Clutch size and body size at first reproduction in Daphnia pulicaria at different levels of food and predation

Field data from seven alpine lakes in Serra da Estrela, Portugal, show that reproduction in Daphnia may be as efficiently controlled by fish predation and copepod predation on eggs in brood cavities as it is by food limitation. Body length and clutch size estimates in Daphnia pulicaria revealed high inter- and intra-population variability in maturation size (body size at first reproduction), and in number of eggs per clutch. Daphnia at first maturation in lakes stocked with rainbow trout were half the size of those found in fishless lakes (body length of 0.86-0.95 and 1.55-1.81 mm, respectively). The mean number of eggs per clutch was reduced to a similar degree by food limitation, predation by fish and copepod predation on eggs in brood cavities. but the underlying mechanisms of this reduction were different. Food limitation caused smaller clutch sizes in all individuals, so variation remained the same. Fish predation caused the selective removal of individuals with maximum clutches, so variation decreased. Copepod predation caused removal of eggs from brood cavities of randomly infested females, so that variation increased, particularly at a high food level when non-infested females carried large clutches of eggs.