SEASONAL PATTERNS IN THE MORTALITY OF DAPHNIA SPECIES IN A SHALLOW LAKE

To assess the impact of predation by young-of-the-year (0+) fish on th e population dynamics of Daphnia species, we made independent estimati ons of the mortality of Daphnia species during the year, and of the pr edation pressure exerted by the juvenile fish. Mortality of daphnids w as computed using a model that allowed us to differentiate between dif ferent size-classes, while total fish consumption was estimated from t he temperature-dependent daily weight increase and the population deve lopment of the 0+ fish. The predation pressure on the different size-c lasses of Daphnia species was estimated by combining the total fish co nsumption with estimates of the selective feeding behaviour of the fis h. To make the estimates of fish consumption independent of our curren t (1989-1991) zooplankton data set, we estimated fish species-specific and fish length-dependent selectivity indices on zooplankton using a different data set (1976-1977). Daphnia population densities usually i ncreased in spring and decreased rapidly in early summer. Predation by 0+ fish was not severe enough to explain the large mortality that cau sed the summer decline; later in the year most of the mortality in the larger size-classes of the daphnids (>1.0 mm) could be explained by f ish predation.