In contrast to terrestrial environments, the open ocean has a dynamics whose timescales overlap with the demography of the organisms it hosts. In particular, so called meso-and submeso-scale processes (1-100 km, days to weeks) have been shown to play a key role in structur-ing the distribution of phytoplankton, which form the large majority of the base of the trophic chain [2]. However, how the (sub)mesoscale turbulence affect higher trophic levels, which have typically swimming capabilities , is largely not known. Here we explore the capability of frontal system to aggregate swimming organisms (fish) by analyzing an idealised model of the stretching region which is often found in between mesoscale vortices. The rationale behind this approach is that an optimal niche for fish, defined in terms of physical properties or prey availability, may shrink with time under the coupled effect of stretching and diffusion. If the shrinking speed is less than the fish swimming capability, fish schools originally dispersed over a wide region may move inside a smaller area, and therefore increase their local density. The model is parameterised for one of the most abundant mesopelagic fish, the myctophid [4], particularly important in the Southern Ocean, using physical conditions representative of their environment.