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North American signal crayfish

What action is going on for this species?

The following will give you information on what work has been taking place through the Species Action Framework:

Species background

The North American signal crayfish (Pacifastacus leniusculus) is a lobster-like invertebrate which can grow in excess of 16cm long and is found in freshwater habitats.

Why is this on the Species Action List?

The signal crayfish meets criterion 2 of the Species Action Framework, as an invasive non-native species which presents a significant risk to biodiversity.

Its entry into Scotland is restricted by the Import of Live Fish (Scotland) Act 1978, Wildlife & Countryside Act 1981, as amended, Conservation of Native Freshwater Fish Stocks: The Prohibition of Keeping or Release of Live Fish (Specified Species) (Scotland) Order 2003 and public bodies have a responsibility to ensure that the protection of biodiversity is considered in all of their activities under the Nature Conservation (Scotland) Act 2004. This applies to those public bodies involved in the regulation of, for example, the transportation of water or fish. 

Habitat, distribution and abundance

Native to North America, signal crayfish have been present in northern Europe since the early 1900s.  They were introduced to Britain in the 1970s and are now commonly found in waterways throughout England and parts of Wales.

They were first formally recorded in Scotland during 1995 and have since been recorded in 15 sites.  These range from Galloway in the south to Inverness-shire in the north.

Signal crayfish are highly adaptable organisms which can modify aquatic environments by eliminating aquatic vegetation and by burrowing into riverbanks or the shallow littoral areas of ponds and lochs.

General ecology

Signal crayfish typically grow up to a length of 16cm, although larger specimens have been recorded.  They reach maturity after two to three years once they have attained a length of 6cm.  Eggs are produced during October and females carry live young.  Signal crayfish may live for up to 16 years.  They are generally considered to be nocturnal and consume aquatic plants and invertebrates.  They can also predate amphibians and fish and can restrict fish species from their preferred habitats.

History of invasion and expansion

Signal crayfish have been present in Britain since they were first imported from Sweden during the 1970s.  Subsequent escapes from farms and deliberate illegal releases have resulted in the establishment of new feral signal crayfish populations over large areas of England and Wales.

Several records exist of signal crayfish introductions to Scotland during the 1980s and there is some anecdotal evidence to suggest that crayfish were translocated to a variety of locations at that time.  They were first formally recorded in Scotland during 1995, since when a total of 15 populations have been identified. These extend from the Kirkcudbrightshire Dee in Galloway and the River Clyde in Lanarkshire to the River Nairn near Inverness.  Populations have also been recorded in the catchments of some of Scotland's most valuable river systems, such as the Tay, Tweed and North Esk.  New populations are now being reported at a frequency of two or three per year.

Impacts on other biodiversity and conservation interests

Signal crayfish have a significantly adverse impact on native freshwater flora and fauna in running and standing waters.  They can do this by consuming large quantities of plants and invertebrates, and by either predating or displacing amphibians and fish.  Signal crayfish can also modify aquatic environments, by burrowing into the banks or rivers and ponds.

In ponds, this behaviour can undermine the littoral zone and result in increased turbidity.

In running waters, extensive burrows may destabilize the riparian zone, leading to increased rates of bank erosion, the shallowing of streams and the compaction of salmonid and lamprey spawning grounds.

The species' impact on freshwater pearl mussel is unknown but is likely to be significant external site .

The potential for signal crayfish to act as a vector for transmission of diseases within or between catchments cannot be discounted.

The 'Species Lead' at Scottish Natural Heritage   Tel 0141 951 4488

Last updated on Tuesday 19th November 2013 at 11:40 AM. Click here to comment on this page