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Adaptive significance of vertical migration behaviour of Skistodiaptomus oregonensis Ghan, David


I have studied the adaptive significance of vertical migration of zooplankton in 2 populations of the copepod Skistodiaptomus oregonensis that migrate and 2 populations that do not migrate. Vertical migration of the copepods is associated with the presence of pelagic sticklebacks. This observation is consistent with the hypothesis that the adaptive benefit of vertical migration by S. oregonensis is to avoid stickleback predators. The alternative hypotheses including avoidance of juvenile sockeye predators, foraging efficiency, bioenergetic efficiency, or combined foraging/bioenergetic efficiency are not supported by the comparisons of migratory behaviour of S. oregonensis in the 4 lakes. Both the depth and timing of S. oregonensis migration are consistent with the hypothesis that copepods are avoiding predation by sticklebacks. Light intensities at the depth at which S. oregonensis reside during the day are sufficiently low to reduce predation risk from visual foraging stickleback and the timing of ascent at dusk and descent at dawn are such that S. oregonensis remain at light intensities that reduce risk from stickleback. S. oregonensis are at the surface at dusk and dawn at the time that juvenile sockeye feed in the surface habitat. Vertical migration appears to be a trade-off with resource acquisition. Phytoplankton are less concentrated in the deep habitat where S. oregonensis reside during the day. Furthermore, migrating copepods contain less phytoplankton as food in their guts than do non-migrating individuals. In vertical columns in the laboratory, the presence or absence of sticklebacks does not influence the vertical distributions of S. oregonensis collected from lakes with either migratory or non-migratory populations. This indicates that the migration phenotype is fixed rather than being a flexible behaviour induced by environmental cues. I developed a dynamic optimization model to predict the optimal depth decisions for S. oregonensis based on depth dependent lake food and temperature conditions, fish abundance and predation rates, and S. oregonensis bioenergetics. The model predicts that vertical migration should occur to avoid sticklebacks under a broad range of modelled conditions, but with a fitness cost due to feeding opportunity costs. This demonstrates quantitatively that it is tenable to hypothesize that vertical migration involves a tradeoff between stickleback avoidance and feeding opportunity. Taken together, these results are consistent with the view that different migration behaviours in these populations are a result of divergent evolution driven by environmental variation affecting the optimal solution to the predation risk/resource acquisition trade-off.

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