Facts: Canker sores
Our native river crayfish (Astacus astacus) immigrated to Sweden after the ice age about 10,000 years ago. The signal crayfish (Pacifastacus leniusculus) is a species from western North America that was deliberately introduced to Europe – Sweden! – in 1960. It brought with it the fungus-like organism Aphanomyces astaci that causes crayfish plague, a disease that is always fatal to river crayfish.
The introduction was a deliberate attempt to improve the situation for the crayfish fishery. Between 1960 and 1994, signal crayfish were released into thousands of waters in the country following a parliamentary decision, which meant a disaster for the river crayfish. Today, all releases of signal power are prohibited, but the damage has already been done. About 98 percent of the crayfish stocks have today been wiped out.
Source: Swedish University of Agriculture
— For a long time, huge amounts of signal power were deliberately spread, through Riksdag decisions. In fact, permits were easily obtained right up until 1994. A total of 5,000 permits were issued. That is the policy we see the result of today, says Lennart Edsman, force expert and researcher at the Fresh Water Laboratory at the Swedish University of Agriculture.
This year, crayfish plague has been reported from two new watercourses that were previously free of the disease: Skellefteälven in Västerbotten and Funäsdalssjön in Härjedalen. It reflects that the plague is now conquering more and more waters in northern Sweden. Norrbotten is now the only county in Sweden that has not been affected by the plague.
— We don’t know if the disease came via signal crayfish. It is likely, but crayfish can be spread in many other ways, through infected fishing gear or through the transfer of water from an infected watercourse. Even excavators can spread the infection, says Edsman.
About 98 percent of crayfish stocks are now wiped out. Signal crayfish not immune
Canker sores are caused by the spores of the fungus-like organism Aphanomyces astaci. It causes very high mortality among all species of crayfish from Europe and Asia. The signal crayfish, introduced from western North America, like other species of crayfish from the western hemisphere, has a high resistance to the plague – although it is not immune. It carries the disease with it and spreads it to the crayfish.
— It is a myth that the signal crayfish are immune to crayfish plague. They also get plague, but are normally not as affected. Although in recent years the catches have been poor and it is likely that this is due to crayfish plague. Something has made them more sensitive so they die, says Edsman.
However, constant release has meant that the signal crayfish is now found all over southern and central Sweden, with the exception of Gotland. In Sweden, there are now only around 600 stocks of crayfish left. About 98 percent of the stocks have been eliminated since the crayfish plague came to Sweden.
— There is only one way to solve the problem: Stop putting out signals! One can only hope that Norrbotten will make it. The threat has existed for a long time, says Edsman.
Signal crayfish from lake Erken in Norrtälje. Our native river crayfish is now very rare in the country. Crayfish on the Gotland ferry
The worst outbreaks of crayfish occurred in the 1980s. During 1980–1984, 1,577 outbreaks occurred, an increase of over 300 percent over the previous five-year period. One reason was that the number of permits for stocking had increased significantly after 1969. Only in 2016 was there a total ban on the stocking and cultivation of signal crayfish in Sweden.
— But the illegal releases continue. Some expose them to pure bullshit, others think it’s fun. People believe that the signal crayfish grow faster, that the catches are bigger, and that they are immune to crayfish plague. All these are myths. On Gotland, we have discovered five illegal releases. So there must have been people sitting with buckets full of signal crabs on the Gotland ferry!