Euphausia superba, is a keystone species in the ecosystem of the Southern ocean. It is the principal phytoplankton consumer. Both food and predators are most abundant at the surface, thus krill during the day go to the depth of the ocean and return to the upper layers at night, to feed at a time when the predatory risk is lowest (Gaten et al., 2008). The analysis of the gut contents showed that during winter, krill could prey upon meso- and microzooplankton (Perissinotto et al. 1997).