Abstract
Monitoring marine top predators is fundamental for assessing the health and functioning of open ocean ecosystems. Although recently tracking observations have substantially increased, factors determining the horizontal exploration of the ocean by marine predators are still largely unknown, especially at the scale of behavioral switches (1–100 km, days-weeks). It is commonly assumed that the influence of water movement can be neglected for animals capable of swimming faster than the current. Here, we challenge this assumption by combining the use of biologging (GPS and accelerometry), satellite altimetry and in-situ oceanographic data (ADCP and drifting buoys) to investigate the effect of the mesoscale ocean dynamics on a marine predator, the southern elephant seal. A Lagrangian approach reveals that trajectories of elephant seals are characterized by quasi-planktonic bouts where the animals are horizontally drifting. These bouts correspond to periods of increased foraging effort, indicating that in the quasi-planktonic conditions energy is allocated to diving and chasing, rather than in horizontal search of favourable grounds. These results suggest that mesoscale features like eddies and fronts may act as a focal points for trophic interactions not only by bottom-up modulation of nutrient injection, but also by directly entraining horizontal displacements of the upper trophic levels.
Highlights
That nektonic animals are able to overcome oceanic currents does not imply that their movement is not directly influenced
Campagna et al.[37] present a striking example of elephant seals whose long trajectories (> 500 km) closely resemble that of a drifter released almost simultaneously in their proximity. Is this example merely anecdotal or should the common assumption be revised? This question is important in marine ecology because the assumption that a swimming behavior is not directly affected by the ocean currents stands at the core of the analysis of search behavior of fast swimming animals - the majority of tagged species
Our study focuses on the scale at which switches in the patterns of foraging behavior of marine predators are observed (~10 km)
Summary
That nektonic animals are able to overcome oceanic currents does not imply that their movement is not directly influenced. Southern elephant seals are a model species to address our research aims During their long-range foraging trips, these animals encounter different oceanic regimes, and in particular highly energetic features emerging between the Polar and the Sub-Antarctic fronts. Mesoscale turbulence is a major determinant of the distribution of consumers as zooplankton[52] and micronekton[53] Top predators such as whales[13], squid[54], king penguins[16], sea-turtles[55], frigatebirds[15,20], elephant seals[35], fur seals[56,57] and albatrosses[58] have been observed to co-localise with mesoscale structures but how much these animals actively track these features and how much they are entrained by them is part of the open question which we address here. In this study we find that, in contrast to what is often assumed, all the analyzed trajectories are characterized by bouts that are largely dominated by the currents advection and we develop a Lagrangian method to quantify the contribution of horizontal oceanic currents to an animal trajectory
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