Planet Earth

North to the Fish, South to the Beach

By Lori OliwensteinJan 1, 1996 6:00 AM


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You might think an animal that tips the scales at two tons or so would be easy to track. But northern elephant seals, when they’re not lying around on a California beach making babies or shedding hair, are essentially invisible: they spend 90 percent of their time underwater, diving for food. The average dive lasts about 20 or 25 minutes, says Brent Stewart, a marine biologist from the Hubbs Sea World Research Institute in San Diego. It’s followed by about 3 minutes at the surface-- they’re diving all the time, and that goes on for months. But we didn’t really know where.

This past year Stewart announced that he’d found out. He and Robert DeLong of the National Oceanic and Atmospheric Administration in Seattle reported that elephant seals make two long-distance journeys each year, traveling farther than any mammal on record.

Stewart and DeLong attached satellite-linked radio transmitters to a few seals, but they knew that tracking system wouldn’t be terribly reliable: elephant seals aren’t often at the surface long enough for a satellite fix. So most of the researchers’ data came from a second tracking device, one that measured the sunlight intensity whenever the animals did come up for air. By noting the time on the recorder at which the highest light intensity occurred--an indication of local noon--the researchers could determine the animals’ longitude. The same device also measured the water temperature--an indication of latitude--and the depth of the animals’ dives.

Stewart and DeLong were able to retrieve intact recorders from 39 seals that breed on San Miguel Island, off Santa Barbara. They found that elephant seals like to take separate vacations: while the females head for spots throughout the north central Pacific, the males travel even farther, to the Gulf of Alaska and the eastern Aleutian Islands. And both sexes do this twice a year, something no other vertebrate does. They breed on San Miguel in winter, then head north; come back at separate times in spring and summer to molt, then head north; and finally return the following winter to give birth and immediately mate again.

The seals migrate north to eat: there are more fish in the cooler waters. The mystery is why they bother to return to San Miguel twice a year when there are closer beaches. The females journey more than 11,000 miles in the 300 days they are at sea each year; the males spend more time on the beach but still log 13,000 miles. Why can’t they stand to travel together? As usual, says Stewart, science creates a lot more questions than it answers.

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