We investigated the causes of a large increase in abundance of small copepods, in particular Oithona similis, that was observed during the iron fertilisation experiment EisenEx in the Southern Ocean. Oithona spp. individuals showed a pronounced migratory response and shifted their vertical distribution towards the progressively phytoplankton-enriched surface layer in the bloom area, while outside, in the area with dilute food concentration, a substantial number of individuals resided in deeper layers. This deep-dwelling behaviour affected an increased drift relative to scarce food in the surface layer, whereas upward migration led to a gradual accumulation of animals in the bloom area. Our simulation study takes into account the particular flow field and the migratory response of Oithona spp. and shows that it can explain most of the abundance increase in Oithona spp. observed during EisenEx. The migratory behaviour of Oithona spp. may be considered as a food-finding strategy to cope with the patchy, mostly poor food environment of the Southern Ocean. © Inter-Research 2009.