It has previously been arguefd that biologically generated turhulence is limited to the scale of the individual animals involved, which would make turbulence created by highly abundant centimetre-scale zooplankton such as krill irrelevant to ocean mixing. Here we show that the collective vertical migration of centimetre-scale swimmers-as represented by the brine shrimp Artemia salina--generates aggregation-scale eddies that mix a stable density stratification, resulting in an effective turbulent diffusivity up to three orders of magnitude larger than the molecular diffusivity of salt.