Eliminering av signalkräfta på Gotland

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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– 2009.

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.