Life at sea

What do they eat?

Three main groups of prey predominate in the diet of marine predators, namely crustaceans, cephalopods and fish. Prey are determined by the analysis of stomach contents for seabirds and scats for pinnipeds. Difficulties arise from the high level of digestion of the samples. Consequently, prey identification relies almost exclusively on hard parts such as exoskeletons of crustaceans, chitinous beaks of cephalopods, and otoliths and bones of fish (Cherel et al. 2002). Items were compared to those of a reference collection including the key species of the pelagic and benthic ecosystems of the Southern Indian Ocean.
The direct method of food analysis is limited by the fact that parent seabirds bring back food to the colony during the chick-rearing period only. We therefore use it in conjonction with indirect methods to gain further information on their food and feeding ecology. The stable isotope technique on blood and feathers allows the determination of the trophic level (nitrogen) and foraging areas (carbon) (Cherel et al. 2000) of the animals, and the use of lipids of adipose tissue and stomach oil as trophic markers allows the determination of their prey (Raclot et al. 1998).
 

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Where do they forage?

Our team was the first to use satellite tracking to follow the foraging trips of individual birds, i.e. wandering albatrosses from Crozet Islands during the austral summer 1988-1989 (Jouventin & Weimerskirch 1990).
 

This technique is now used by many scientists on different species of pinnipeds, penguins, petrels and albatrosses. Miniaturisation of the transmitters over the last 15 years allows now to equip smaller animals, for example white-chinned petrels (Catard et al. 2000). We also recently deployed two new kinds of loggers, GLS devices (geolocation system) that permit to track seabirds during long periods (6-12 months) (Weimerskirch & Wilson 2000), and GPS devices that locate animals very accurately (Weimerskirch et al. 2002).   We know now the foraging areas with their spatio-temporal changes for many top predators. This a key step for a better understanding of their life history, but also for their conservation (Weimerskirch et al. 1999).

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How do they locate and catch prey ?

Satellite telemetry determines foraging areas, not the exact location where the animals feed, and it gives no information on the use of the water column by diving organisms.
 

Answering to the first question requires the simultaneous use of different electronic devices on the same individuals. Satellite tracking together with probes recording stomach and water temperatures indicate when the animal ingests prey (Weimerskirch et al. 1994), and when it flies or settles at the surface (Weimerskirch & Guionnet 2002) during the foraging trip.  Answering to the second question requires the use of pressure sensors that record the underwater foraging pattern of diving animals  (Tremblay & Cherel 2000).

For each dive, we know its duration, depth, descent and ascent rates, and post-dive interval at the sea surface to recover. At the scale of a foraging trip, we can thus investigate spatio-temporal changes, for example differences in diving behaviour associated with the day/night cycle.
 

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