Summary

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.