摘要
A lot of water reservoirs offer good natural conditions for crayfish breeding. Today there are four crayfish species present in Latvia--the European species noble crayfish （Astacus astacus）, narrow-clawed crayfish （Astacus leptodactylus）, the North American signal crayfish （Pacifastacus leniusculus） and spiny-cheek crayfish （Orconectes limosus）. In general, crayfish were found in 258 localities--lakes （175）, rivers and streams （66）, reservoirs, ponds and gravel-pits （17）. A. astacus in Latvia is the dominant crayfish species distributed in all regions. Narrow-clawed crayfish was registered since 1960s. North American signal crayfish was introduced due to its resistance to diseases, but spiny-cheek crayfish arrived from Lithuania waters spontaneously. The main threat for crayfish population is crayfish plague, initiated by mucus Aphanomyces astaci. Astacus astacus was more susceptible species in comparison with Pacifastacus leniusculus and Orconectes limosus. Though the North American signal crayfish and spiny-cheek crayfish were not only resistant, they were the carriers of this disease. The physical habitat alterations, pollution and overfishing are significant during the first year breeding period. Crayfish might serve as bacteria carriers and can cause substantial fish diseases （such as aeromonosis, flavobacteriosis）.
