Density variation of the zebra mussel Dreissena polymorpha in Lake Zürich, from 1976 to 1988

A population explosion of the zebra mussel Dreissena polymorpha in Lake Zurich in the early seventies was followed by a decline, most noticeably in the littoral zone (Burla and Lubini-Ferlin, 1976). To check for further density changes, zebra mussels were counted every winter from 1978 to 1988, and larval settlement was recorded every summer from 1976 to 1988. During this period, mussel densities ranged from 11 to 1500 ind. m−2; juvenile densities from 11 to 108 ind. cm−2. Counts of newly settled juveniles and counts of established mussels were positively correlated between years. This suggests that the number of larvae in any given year depends on the size of the breeding population of mussels. Juvenile counts from different sites were more similar than mussel counts in any given year, indicating that within-lake dispersal of larvae is high. In both newly settled juveniles and established mussels, density variation occurred in what appeared to be two full cycles of 5 years. Mussel densities were positively correlated with total bird counts of three known predators of D. polymorpha, the Coot (Fulica atra), Tufted duck (Aythya fuligula), and Pochard (A. ferina). Higher mussel densities were observed near Zollikon and Goldbach than at other sampling sites. This may be a consequence of a steeper, stonier slope of the lake bed at these sites. Twenty-eight years after it was first recorded in Lake Zurich, D. polymorpha appears to be firmly established in the lake.