Fine-scale structures as spots of increased fish concentration in the open ocean

Alberto Baudena,Enrico Ser-Giacomi,Yves Cherel,Xavier Capet,Cedric Cotté,Donatella D’Onofrio,Francesco D’Ovidio

doi:10.1038/s41598-021-94368-1

Abstract

Oceanic frontal zones have been shown to deeply influence the distribution of primary producers and, at the other extreme of the trophic web, 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 Indian sector of the Southern Ocean. First, we demonstrate that higher fish concentrations occur more frequently in correspondence with strong Lagrangian Coherent Structures. Secondly, we illustrate that, while increased fish densities are more likely to be observed over these structures, the presence of a fine-scale feature does not imply a concomitant fish accumulation, as other factors affect fish distribution. Thirdly, we show that, when only chlorophyll-rich waters are considered, front intensity modulates significantly more the local fish concentration. Finally, we discuss a model representing fish movement along Lagrangian features, specifically built for mid-trophic levels. Its results, obtained with realistic parameters, are qualitatively consistent with the observations and the spatio-temporal scales analysed. Overall, these findings may help to integrate intermediate trophic levels in trophic models, which can ultimately support management and conservation policies.

Highlights

Oceanic frontal zones have been shown to deeply influence the distribution of primary producers and, at the other extreme of the trophic web, top predators
Acoustic Fish Concentration (AFC) were separated in two groups: those falling over a front, identified by Finite-Size Lyapunov Exponent (FSLE) values over a threshold, and those falling outside it
This result demonstrates that high fish concentrations occur preferably over fronts detected with the diagnostics employed

Summary

Introduction

Oceanic frontal zones have been shown to deeply influence the distribution of primary producers and, at the other extreme of the trophic web, top predators. One of the origins of the heterogeneity of biotic fields is the dynamic nature of ocean environments, which transports water masses and affects their properties on a large range of temporal scales, including those of ecological relevance. In this regard, the mesoscale and submesoscale processes[7,8], commonly referred to together as “fine-scales” (from a few to hundreds of kilometers) influence importantly the ecological landscape. By shaping and elongating water patches, Lagrangian Coherent Structures have been demonstrated to set the frontiers of phytoplanktonic patches in terms of chlorophyll c oncentration[10], and even functional t ype[15]. The concentration of predators foraging efforts has been observed in association with

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