Rafting behaviour of seabirds as a proxy to describe surface ocean currents in the Balearic Sea

A Sánchez-Román,R Fablet,D Oro,E Mason,J M Arcos,S Ruiz,L Gómez-Navarro,A Pascual

doi:10.1038/s41598-018-36819-w

Abstract

Spatio-temporal variability of surface geostrophic mesoscale currents in the Balearic Sea (western Mediterranean) is characterized from satellite altimetry in combination with in-situ velocity measurements collected, among others, by drifting buoys, gliders and high-frequency radar. Here, we explore the use of tracking data from living organisms in the Balearic Sea as an alternative way to acquire in-situ velocity measurements. Specifically, we use GPS-tracks of resting Scopoli’s shearwaters Calonectris diomedea, that act as passive drifters, and compare them with satellite-derived velocity patterns. Results suggest that animal-borne GPS data can be used to identify rafting behaviour outside of the breeding colonies and, furthermore, as a proxy to describe local sea surface currents. Four rafting patterns were identified according to the prevailing driving forces responsible for the observed trajectories. We find that 76% of the bird trajectories are associated with the combined effects of slippage and Ekman drift and/or surface drag; 59% are directly driven by the sea surface currents. Shearwaters are therefore likely to be passively transported by these driving forces while resting. The tracks are generally consistent with the mesoscale features observed in satellite data and identified with eddy-tracking software.

Highlights

Most of the studies in the literature are focused on: (i) analysis of long range flights, where birds are travelling at high speeds over long periods; (ii) the identification of foraging hotspots or; (iii) the development of habitat models
Recently[4], used the global positioning system (GPS) to track breeding Northern Gannets Morus bassanus in order to study the incidence of rafting behaviour on foraging trips; rafting was observed in 62% of the breeding foraging trips that were monitored
We compare the satellite-derived velocity patterns in the Balearic Sea with the paths followed by Scopoli’s shearwaters when they are at rest on the sea surface and act as passive drifters in an attempt to determine whether animal-borne GPS data can be used as a proxy to describe sea surface currents in the area

Summary

Introduction

Most of the studies in the literature are focused on: (i) analysis of long range flights, where birds are travelling at high speeds over long periods; (ii) the identification of foraging hotspots or; (iii) the development of habitat models. We compare the satellite-derived velocity patterns in the Balearic Sea (western Mediterranean Sea) with the paths followed by Scopoli’s shearwaters when they are at rest on the sea surface and act as passive drifters (emulating drogue-less drifting buoys) in an attempt to determine whether animal-borne GPS data can be used as a proxy to describe sea surface currents in the area Such drifters, without a holey-sock drogue (usually placed at around 15 m depth) are strongly affected by the direct local wind and, to a lesser extent, by surface waves acting on the upper part of the drifter that protrudes above the sea surface[16]. The final goal of this work is to contribute to the understanding of marine system’s dynamics and their spatio-temporal evolution

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Conclusion