Rushing across a temple parking lot, British angler Rick Humphreys yells, "We've got a fish."

He jumps into a small motorboat on the Mae Klong River in time to see Wirat Moungnum bring the prize to the surface: a rare giant freshwater stingray that weighs as much as 44 pounds.

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"It's a start," Humphreys says almost apologetically. The specimen is a tenth the size of the largest rays. "There are a lot bigger ones than that."

Humphreys and his partner, Wuttichai Khuensuwan, have caught 40 rays on the Bang Pakong and Mae Klong Rivers, with the largest weighing in at 485 pounds.

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The next day, they have better luck on the Mae Klong.

The rod bends almost into the water, and Wirat struggles for almost half an hour as the ray dives under the boat and across the bow.

It finally is brought to the surface, revealing its big bulging eyes and dark, coarse skin. Its tail alone is 12 feet long.

Hogan, 34, has heard the stories of Cambodian fishermen catching rays that weighed more than 1,100 pounds with wingspans of 14 feet. But so far they are just stories. If he can confirm them, his find could eclipse the world record now held by the Mekong giant catfish: a 646-pound specimen caught in 2005 in Thailand.

"It could be the largest fish in the world and we know next to nothing about it," Hogan says. "I've spent five years on the Mekong looking for rays and only saw two or three. They were nowhere near the size I'd heard about."

Hogan said he was drawn to the freshwater ray, known scientifically as Himantura chaophraya, because so little is known about it.

Listed as vulnerable by the International Union for Conservation of Nature, it is believed to be found in rivers from Thailand to northern Australia. Scientists discovered it only 18 years ago, and its population is unknown.

Name: Giant freshwater stingray (Himantura chaophraya)

Maximum Size: 197 inches (500 centimeters), 1,323 pounds (600 kilograms), body diameter 95 inches (240 centimeters)

Habitat: Mekong River Basin, Southeast Asia

Endangered Status on IUCN Red List (2006): Vulnerable

Population Size: Unknown

Major Threats: Overfishing and pollution