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. 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.
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.
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)
