An isolated population of the white-clawed crayfish (

An isolated stock of Austropotamobius pallipes is studied in a small water system draining into a tributary to the river Alpenrhein in the Principality of Liechtenstein. The population is known to originate from one single stocking activity of 109 specimens in 1985. Crayfish stock 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 longitudinal 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 shows a heterogeneous micro-scale pattern corresponding to the distribution of structures and substrates in the waters. The population was estimated to at least 2.000 individuals. No subsidiary stocks were found in the ditch system. Parasitic growth of Branchiobdella pentodonta is very high and affects all individuals. Fish stock is confined to the species bullhead Cottus gobio , minnow Phoxinus phoxinus and brown trout Salmo trutta f. fario in low densities. Fish prédation 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 situation and management are discussed