Sensor-equipped elephant seals are helping scientists survey the ice-covered oceans surrounding Antarctica - and in some ways the animals do a better - and cheaper - job than traditional methods.

Scientists typically collect data about Antarctica's waters using satellites, buoyant floats and ship expeditions. But during the winter, increased amounts of sea ice renders the region virtually impermeable to all three.

Using tagged elephant seals, researchers collected 30 times more data about the sea ice in oceans off Antarctica than was possible using conventional methods.

"This is the first study to look at the entire (ocean off Antarctica) and to show the scale over which these kinds of data can be collected," said study team member Daniel Costa, a marine biologist at the University of California, Santa Cruz.

The research appears this week in the Proceedings of the National Academy of Sciences.

LIVING SUBMARINES

Costa and his colleagues attached electronic sensor tags to 58 southern elephant seals wintering in Antarctica. The tags - each about the size of a remote garage-door opener - measured temperature, pressure, salinity and the animals' positions when they surfaced. The data was beamed to a satellite and then relayed to researchers.

The tags, which were glued to the seals' heads using epoxy while the animals were anesthetised, remained attached for several months before falling off during seasonal sheddings.

While diving as deep as two kilometres in search of food, the living "submarines" collected information about the thickness and formation rates of sea ice.

The seals made a total of 4520 measurements, 90 per cent of which were acquired during the autumn and winter months. By comparison, only 148 autumn-winter measurements were obtained by ships and floats between 2004 and 2005.

OCEAN CIRCULATION

Sea ice is a crucial part of the Antarctic ecosystem. Algae grows in and under it and, in turn, feeds krill, tiny shrimp-like organisms that form a major link in the polar food chain.

"The abundance of krill is directly related to the extent of the annual sea ice," Costa said.

Sea ice can also affect global climate because water releases salt when it freezes. This changes water density and thus ocean-circulation patterns.

Environmental data gathered by animals are already being incorporated into models of ocean-circulation patterns by oceanographers such as John Klinck at Old Dominion University in Virginia.

"From my point of view, it's phenomenal," said Klinck, who was not involved in the study. "This is giving us information that we can't get any other way."

CHEAPER LABOUR

Costa estimates that the use of seals as ocean surveyors is significantly cheaper than conventional methods.

An Antarctic survey ship, for example, costs as much as US$50,000 per day to operate, and can gather perhaps 10 daily measurements.

In contrast, the total cost of a tagging a seal, satellite time and hiring workers is about US$20,000, and the seals can make up to 10 daily measurements for as long as 200 days, Costa said.

Greg Marshall is a marine biologist and vice president of remote imaging at the National Geographic Society.

Marshall invented Crittercam, a device that can be worn by sharks, sea turtles and other animals and which can record video and audio and also collect environmental data.

Projects such as Costa and colleagues' provide an interesting and important window into the ocean environment, Marshall said.

"The ocean is changing, and these guys are doing a great job of establishing the baseline against which we can evaluate further changes."

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