In this dissertation, I address how small-scale patterns of habitat use by motile zooplankton can affect the risk of encounter with visual predators. A novel Optical-Acoustic Submersible Imaging System (OASIS) is used to study a population of Euphausia pacifica Hansen in Saanich Inlet, British Columbia. A series of field, laboratory and modeling studies indicate that this instrument can resolve the three-dimensional locations of individual fish and zooplankton, and can differentiate size classes of euphausiids.

E. pacifica undergoes diel vertical migration (DVM), migrating away from food-rich surface waters during the day when the risk of attack by visual predators is highest. Analysis of spatial distributions at scales of 100 m and 10's of cm reveals that, although the euphausiids aggregate in the vertical dimension, they are approximately randomly distributed in the horizontal dimension, indicating that they do not form social aggregations. DVM and social aggregation may be redundant predator avoidance behaviors, and social aggregation may be rare in open water zooplankton. Acoustic tracking of euphausiids provides no evidence that euphausiids alter their swimming behavior or spatial distribution when in the proximity of a fish, suggesting that they are unable to detect a nearby predator. The euphausiids avoided moving directly up or down, even during periods of dusk ascent. I hypothesize that oblique swimming trajectories may reduce vulnerability to visual predators by allowing the euphausiids to maintain ventrally directed bioluminescent camouflage during vertical migration.

A trade-off model predicts that if DVM is a mechanism to balance the conflicting demands of energy gain and predator avoidance, the timing of DVM should vary with body size. Smaller, less optically conspicuous animals should ascend earlier and descend later than larger, more vulnerable animals. Twilight echo counting measurements confirm this prediction, as smaller-bodied euphausiids consistently ascended earlier and descended later than adults. The proposed mechanism for size-dependent timing of DVM is based on the potential for increased energy gain in surface waters and spatio-temporal gradients in the risk of attack by size-selective visual predators. Since these conditions are often met in pelagic environments, size-dependent timing of zooplankton DVM is likely to be widespread.

Alex.De.Robertis@noaa.gov