Predicting elephant seal routes in the meandering polar front | Polarjournal
Here, a southern elephant seal that is equipped with several tags, while coming to breed on the northern river beach of the Kerguelen archipelago, by scientists from the French Polar Institute. Image: Camille Lin

To reconstruct the fishing activities of southern elephant seals, researchers attach sensors to their backs, imagine their lifestyles, and then see if the two coincide. Hassen Allegue notes that the theory of “optimal foraging” may not be so accurate.

Being able to guess where the southern elephant seals are eating in the ocean is a bit like trying to predict the road traffic for the next vacation. It is necessary to use a mathematical model that describes the traffic well and, above all, to take several factors into consideration. Hassen Allegue and his colleagues remind us of the need to better describe the movement of elephant seals in their latest study published in Movement Ecology.

Most car drivers use GPS devices that provide information on speed, location, and sometimes even vehicle type or more. Mathematical recipes analyze this information and describe the possible behaviors of motorists during the upcoming vacations. In the same way, researchers working on swimming animals use predictive tools and equip their little protégés with a device that, like GPS, allows them to track the speed, acceleration, location or depth of their dive.

However, understanding the movement of a vehicle on road networks is simpler than understanding the movements of a marine animal underwater. Our knowledge of the “rules of the road” for these animals is relatively limited. The researchers therefore devised a theoretical code called “optimal foraging” for these marine mammals, which spend three-quarters of their lives hunting.

According to the theory of “optimal foraging”, an elephant seal optimizes its foraging by favoring areas where prey is concentrated. A bit like the dream place to spend your vacations, which gathers calm, atmosphere, beach, mountain, terraces… in the same place.

The average depth and duration of the elephant seal dives is 300-400 meters for 20-30 minutes. But Hassen Allegue (here at work) has already recorded dives of more than an hour and a half. Image: Camille Lin

Each elephant seal then uses two modes of movement, one fast and rather linear to find the right place to feed. Then, it rocks, chasing its prey one after another, moving more slowly along tortuous curves. “In fact, it is not always so true! I’m not saying it’s totally false, but it depends on the case,” the researcher cautions.

“Mark Hindell, an Australian scientist who also works on Kerguelen elephant seals, shows that the model works near a continental shelf. Only, it seems that elephant seals don’t drive the same way when they are in the middle of the ocean.”

Hassen Allegue

For example, off a continental shelf, the pattern of movement of these mammals is probably more elongated, in the direction of the currents. “A study shows that to save energy, elephant seals use currents to move,” he explains. When these aqueous streams carry nutrients, there may be fish, and therefore food for them. “In this configuration, the elephants certainly do not have the same mode of movement, and perhaps do not make such sinuous journeys as they do above the continental shelf, to feed. If we only look at their speed of movement, we can’t tell if they are hunting or not,” he adds.

“To understand and protect a well-defined population of animals, you have to study them closely and understand how they behave. The general theory is not necessarily going to apply to every situation.”

Hassen Allegue

The researcher adds, “Some individuals are more reckless than others.” The character of the animals is also to be taken into account. The most daring risk being attacked by an orca, overtaken by a competitor, stuck under the ice. They may also end up in the wrong place at the wrong time, because areas of nutrient abundance disappear as quickly as they appear from the ocean currents. But if their strategy pays off, they benefit from more productive locations. The bold have certainly not adopted the same rules of the road as the more cautious.

Now, another question arises, do elephant seals ever take time off from their hunt? Well yes, on the beaches of the Southern Ocean, where they renew their epidermis, lying on top of each other. The “GPS” attached to the backs of these animals by the researchers then become detached from their bodies. That’s when the scientists come to recover them, because they contain precious information, essential to decipher the navigation code of these rather endearing marine mammals.

Camille Lin, PolarJournal

Link to study: Hassen Allegue, Denis Réale, Baptiste Picard and Christophe Guinet, Movement Ecology, 2023, Track and dive-based movement metrics do not predict the number of prey encountered by a marine predator,

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