Abstract

A series of experimental catches, carried out in 1995 in lakes and rivers of Southern Lithuania, confirmed the distribution of a new species Orconectes limosus. Crayfish of this species are spreading in the basin of the largest Lithuanian river-the Nemunas. Thus four species of crayfish are in Lithuania at present. The noble crayfish, Astacus astacus, is native. In the late 19^th century Astacus leptodactylus began spreading from the East. The American crayfish Pacifastacus leniusculus was introduced in same isolated lakes in 1972. The distribution of O. limosus was determined in 1995 in several lakes and rivers related with each other and having a direct connection with the river Nemunas. The river Nemunas flows into southern Lithuania from Byelorussia, and there a large tributary flows into it from Poland. The data about crayfish distribution in each of 44 administrative regions and the main river basins were obtained by means of questioning, questionnaires and experimental catches. The data show, that the number of water bodies inhabited by crayfish is increasing, crayfish are spreading to new water bodies and reappearing in the ones formerly inhabited. Unfortunately, local inhabitants usually do not distinquish between crayfish species. The degree of crayfish abundance in a region or river basin is expressed by dividing surface area of water bodies inhabited by crayfish by the surface area of all the water bodies.

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