July 17, 2001
Contact: Tim Stephens (831) 459-2495; stephens@cats.ucsc.edu
DEEP-DIVING SEA LIONS PUSHED TO EDGE OF PHYSICAL LIMIT, STUDY SHOWS
For Immediate Release
SANTA CRUZ, CA--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, a professor of ecology and evolutionary biology at the University of California,
Santa Cruz.
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.
Editor's note: Reporters may contact Costa at (831) 459-2786 or costa@biology.ucsc.edu.
Images can be downloaded from the web at www.ucsc.edu/news_events/download/.
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