Abstract
The aim of the study was to determine the impact raised water level in the winter anoxia Lake Oświn (northeastern Poland) had on the population of narrow-clawed crayfish, Astacus leptodactylus (Esch.). Raising the water level in the lake increased the surface are from 360 ha to 900 ha and improved habitat conditions. The species diversity of vegetation increased, as did the area of lake bottom covered by it (from 22 to 51%). The threat of winter anoxia was also reduced. The improved habitat conditions led to increased crayfish population density expressed through the mean catch efficiency, which increased by 252%. The share of individuals measuring more than 10 cm increased from 76% in 1991 to 95% sixteen years after raising the water level. During this period (1991-2007) the mean total length of crayfish increased significantly statistically (P < 0.05) from 10.5 to 12.7 cm. This means that during this period, the life expectancy of the dominant crayfish age group increased by about two to three years.