Vertical migrators see the light?1

Abstract

Enright and Honegger (1977) observed that stage V and adult Calanus sometimes migrate into the upper water strata an hour or more before sunset. Enright (1977) proposed a metabolic model to account for this variation from the more typical pattern, in which vertical migration occurs roughly at sunset. This hypothesis is criticised and another proposal is put forward, this being that under certain circumstances, late afternoon feeding in the surface waters by diurnal migrators may also be consistent with a strategy of predator avoidance. Enright (1977) assumes that visual predation on zooplankton is most efficient during daylight and least efficient at night, however bioluminescent organisms mean darkness is frequently punctuated by flashes of light and active zooplankton may be more vulnerable to visually orientating predators. Therefore vertically migrating zooplankton might best avoid these predators by entering the upper waters and grazing most intensely in late afternoon.