One of the recent theories for the adaptive value of vertical migration (McLaren 1963) states that migrants gain an energetic advantage over non-migrants because by alternating between areas of high and low temperatures they are able to partition energy into growth more efficiently than non-migrants. The energetics of the vertical migration of fourth-instar Chaoborus trivittatus larvae in Eunice Lake, British Columbia were studied to identify and quantify this hypothesized energetic advantage. Fourth-instar C. trivittatus larvae undergo a regular, synchronous, diel vertical migration which exposes them tc a wide range of temperature and prey density. Feeding occurs primarily at night, and near the surface. Although all 20oplankton in Eunice Lake are potential prey, Diajgtomus kenai constitutes the majority of fhe biomass in the diet of fourth-instar larvae. In Eunice Lake C. trivittatus larvae grew more slowly than their potential growth rate because of low food availability. Several energetics parameters including carbon assimilation efficiency, the effect of temperature on respiration rate, and the effects of ration size and temperature on larval growth were measured in the laboratory. Carbon assimilation efficiency of both copepods and cladocerans by C. trivittatus is about 68%. Respiration rate increases linearly with temperature over the range 5-25°, although there is a suggestion of a plateau in oxygen consumption ever the temperature range the larvae are exposed to during their migration. Temperature and ration size interact to determine larval growth rate; fluctuating temperatures limited growth regardless of prey density while at 20° prey density limited the growth rate. Empirical data from the field and laboratory were incorporated into a generalized computer simulation model of the energetics of a vertically migrating larva. The model was used to examine the effects of various migration patterns, physical parameters, and biological parameters on larval growth. Analysis of several possible migration strategies showed that, on an energetics basis alone, growth will be maximized by either staying near the surface where there is feed, or by vertically migrating with a physiologically determined periodicity based on individual feeding history. The results of laboratory growth experiments and computer simulations agreed with these two alternative strategies. However, C. trivittatus larvae in Eunice Lake do not follow either of these patterns. No alternative hypothesis to explain their migration pattern is attractive?, and I conclude that it is a relict of previous selection for this pattern in lakes containing diurnal vertebrate predators. / Science, Faculty of / Zoology, Department of / Graduate