DEEP-SEA DIVERS

To find food, Australian sea lions spend nearly 60% of their time at sea diving to depths of up to 100 m.

There are - as they say - a lot of fish in the sea, so deciding where to dine is no easy task for seals and sea lions. Antarctic fur seals prefer to feed on small fish and krill, keeping most of their foraging dives to less than 20 meters in depth. With this strategy, the fur seals don't have to travel very far to find food, but their food source is often patchy and unreliable. Australian sea lions take a different approach, diving up to 100 meters down to the ocean floor to feed on a more predictable supply of bottom-dwelling organisms.

As part of his research about the foraging strategies of marine mammals, Academy fellow Dan Costa has been analyzing these differing diving behaviors. So far, his results have shown that Australian sea lions are pushing their physiological capabilities by taking such long, deep dives. In the search for food, they regularly stay underwater so long that they use up their oxygen supplies and must use anaerobic (oxygen-free) metabolism to keep swimming. This process creates a byproduct called lactic acid that, among other things, makes muscles sore.

Costa has noted that while shallow divers like Antarctic fur seals have enjoyed population growth over the past few decades, the deep-diving Australian sea lion population has declined. One possible explanation is that Australian sea lion pups may have a lower survival rate than fur seal pups, since they cannot handle the deep dives until they are older. Costa is now studying the development of diving skills in Australian sea lion pups.

Map by Colleen Sudekum
Resting sea lions equipped with dive recorders to gather data. Photo: Dan Costa
Costa equips the sea lions he studies with dive recorders that provide data on the depth and duration of each dive. Photo: Dan Costa.

Costa will be speaking about his research on Australian sea lions at the Academy's Bioforum forum on
November 1, 2003
Marine Mammals: Exceptional and Endangered.
For more information: http://www.calacademy.org/education/bioforum/