Crayfish Check: Initial Investigation for Invasives Shows All Clear

With works on the dam by the Yarner Office underway, now is a great time to enjoy the new bird hide on the reservoir; across the road from the Middle Trendlebere car park. The hide has been constructed by volunteers, with funding from the Moor than meets the eye scheme.

As well as funding the hide, the scheme is also funding work to boost the reservoir’s wildlife value. In order to safeguard against the work disturbing important species, and to ensure the project is successful for wildlife, surveys were performed to assess the current ecological state of the reservoir.

Part of this surveying was for crayfish. Crayfish are globally distributed freshwater crustaceans, though the only UK-native species is the White-Clawed Crayfish (Austropotamobius pallipes). A.pallipes can live for 10 years, grows up to 12cm long, and is omnivorous; feeding on a range of both live and decaying matter. They live in a variety of cryptic habitats, using rocks and submerged vegetation as cover.

Native crayfish
A.pallipes (photo by David Holdich in Holdich D (2003). Ecology of the White-clawed Crayfish)

A.pallipes was once widespread across Europe, reaching its highest densities in the UK. However, its range and numbers have experienced significant decline since the 1970s. Due to this 50-80% reduction in distribution, A.pallipes is listed as endangered on the IUCN Red List, and is protected under UK and EU legislation. Surveying for this endangered species was therefore imperative before starting work on the reservoir.

A.pallipes UK distribution in the 1970s (left) compared with the 1990s (right) (images from Holdich DM and Rogers WD (1997). Austropotamobius pallipes in the British Isles – distribution, threats and legislation)

The decline of A.pallipes is, in part, due to habitat loss and pollution – the species is particularly sensitive to water quality- but is also due to the introduction of American Signal Crayfish (Pacifastacus leniusculus). P.leniusculus was introduced to Europe in the 1960s for food, and has rapidly spread throughout freshwater habitats. P.leniusculus is a carrier of Crayfish Plague, which is lethal to A.pallipes. P.leniusculus is also larger and more predatory than A.pallipes; competing for food and actively predating on A.pallipes. Once P.leniusculus enters a population, A.pallipes faces extinction through competitive exclusion within 3-4 years.

invasive crayfish
P.leniusculus (photo by Sue Scott in Holdich D (2003). Ecology of the White-clawed Crayfish)

As well as being damaging to A.pallipes, P.leniusculus has wider ecological impacts because it feeds on a plethora of other freshwater species; from macrophytes, to freshwater invertebrates, to young fish. Surveying was therefore imperative to assess the population of P.leniusculus, as this would have significant bearing on any ecological work.

Surveying for crayfish involved putting out funnel-traps baited with meat, which were checked daily for any captures. This surveying occurred throughout November, but no crayfish were found. This was perhaps to be expected; the reservoir is quite an isolated body of water making colonisation by crayfish difficult; the water in the reservoir is acidic and turbid, whereas crayfish prefer clear, alkaline waters as their carapace is made from calcium carbonate and their gills can be damaged by suspended sediment; plus, crayfish activity is low between November and April, so even if crayfish were present their absence from the traps may have been due to decreased activity. Further surveying for crayfish will occur in the late spring, to ensure crayfish are absent from the reservoir.

Cray fish Trap
Baiting the crayfish funnel-traps (photo by Kate Smith)

If future surveys confirm that crayfish are absent, habitat creation on the reservoir will be able to go ahead without the need for considering impacts on the endangered A.pallipes or the need for control of the invasive P.leniusculus. In years to come, the reservoir should become a really great spot for wildlife, and something quite different to the rest of the reserve, making the new bird hide an even better place to enjoy nature.

Written By Tristan Colaço (Conservation Assistant)

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