  Meso- and submesoscales (fronts, eddies, filaments) in surface ocean flow
have a crucial influence on marine ecosystems. Their dynamics partly control
the foraging behaviour and the displacement of marine top predators (tuna,
birds, turtles, and cetaceans). In this work we focus on the role of
submesoscale structures in the Mozambique Channel on the distribution of a
marine predator, the Great Frigatebird. Using a newly developed dynamical
concept, namely the Finite-Size Lyapunov Exponent (FSLE), we have identified
Lagrangian coherent structures (LCSs) present in the surface flow in the
Channel over a 2-month observation period (August and September 2003). By
comparing seabirds' satellite positions with LCSs locations, we demonstrate
that frigatebirds track precisely these structures in the Mozambique Channel,
providing the first evidence that a top predator is able to track these FSLE
ridges to locate food patches. After comparing bird positions during long and
short trips, and different parts of these trips, we propose several hypotheses
to understand how frigatebirds can follow these LCSs. The birds might use
visual and/or olfactory cues and/or atmospheric current changes over the
structures to move along these biological corridors. The birds being often
associated to tuna schools around foraging areas, a thorough comprehension of
their foraging behaviour and movement during the breeding season is crucial not
only to seabirds' ecology but also to an appropriate ecosystemic approach of
fisheries in the Channel.
