Fine-scale fronts as hotspots of fish aggregation in the open ocean
Oceanic Lagrangian Coherent Structures have been shown to deeply influence the distribution of primary producers and, at the other extreme of the trophic chain, top predators. However, the relationship between these structures and intermediate trophic levels is much more obscure. In this paper we address this knowledge gap by comparing acoustic measurements of mesopelagic fish concentrations to satellite-derived fine-scale Lagrangian Coherent Structures in the open ocean. The results demonstrate unambiguously that higher fish concentrations are significantly associated with stronger Lagrangian Coherent Structures, and we observe that these features represent a limiting condition for high fish concentrations. A model, specifically built for mid trophic levels with realistic parameters, provides a possible mechanism of fish aggregation, and is coherent with the observations. These results may help to integrate intermediate trophic levels in trophic models, which can ultimately support management and conservation policies.