The temporal-spatial pattern of vertical migration by the freshwater copepod Skistodiaptomus oregonensis relative to predation risk

Abstract

Skistodiaptomus oregonensis vertical migration in Kennedy Lake and Paxton Lake may be an adaptation to avoid predation by threespine stickleback (Gasterosteus aculeatus). In Kennedy Lake, juvenile sockeye salmon (Oncorhynchus nerka) and the mysid Neomysis mercedis are also predators that potentially drive the vertical migration. A corollary of the hypothesis that predation selects for vertical migration is that the extent and timing of the vertical migration decreases predation risk. This corollary is tested for each of the potential predators. Laboratory experiments indicate that stickleback feeding rate decreases below 1.6 µE ·s-1 ·m-2. In Kennedy and Paxton lakes, S. oregonensis occupied depths below this light intensity during the day. Furthermore, as S. oregonensis ascended at dusk and descended at dawn, they remained within light intensities that reduced stickleback predation rate. In Kennedy Lake, hydroacoustic data coupled with information in the literature on feeding behaviour of juvenile sockeye indicate that this species move to surface waters to feed before sunrise and after sunset. Vertically migrating S. oregonensis are near the surface during the twilight feeding periods of juvenile sockeye. The timing of S. oregonensis vertical migration also does not reduce the time it is in contact with the predator N. mercedis.