While diving as deep as 1.2 miles (2 kilometers) in search of food, the living "submarines" collected information about the thickness and formation rates of sea ice.
The seals made a total of 4,520 measurements, 90 percent 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.
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.
(Related: "Antarctic Icebergs Teeming With Life, Study Says" [June 21, 2007].)
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."
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 U.S. $50,000 per day to operate, and can gather perhaps ten daily measurements.
In contrast, the total cost of a tagging a seal, satellite time, and hiring workers is about $20,000, and the seals can make up to ten 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, which owns National Geographic News.
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.
(Related: "Fish Steal Food From Rare Seals, Undersea Camera Shows" [November 27, 2007].)
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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