The Noble crayfish,
Astacus astacus (also called the European crayfish or the broad-fingered
crayfish) used to be the most common crayfish species in Sweden and Europe. It is
susceptible to the crayfish plague,
Aphanomyces astaci, and since 1907, when the first
outbreak of crayfish plague was noted in Sweden, the Swedish Noble crayfish population has
decreased with 97 %. The pattern is the same in all of Europe. To compensate for the
diminishing Noble crayfish populations, Signal crayfish, Pacifastacus leniusculus was
introduced into Europe (in Sweden in 1960). However, despite its resistance to crayfish
plague, it still acts as a carrier of the disease. Thus, the introduction of the Signal crayfish may
have accelerated the extinction rate of native crayfish populations. In Europe, the IUCN Red
List of Threatened Species classifies the Noble crayfish as vulnerable. In Sweden the
prospects for the crayfish are even worse, and the Official Swedish Red List classifies the
Noble crayfish as Critical Endangered (based on the IUCN Red List criteria), facing an
extremely high risk of extinction in the wild in the immediate future.
The Swedish island of Gotland, situated in the Baltic Sea, is the first protected area in Sweden
for the Noble crayfish. No known outbreaks of crayfish plague have been confirmed on
Gotland, and Gotland is considered one of the very few areas left in Sweden that are not
exposed to the crayfish plague. However, on Gotland there are three confirmed water bodies
where Signal crayfish has been illegally introduced. The County Administrative Board on
Gotland was given the formal commission to exterminate any occurring populations of the
Signal crayfish, as they can be carriers of the crayfish plague. This report contains of the
actions taken to eliminate the Signal crayfish populations on Gotland during the years 2007–
The three localities where Signal crayfish occurs, including the ponds and the surrounding
environment, was firmly investigated concerning the physical property and the composition of
the ecosystem. The ponds where then treated with a pesticide containing the active substance
deltamethrin, with the aim of extinguishing all Signal crayfish. Deltamethrin is a synthetic
pyrethroid insecticide, widely used on crops, as it is stable, less volatile and environmentally
compatible. The toxicity on mammals, including humans, and bird is relatively low as
compared to other pesticides, while it has been shown to be lethal to arthropods in very low
concentrations. The ponds were treated with a dose aimed to reach a concentration of 0.5 μg
deltamethrin/l to obtain lethal doses.
The outcome of the extermination of the signal crayfish in the three localities on Gotland was
successful so far. No live crayfish was found after the treatment, and all crayfish that were
placed in cages in the ponds to control for the effectiveness of the pesticide treatment died.
The concentration of deltamethrin declined rapidly and reached values below 0.1 μg/l, the
European Union drinking water limit, within one or a couple of weeks. The final succes of
these actions is still uncertain as surviving individuals may take several years to detect.
Inspections at all sites during the following years need to be carried out in order to determine
if the Signal crayfish was completely exterminated from Gotland.
2010. , 51 p.