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 avoi ding size-selective visually hunting predators and maximizing energy gain, smaller bodied prey should enter surface waters earlier and leave later tha n larger, more visually conspicuous organisms. Conventional sampling techno logies lack the temporal resolution to test this prediction. Here, we repor t on the first test of this prediction using a new submersible optical-acou stic imaging system capable of resolving the timing of migration of the eup hausiid crustacean Euphausia pacifica Hansen. Smaller bodied animals consis tently 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-depende nt risk of attack by visual predators alters the tradeoff between feeding a nd predator avoidance, supporting the predator-avoidance hypothesis for DVM .