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Great white sharks in spotlight at Guadalupe Island

October 29, 2008 |  1:24 pm

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The great white shark in the photo is among dozens that reside seasonally at Guadalupe Island, a large volcanic land mass 250 miles southwest of San Diego and about 160 miles west of Baja California.

It was e-mailed to Outposts by Michael Domeier, president of the Marine Science Institute in Fallbrook, Calif. Domeier has just released results of an eight-year study, during which 56 white sharks were tagged at Guadalupe and their movements and behavior were monitored.

Based on the findings, Guadalupe's sharks migrate — beginning as early as Dec. 21 but spread over a 19-week period — to a vast pelagic habitat in the mid-Pacific, where they spend an average of 140 days, diving, at times, to 3,000 feet in an apparent search for food.

They then swim back to Guadalupe. The males arrive beginning about July 22, coinciding with the pupping of Guadalupe fur seals. The females arrive several weeks later.

More interesting is that white sharks from Northern California migrate to the same offshore area — some from both groups even travel as far west as Hawaii — but there is no known intermixing. No tagged shark from Northern California has ever visited Guadalupe, and vice versa.

Whereas the Northern California study, carried out by Stanford University's Hopkins Marine Station, referred to the area as the White Shark Cafe, Domeier and co-author Nicole Nasby-Lucas label it the Shared Offshore Foraging Area, or SOFA.

But precisely why the apex predators visit the cafe or SOFA remains a mystery.

Domeier suggests it's to hunt rather than reproduce, based on diving patterns, the fact that tagged sharks were widely dispersed at the SOFA, and that they do not return to the island noticeably thin.

What food could they find at such depths? Squid, maybe swordfish and other sharks. Those visiting Hawaii would be there during the humpback whale calving season.

But if white sharks are not mating at Guadalupe Island, where male sharks with reddened claspers have not been seen, then where does mating occur?

It could be at the SOFA, but nobody knows that. Nor is it known where white sharks are born, though some presume pupping occurs along the coast.

In any event, we now have a little more light shed on one of the planet's  most famous, yet mysterious creatures. And we also know the safest  time to go for a swim at Guadalupe: Well after Dec. 21 and long before July 22.

-- Pete Thomas

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Photo: White sharks, at times lured for tagging studies by tuna tied to a rope, are currently gathered in large numbers at Guadalupe Island off Baja California. Credit: Michael Domeier

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