EVALUATION OF CHANGES IN THE ABUNDANCE OF THREE CATCHABLE CRAYFISH SPECIES IN LAKE POB£ÊDZIE (NORTHERN POLAND)

The aim of the study was to learn about the character and dynamics of changes in the abundance of the catchable population of the signal crayfish, Pacifastacus leniusculus (Dana), the spiny-cheek crayfish, Orconectes limosus (Raf.), and the noble crayfish, Astacus astacus (L.), that all inhabit Lake Pob³edzie (northern Poland). During the study period of 1996-2005, the effectivity of crayfish catches conducted with Evo traps in the lake rose from 1.30 to 5.67 specimens trap -1 night -1 . This indicates that crayfish abundance increased progressively. The noble crayfish was observed only in the 1999-2002 period and succumbed to growing pressure from both of the American species. Studies have excluded the existence of the crayfish plague caused by Aphanomyces astaci Schikora. The abundance of the signal crayfish, which was introduced in 1992, increased throughout the study period. The abundance of the spiny-cheek crayfish rose from 1999 to 2002. Although a slight decrease was noted in 2003, not until 2004 was a rapid decline noted in this species. A similarly dramatic decline has been noted since 2002 in the abundance of spiny-cheek crayfish in many other Polish waters. This was probably the result of an outbreak of an infectious disease particular to this species. The catch effectivity of signal crayfish (99.5% share of catches) in 2005 was 5.00 specimens trap -1 night -1 and was higher than the catch effectivity achieved with this species in Californian and Swedish lakes.