OCCURRENCE OF NATIVE AND INTRODUCED CRAYFISH IN NORTHEASTERN GERMANY AND NORTHWESTERN POLAND.
On the basis of monitoring undertaken in 300 lakes since 1994, the recent stocking situation for freshwater crayfish in northeastern Germany (Uckermark) and northwestern Poland (Szczecin area) is described. The two native species Astacus astacus (L.) and Astacus leptodactylus (Esch.) were found in five and two sites, respectively, while the introduced species Orconectes limosus (Raf.) and Pacifastacus leniusculus (Dana) were found in 214 and two sites. No crayfish were present in 77 sites. Historical stocking data from 1959 for 92 sites in the area of Szczecin and Koszalin, northwestern Poland taken from the litterature were compared with the recent stocking situation according to our own investigations. In 1959, a total of 22 sites contained A. astacus and 13 sites contained A. leptodactylus. Both species were absent after 1994. In contrast, the number of stocks for O. limosus has increased from 57 in 1959 to about 90 after 1994. Differences in juvenile growth and female fecundity studied in two lakes are suggested as a potential mechanism for extinction of native crayfish by introduced species, apart from infection by crayfish plague. Juvenile (1 year old) O. limosus showed a higher weight (8.03 ± 0.12 g) and length (64.7 ± 0.9 mm) than juvenile A. leptodactylus (2.11 ± 0.05 g and 44.8 ± 6.9 mm, respectively). The correlation between number of ovules and female total body length indicates a significantly (p = 0.004) higher number of eggs at the same length for O. limosus (regression coefficient : + 89.5) than for A. leptodactylus (regression coefficient : + 36.4).