| Popis: |
The noble crayfish, Astacus astacus, is the only native species in the Baltic and Nordic countries. Crayfish plague struck the native stocks in the various countries under consideration during the period 1890–1971 and reduced crayfish abundance and distribution. Later, man made pollution and habitat changes have also reduced both crayfish production and the potential for crayfish production. Many of the remaining stocks of noble crayfish in Norway, Sweden, and Finland result from stocking into new areas during this century. To substitute the loss of the noble crayfish, the crayfish plague resistant, North American, signal crayfish, Pacifastacus leniusculus, was introduced into Sweden in 1960. Later the signal crayfish was introduced into Finland, Latvia, Lithuania, and Denmark. The narrow-clawed crayfish, Astacus leptodactylus, has expanded, due to a combination of stocking and further natural dispersion, from its former area of distribution in southeastern Europe, and is now found in Lithuania, Latvia, Finland, and Denmark. The North American, introduced, spiny-cheek crayfish, Orconectes limosus, has expanded widely from the first introduction in the Oder River in Germany and it has recently invaded Lithuanian rivers and lakes. At present, Norway and Estonia are the only countries without introduced crayfish. The interest for conservation of the noble crayfish has increased in all Nordic and Baltic countries and measures are planned and partly implemented to improve protection of remaining stocks and restore former crayfish localities. The expansion of the introduced North American crayfish species do, however, represent a threat since they both out-compete the noble crayfish and are carriers of the crayfish plague fungus. Also the expansion of the more competitive European narrow-clawed crayfish represent a threat to the native crayfish. Present catch of noble crayfish from the wild, in the Baltic and Nordic countries, is less than 200 tons, compared to historic records of more than 2000 tons. |
