for National Geographic News
This is the fifth story in a continuing series on the Megafishes Project. Join National Geographic News on the trail with project leader Zeb Hogan as he tracks down the world's largest fishes.
From a spotter airplane buzzing off the coast of Baja California, it's hard to miss the dark shape of a giant whale shark moving through the emerald green waters below.
Whale sharks are the world's largest living fish species, growing up to 40 feet (12 meters) long.
They move near the surface, feeding on the plankton and krill that mass in these waters during the winter months.
The Bay of La Paz, though busy with fishing boats and divers, is a safe zone for these rare and threatened animals.
But around the world, shark populations have declined dramatically in recent years, mainly due to overfishing.
Most at risk are migratory sharks, including whale sharks, which are known to travel more than 8,000 miles (12,875 kilometers) across the Pacific Ocean, from Mexico to the Tonga Islands, according to Zeb Hogan, a fisheries biologist with the University of Reno in Nevada.
"Every time a migratory shark moves from one spot to another, there's a greater chance that it might be targeted by fishermen or subject to habitat destruction," Hogan said.
Of the more than 1,000 species of sharks and rays, 145 are known to be migratory.
Eighteen percent of these thousand species are threatened with extinction, according to Hogan, compared to 45 percent of the migratory sharks and rays.
At a UN-sponsored conference on migratory sharks held in the Seychelles last month, three species—whale sharks, basking sharks, and great white sharks—were singled out as being in urgent need of protection.
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