Fish predation and offspring survival in the prosobranch snail Viviparus ater

In this study we identified some of the predators of the freshwater snail Viviparus ater and estimated offspring survival to the end of the first summer in a natural population. Newborn V. ater were eaten by the fish Barbus barbus, Rutilus rutilus, Scardinius erythrophtalmus and Tinca tinca. Out of 137 guts of Abramis brama caught in Lake Zürich 1 contained shell fragments and an operculum of a newborn V. ater. On a 40×40 m grid near Goldbach, Lake Zürich, we counted 678 adult females of V. ater in June 1988, which together gave birth to approximately 13 300 offspring throughout the summer. In October we found 1348 V. ater of age class 0 on the grid, i.e. approximately 10% of the year's young had survived to the end of their first summer. The grid had been divided into two sections, A and B. In section A, 72 carthenware tiles had been placed as shelters for V. ater. Offspring survival was slightly but significantly higher here (10.9%) than in section B (9.6%). The observation that offspring survival in the natural habitat was 1 order of magnitude lower than in cages suggests that predation is an important cause of mortality of newborn V. ater. Average population density was 2 individuals m^-2 There was a migration from shallow (1-4 m) to deeper water (5-9 m) in September. The average distance between weekly recordings of tagged V. ater was 4.7 m in males and 3.0 m in females.