Diel Vertical Movements and Feeding Activity of Zooplankton in St. Georges Bay, N.S., Using Net Tows and a Newly Developed Passive Trap

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Abstract
Newly developed plankton traps, designed to passively collect vertically mobile plankters, sampled ascending plankton but failed to collect many species during descent. This discrepancy may be behavioral with passively sinking organisms reacting to the trap surface with an upward avoidance reaction. Simultaneous use of conventional net tows and semiautomated traps allowed us to interpret the vertical movements of plankton more fully than by either method alone. Asynchronous vertical movement of the Pseudocalanus population is suspected because the percentage of trapped animals with food in their guts was usually higher in the downward moving fraction of the population. Migratory behaviors ranged from dusk and dawn ascent with midnight sinking to reverse migrations where the night level inhabited is deeper than the day depth. Noctural dispersal of herbivore and omnivore populations over depth probably reflects predator avoidance by presenting less dense aggregations to vertically mobile predators. Sightless predators reside in deeper waters than their prey during daylight presumably because they are larger and more vulnerable themselves to visual predation. Visual predators descend to greater depths than their prey at night. All the migration patterns observed can be explained in evolutionary terms simply by competition for food and avoidance of predators.