May 7, 2013

Sea elephant oceanographers: a 10-year success story

For the last 10 years, more than 200 elephant seals have been patrolling the southern seas wearing some strange headgear: an Argos transmitter instrumented with temperature, salinity and pressure sensors. The data these sensors acquire are fed in near-real time into the Mercator operational oceanography model and also tell us more about the animals' ecology.

31 May 2013

Antarctic Circumpolar Current

Fitting elephant seals with an Argos transmitter has become a routine operation for the team from the Chizé biology research centre (CNRS) led by Christophe Guinet. At least it has for 300-kg female elephant seals, because males—weighing up to 2,000 kg—can still prove something of a handful. Fifteen seals of both sexes were recently fitted from January to March in the Kerguelen Islands and then released back into the ocean, where they will spend the next 7 to 8 months foraging for food. “Males and females look for their prey—fish and squid—in different places,” explains Christophe Guinet. “Females favour the open ocean, whereas males tend to forage in the maze of sea ice and icebergs of the Antarctic continent. In this way, we obtain complementary data.”

The data we are talking about here concern the Antarctic Circumpolar Current, the strongest current in the world. The transmitters are all equipped with temperature, salinity and pressure sensors. Some of them also have a fluorescence sensor to estimate concentrations of phytoplankton, or a sensor to measure dissolved oxygen. Every time the animal surfaces to breathe, the antenna on its head uplinks the data recorded during dives to satellites overhead.

Animal ecology and space oceanography

Concentrations of dissolved oxygen recorded by an elephant seal from November to January 2011 between the Kerguelen Islands (right) and the Antarctic (left). The animal dived to average depths of 500 m, but regularly descended to 800 m and once even as far down as 1,050 m. Credits: CNRS/CEBC..

From 2004 to 2011, 125,000 temperature profiles and 28,000 salinity profiles were thus collected by 154 elephant seals and fed in near-real time into Mercator, the French-European operational oceanography model. During the same period, conventional methods using oceanography vessels recorded 10 times fewer profiles. And Argo profiling floats are ineffective in the southern seas, where they get stuck under the sea ice, crushed by icebergs or swept out to sea by the circumpolar current. “But we first had to test out our method,” recalls Christophe Guinet. “And we were able to do that thanks to the elephant seal oceanographers programme backed by the French national research agency ANR, the Fondation Total and CNES, with logistic support from the Polar Institute.”

But what the ecologist is looking to achieve today is to gain insight into the feeding strategies of these large seals. “We watch the elephant seals' movement with respect to their prey and to the currents,” he explains. “And we do that down to a scale of around 1 km.” Here again, satellites play a key role, as imagery from the Envisat satellite up to 2012 and from the U.S. Terra and Aqua satellites allows scientists to determine chlorophyll concentration in the Southern Ocean.

And of course chlorophyll tells us where the phytoplankton and therefore fish and squid on which the elephant seals feed are to be found. “Today, we can even discriminate phytoplankton groups in satellite imagery,” says Christophe Guinet, who has been working on these aspects with Séverine Alvain at the oceanology and geoscience laboratory in Lille. To track currents, researchers use data from altimetry satellites like TOPEX-Poseidon and the Jason series, and from the Mercator model. “Our work lies somewhere between animal ecology and space oceanography, which is a fantastic tool for describing and understanding these animals' behaviour in the most remote ocean on earth. We live in really exciting times!” concludes Guinet enthusiastically.