Evidence for Indirect Effects of Fish Predation on Maternal Lipid Investment in Holopedium gibberum

Abstract

We examined selected aspects of the reproductive dynamics of Holopedium gibberum in 25 lakes in Algonquin Park, Ontario. Using an index based on Daphnia size, we divided the lakes into two groups characterized as high and low fish predation. Overall, egg volumes were found to be a good predictor of energy reserves in stage 1 eggs of Holopedium. However, small eggs, which were associated with small females typical of the high predation lakes, had relatively less fat than large eggs, suggesting that fish predation indirectly reduces starvation resistance of Holopedium neonates. In addition, in high-fish lakes, mean adult body size, minimum size of ovigerous females, clutch size, and fat volumes of Holopedium were smaller than in lakes characterized as low fish predation. Maternal lipid investment (MLI) into eggs of Holopedium was compared between the two lake types. We found striking differences in the seasonal pattern of absolute MLI between the two lake groups, mostly mediated through differences in body size. The greatest difference in absolute MLI between the two lake types occurred in May; females from the low-predation lakes invested 4 times as much energy reserves into reproduction as compared with females from the high-predation lakes at this time. Relative MLI (energy reserves per microgram female weight), comparing only individuals which overlapped in size, was unaffected by lake type. Thus a constant proportion of an ovigerous female's weight is invested in reproduction with a seasonal component that matches standard algal abundance for many shield lakes, suggesting that relative MLI is determined more by the exigencies of cladoceran life histories than by the presence of fish. Nevertheless, absolute energy investment is depressed in the high-predation lakes and we suggest that clonal replacement mediated by size-selective predation by the fish can account for the differences between the lake types.