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The selective advantage of diel vertical migration behavior in juvenile sockeye salmon and kokanee (Oncorhynchus nerka) Levy, David Alan


The widespread occurrence of diel vertical migration behavior in pelagic aquatic organisms suggests that there is a selective advantage of the behavior compared to a static vertical depth distribution in the water column. Juvenile sockeye salmon (Oncorhynchus nerka) are active diel vertical migrators within pelagic lacustrine habitats, usually migrating over vertical distances covering tens of meters. The migrations are timed such that the animals occupy shallow depths during crepuscular and nocturnal periods, and spend daytime periods relatively deep in the water column. Hydroacoustic surveys were undertaken in fifteen British Columbia sockeye and kokanee lakes in order to compare predictions from selective advantage theories with the performance of the animals under field conditions. Juvenile sockeye undertook diel vertical migrations in all systems studied with the exception of Nimpkish Lake, where the migrations were periodically reversed, and Chilko Lake, where the migrations were periodically relaxed. None of the single factor theories correctly predicted all of the features of juvenile sockeye diel vertical migration. A multifactor theory, which interpreted the migration as a three-way compromise between foraging, predator avoidance, and the optimization of nocturnal metabolic efficiency, provided the most realistic explanation for the selective advantage of the behavior. The field observations also supported a hypothesized sensory mechanism for the behavior involving light and temperature controlling the day and night depth positions respectively. Two distinct kokanee diel migratory patterns were observed within Okanagan Lake. One group of animals migrated in a similar fashion as juvenile sockeye, while the second group undertook a reversed diel vertical migration. Target strength estimates from a high frequency echosounder suggested that small juveniles comprised the first group, while larger sub-adults comprised the second. The field observations are consistent with an ontogenetic shift in kokanee diel vertical migratory behavior within Okanagan Lake. Crustacean zooplankton prey organisms of juvenile sockeye maintained static diel vertical distributions within three Eraser River system sockeye lakes. Within Babine Lake, the cladoceran Bosmina coregoni undertook reversed diel vertical migrations probably in response to the vertical migrations undertaken by the predatory copepod Heterocope septentrionalis. Within Nimpkish Lake, where juvenile sockeye undertook reversed diel vertical migrations, B. coregoni as well as other planktonic Crustacea undertook diel vertical migrations. The correspondance between vertical migratory patterns in zooplankton and planktivores suggests a tight coupling in the diel migratory behavior of adjacent trophic levels with potentially important consequences for pelagic aquatic community structure.

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