Assessment of the importance of fish predation versus copepod predation on life history traits of Daphnia hyalina

Daphnia hyalina is a cladoceran present through the whole year except for late summer in Maranhao, a mesoeutrophic reservoir in central Portugal. Apart from the influence of food, both vertebrate and invertebrate predation pressures seem to have an effect on D. hyalina population dynamics. Enclosure experiments were designed to assess the relative importance of both types of predation. After the summer crash, D. hyalina reached higher numbers in the fishless enclosures than in the lake despite of high predation pressure upon juveniles by Acanthocyclops robustus. Fish predation upon the largest individuals, especially large egg bearing females, was responsible for the lower fertility of the open water population when compared with the enclosure population. In the enclosures an increase in tail spine length was observed. The longer tail spine probably offered protection from copepod predation, allowing at least some of the juveniles to coexist with their potential predator and reach the adult stage, less susceptible to copepod predation.