PREY SELECTIVITIES OF THREESPINE STICKLEBACKS (GASTEROSTEUS-ACULEATUS) AND PHANTOM MIDGE LARVAE (CHAOBORUS SPP) IN NEWFOUNDLAND LAKES

1. Threespine stickleback (Gasterosteus aculeatus) and Chaoborus (C. punctipennis and C. trivittatus) planktivore populations were sampled from several oligotrophic lakes on the Avalon Peninsula of Newfoundland, Canada. Pelagic zooplankton communities in these lakes consist of Daphnia catawba, Holopedium gibberum, Eubosmina longispina, Leptodiaptomus minutus, Epischura nordenskioldi, Cyclops scutifer and rotifers (Conochilus unicornis, Keratella and Kellicottia spp.). 2. Comparison of gut contents of G. aculeatus and crop contents of later instar Chaoborus larvae with lake zooplankton communities indicated that the vertebrate and invertebrate planktivores show contrasting species and size selectivities in their choice of zooplankton prey. Prey selectivities were quantified by means of the selectivity index alpha. 3. G. aculeatus showed positive selectivity for larger zooplankton (greater than 0.76 mm in length), typically the cladoceran D. catawba, while Chaoborus larvae showed positive selectivity for smaller zooplankton (less than 0.50 mm in length), typically the cladoceran E. longispina. While rotifers never featured in the diet of the sticklebacks, these zooplankters were a common component of the chaoborid diet. H. gibberum, with its gelatinous sheath, was rarely found in the gut contents of G. aculeatus and was never found in the crop contents of Chaoborus larvae. 4. Selective predation by these planktivores in the relatively shallow and well-mixed oligotrophic Avalon lakes may influence species composition and size structure of the pelagic zooplankton communities.