AN ISOLATED POPULATION OF THE WHITE-CLAWED CRAYFISH (AUSTROPOTAMOBIUS-PALLIPES) IN THE PRINCIPALITY OF LIECHTENSTEIN

An isolated stock of Austropotamobius pallipes is studied in a small w ater system draining into a tributary to the river Alpenrhein in the P rincipality of Liechtenstein. The population is known to originate fro m one single stocking activity of 109 specimens in 1985. Crayfish stoc k is investigated by direct observation at night and by trap catching including mark and recapture. Essential ecomorphological properties of the waters are recorded in respect to their attraction for crayfish. The stock is spread over two small natural ground water ponds and some connected ditches. The settled area is only 850 metres in total longi tudinal extension and the boundaries of the stock are very sharp over only few metres on all sides following distinct changes of morphometry , structure or water flow which prevent the population from expanding. In addition, isolation from the downstream fauna is given by a local source of water pollution. The distribution of population density show s a heterogeneous micro-scale pattern corresponding to the distributio n of structures and substrates in the waters. The population was estim ated to at least 2.000 individuals. No subsidiary stocks were found in the ditch system. Parasitic growth of Branchiobdella pentodonta is ve ry high and affects all individuals. Fish stock is confined to the spe cies bullhead Cottus gobio, minnow Phoxinus phoxinus and brown trout S almo trutta f. fario in low densities. Fish predation has no impact on the crayfish population. The waters are situated in a natural reserve , neither fish nor crayfish stocks are exploited by man. Stock situati on and management are discussed.