Top marine predators track coherent structures at the surface of the ocean.
Understanding the role of oceanic eddies in the distribution of marine predators remains a crucial issue for ecology as well as for fisheries. A multidisciplinary study conducted by a team of researchers has highlighted the influence of submesoscale structures (associated with eddies) on the displacement and foraging behaviour of frigate birds in the Mozambique Channel.
Located between Madagascar and Africa, the Mozambique Channel is an incredible natural laboratory for studying oceanic mesoscale eddies,* their associated structures and their influence on populations of top marine predators. Every year a succession of 4-7 eddies moves through the channel from north to south along the west coast. Their dynamics partly control the foraging behaviour and displacement of marine predators such as tuna, birds and even turtles. While scientists already knew about the influence of mesoscale eddies (hundreds of km) on the distribution of top predators, the role of submesoscale structures, (tens of km) resulting from the interactions between eddies, has not been understood until now.
Pacific frigate birds (this bird’s scientific name is ‘Fregata minor’) were fitted with ARGOS beacons, which allowed their positioning to be tracked by satellite. Thus, scientists were able to draw a link between the distribution of frigate birds from the colony of the island of Europa (during their breeding season) and these delicate ocean structures. Frigate birds precisely follow these structures in order to navigate and feed.
Scientists, however, don’t understand how frigate birds follow these structures. Some hypotheses have been advanced, such as the use of visual and/or olfactory cues and/or interactions between atmospheric currents and oceanic Lagrangian coherent structures.
In addition, frigate birds show a particular interest in fisheries, because, having a permeable plumage, they cannot dive for food and so rely on the presence of subsurface marine predators such as tuna and dolphin. This study, therefore, brings a better understanding of the displacement and foraging behaviour of frigate birds, essential not only to ecology but also to an ecosystemic approach to fishing in the Mozambique Channel, as it is subject to large-scale tuna catches.
Tew-Kai, E., Rossi, V., Sudre, J., Weimerskirch, H., Lopez, C., Hernandez-Garcia, E., Marsac, F. and Garçon, V., 2009, Top Marine Predators Track Lagrangian Coherent Structures, Proceedings of the National Academy of Sciences, in press. (Translator’s note: correction to in press; 2009 May 19;106(20):8245-50).
Emilie Tew-Kai: CRH / IRD
Vincent Rossi: LEGOS / CNRS
* Intermediate scale: (hundreds of km) extending across the global circulation (depressions and anticyclones: extending across an entire continent, ocean currents, etc..) and systems on very small scales of less than 2 km in diameter.