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July 9, 2001

Deep-diving sea lions pushed to edge of physical limit

By Erica Klarreich

A deep-diving Australian sea lion. Photo: Dan Costa
A new study may help explain why certain species of marine mammals seem particularly vulnerable to changes in their food supply.

Researchers have found that some deep-diving sea lions already work so hard searching for food that their ability to increase the duration of their dives is limited. As a result, they may be unable to cope with food shortages or other environmental stresses.

"They are really pushing their physiological capabilities," said Daniel Costa, professor of ecology and evolutionary biology.

Costa and his coauthors, Nicholas Gales of the Australian Antarctic Division and Michael Goebel of the U.S. Antarctic Marine Living Resources Program of the National Oceanic and Atmospheric Administration, published their findings in the July issue of Comparative Biochemistry and Physiology (Part A).

The researchers looked at three species of marine mammals: the Antarctic fur seal, the Australian sea lion, and the New Zealand sea lion. The fur seal population is thriving, but the two sea lion populations are both struggling. The researchers found that the sea lions regularly stay underwater so long that their oxygen supplies dwindle to the point where they have to use anaerobic metabolism to keep swimming.

Anaerobic metabolism generates energy using a chemical reaction that does not require oxygen. It is useful when oxygen is in short supply, but it cannot be sustained long, for two reasons: It is less efficient than aerobic metabolism, and the reaction creates a byproduct called lactic acid that, among other things, makes muscles sore.

Researchers have devised mathematical formulas to calculate "aerobic dive limits," the amounts of time various marine mammal species can stay underwater before anaerobic metabolism kicks in. Costa's team compared these limits to the amount of time the seals and sea lions actually spent underwater when they dove. To track the animals' diving habits, the researchers used monitors attached to their fur that recorded how deep they dove and how long they stayed underwater. The scientists also measured the animals' oxygen levels by taking blood samples and muscle biopsies.

They found that while the Antarctic fur seals generally stayed underwater only as long as they could dive aerobically, the Australian and New Zealand sea lions typically dove nearly 1.5 times longer than their aerobic dive limit, meaning that they were forced into anaerobic metabolism for about one-third of each dive. They also had to rest longer between dives than the fur seals did.

"They're either doing something we don't understand or they're routinely exceeding their aerobic limit," Costa said. "It's interesting that the populations that are working very hard are the ones that are not doing well."

Scientists differ on how to calculate an animal's aerobic dive limit, but the study's results are nevertheless sound, Costa said. "Even with the most conservative measures of the aerobic dive limit, these animals are pushing or exceeding their limit," he said.

Antarctic fur seals tend to make shallow dives and feed on small prey, such as krill, found near the surface. Australian and New Zealand sea lions, by contrast, seek out larger prey on the ocean floor, where they are believed to catch mainly octopus and large squids. Since they must travel to the ocean floor before searching for prey, the sea lions' dives tend to be nearly three times as long as those of the fur seals. And since the sea lions search for a single large prey instead of many small ones, they may have an extra incentive to stay underwater beyond their aerobic limit when they spot a potential meal.

Analyses like Costa's may help researchers figure out when a population is struggling because of limited food supply, as opposed to disease or entanglement in fishing nets, he said. They may also help policy makers understand how large-scale commercial fishing affects marine ecosystems.

"This study shows that if we take lots of fish away, deep-diving species are more likely to decline than others," he said. "These are complex issues, with complex answers. But I think this is a significant part of the puzzle."

The research was funded by the National Science Foundation's Polar Program and by the National Geographic Society.

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