Abstract

Procambarus clarkii was illegally introduced into Schuebelweiher, a 1.5 ha pond near Zurich, in 1989. Without natural control, the population thrived and exceeded the carrying capacity of the pond within a few years. Massive emigration was observed in 1996. To reduce the number of crayfish and to prevent P. clarkii from further spreading, the responsible fisheries authority trapped the crayfish population intensively (ca. 70 traps ha^-1) during the warm seasons of 1997, 1998 and 1999. Additionally, 70 pike Esox lucius (marked) and 250 eel Anguilla anguilla (unmarked) were released into the pond in spring 1999. Average CPUE in 1997 and 1998 was 3.44 and 3.07, respectively, but dropped to 0.69 after the introduction of predacious fish in 1999. 12 pike were caught during summer 1999 and their gut contents analysed. Only 4 of these pike were marked, and only 3 contained few crayfish remains. This indicates that in Schuebelweiher pike preys little on P. clarkii. Therefore it is likely that the reduction of the crayfish population is a consequence of eel predation, although direct evidence for this mechanism is not available (only 1 eel was recaptured). Trapping had no substantial effect on the size of the P. clarkii population but increased its sex ratio (females/males) and decreased the average size of the crayfish due to the selective removal of the larger and more aggressive individuals.

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