Size-dependent visual predation risk and the timing of vertical migration in zooplankton

Zooplankton commonly exhibit diel vertical migration (DVM), descending from food-rich surface waters during the day. If DVM is a tradeoff between avoiding size-selective visually hunting predators and maximizing energy gain, smaller bodied prey should enter surface waters earlier and leave later than larger, more visually conspicuous organisms. Conventional sampling technologies lack the temporal resolution to test this prediction. Here, we report on the first test of this prediction using a new submersible optical-acoustic imaging system capable of resolving the timing of migration of the euphausiid crustacean Euphausia pacifica Hansen. Smaller bodied animals consistently ascended as much as 30 min earlier and descended up to 45 min later than adults. The timing of vertical migration reflects how the size-dependent risk of attack by visual predators alters the tradeoff between feeding and predator avoidance, supporting the predator-avoidance hypothesis for DVM.