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How the American signal crayfish invaded British waters

The tale of an aggressive American crayfish's invasion to our waters is a sad one and a stark reminder of the importance of biosecurity.

American signal crayfish

American signal crayfish

Man’s stupidity when it comes to moving things around knows no bounds. He decided to bring the North American version 
of the crayfish to Europe 
and then to Britain to 
augment stocks of natives, which were suffering from crayfish plague. Little did 
the importers know the 
American signal crayfish were carrying 
it with them.

The British imports were in the 1970s and little thought was given to the fact that these creatures would happily leave their abode and wander off over the countryside under the cover of darkness like freedom fighters. Like the latter, many succeeded and, from a scattering of sites where they were being kept as potential food, they have colonised much of the country 
— they have been found as far north 
as the Moray Firth in Scotland.

Biodiversity risks

It is a sad tale but today we are still not addressing the problems of introducing alien material to our shores. Trees and other plants are being brought in, both in containers and bare rooted, with all the risks that poses. We come home from walking on foreign soil with no checks on the material on our clothing or boots. One of the latest and serious risks to another of our natives is the South American disease Phytophthora austrocedrae, which has affected stands of juniper plants throughout the country.

Try to enter many other countries with contaminated clothing and see what response you get from border control. Your clothing may well be confiscated if it cannot be cleansed.

Declining fisheries

Anglers are known for seeking excuses for fisheries declining, but in the case of their watercourse becoming home to signal crayfish, they have a good reason to be concerned. This little predator knows no limits when it comes to its food supply. It will eat almost anything 
and research has proved that, without a doubt, its impact is very noticeable when it comes to many native species. Flies, plants, native fish and crayfish, even riverbanks, all come under attack from the signals.

American signal crayfish facts

  • It is true that some species of fish will eat the smaller signals but — and it’s a big but — they do not eat enough of them to hold back the tide. At the same time, the signals are eating even more of the fish eggs and fry.
  • The female signal crayfish will produce 200 to 400 eggs, which she carries with her under her tail, lobster-like, throughout the winter, until they hatch in the spring.
    Once hatched, they go through three moults before they leave their mother. They are advised to leave quickly, as the species is known for cannibalism — she may even eat her own offspring.
  • The adults are a blue to red-brown colour with a light patch at the hinge of the quite impressive claws. It is this pale patch that gives the crayfish its name because it is not unlike a signal flag when the creature raises its claws.
  • Normal size is around 2½in to 3in long, but they can attain 8in and 
can live well into their teens.


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Help from otters

The recolonisation of many river systems by the otter will help a little with control as the otter loves the little morsels. The remains of the bodies can be found on stones and grass banks where the otter has had dinner, with telltale tooth holes in the larger skulls an indication of what consumed it.

From small introductions, this pest — for that is what the American signal crayfish 
is on these shores — has spread across 
25 European counties, wreaking havoc wherever it has gone. It would appear that its work is not done here, given the impact it has already had 
on countless native species.