Vertical migration for horizontal transport while avoiding predators: I. A tidal/diel model

Research into the vertical migration behavior of scallop (Placopecten magellanicus) veligers has led us to examine whether these, and possibly other small zooplankters, may migrate in response to a combination of tidal and diel stimuli. This paper uses Hill's (1991) model to evaluate the horizontal transport effects of such migrations. We demonstrate that most types of vertical migration behavior reported in the literature (e.g. nocturnal, twilight, midnight sink) appear at different phases of the lunar cycle. Moreover, migrating in response to both of these cues may provide horizontal transport advantages if the zooplankter is very small (unable to migrate the full water column depth) and/or has difficulty determining its position in the water column (especially if the behavior also holds it in regions of increased shear). Such behavior need not interfere with other advantages of vertical migration, including avoiding predation, avoiding UV light, searching for patchy food, etc. Tidal/diel migration may have distinct advantages for occupying new habitats or coping with local changes associated with altered current regimes. Because averaging the results of several days, sampling less frequently than every 2 h or sequential sampling of different sites is likely to obscure the tidal portion of a tidal/diel migration, such behavior could be common without being obvious to researchers. Aliasing of the lunar and solar cycles (a 14.8 day period) may allow the detection of tidal period migrations in long-term records with lower sampling frequencies.