extensive habitat enhancement of the first Ark site – a reservoir in Chelmsford District – was required. Crayfish are happiest under a rock or in a crevice where they can shelter from predators, including larger crayfish, and a total of 20 tonnes of bricks and granite was kindly donated by Essex and Suffolk Water and by Essex Wildlife Trust's Wrabness Nature Reserve. Abberton Reservoir Nature Reserve supplied 40 large faggot bundles of mainly Hawthorn and Blackthorn, assembled with the help of a secondary school group and the Trust’s Education Outreach team to provide important woody debris within the new site. The crayfish habitat was installed with the help of Essex Wildlife Trust Reserves staff (and their tractor!) and Writtle College staff, just in time for the planned release into the newly refurbished site.

The translocation begins

The first translocation attempt took place on 7th October 2011. A dedicated team of 16 crayfish surveyors, including many of the Conservation and Reserves team and students from Writtle College, were armed with waders and pond nets ready to catch as many crayfish as possible. Unfortunately, several dead and dying White-clawed Crayfish were discovered, as well as an alarming number of Signal Crayfish that had colonised the donor site more rapidly than first thought. A total of 15 healthy looking White-clawed Crayfish were recovered, but with fewer crayfish than surveyors the translocation was abandoned for the day. Specimens were collected by the Environment Agency for testing for Crayfish Plague. The decline of what was previously regarded as the best population in Essex prompted a good deal of soul searching and fears that we had arrived too late.

A much-needed stroke of good luck came the following week when, having scoured historical records, a large previously unknown population of White-clawed Crayfish was discovered on a small tributary of the River Chelmer. Without further delay, a second translocation attempt was arranged for 14th October. After a long day in the river, a grand total of 212 healthy crayfish was caught, spanning the age ranges from small immature crayfish to large adults. One hundred tiny juveniles were returned to the brook to restock the population. In fading light, the White-clawed Crayfish were released into their new home, where they quickly crawled under the nearest stone. The new Ark site, which is to be joined by at least one further site in the coming year, represents a significant step towards securing the future of White-clawed Crayfish in Essex – it is hoped that populations breeding here can eventually be used to re-stock rivers in their native range.

Help protect crayfish in Essex

White-clawed Crayfish are protected under UK and European law, which means that a licence is required to handle and catch crayfish. Crayfish Plague can be carried to different river catchments by canoes, wellies and damp fishing equipment: wash and dry equipment for several days or disinfect when moving between sites to prevent spreading plague (see www.crayfish.org.uk for further information).

Taking measurements from the population


River Chelmer

WFD water body status


Project type

Translocation project

Benefiting species and/or habitats

White Clawed Crayfish

Project lead

Essex Biodiversity Project, Essex Wildlife Trust, EECOS

Contact for more information

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Project partners

Environment Agency, Writtle College, Essex & Suffolk Water

Project start


Project end


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Design by LTD Design Consultants and build by Garganey Consulting based on an original concept by Herts and Middlesex Wildlife Trust