A thick, polluting haze envelops the Three Gorges Dam, blurring the view of the world's largest hydroelectric station.
But for Zeb Hogan, a fisheries biologist with the University of Reno, in Nevada, seeing the 1.5-mile-wide (2.5-kilometer-wide) dam from the banks of the Yangtze River brings into sharp focus the threats facing the animals he has set out to study: the world's largest freshwater fishes.
"From the point of view of the fish, there's nothing worse than a dam," he said.
"Dams block upstream migration, destroy spawning habitat, and can turn large stretches of river into ecological wastelands."
Earlier this year Hogan launched the Megafishes Project, a three-year effort funded by the National Geographic Society to document the 20-some species of giant fish found around the world. (National Geographic News is owned by the National Geographic Society.)
Megafishes live in rivers and lakes and grow to at least 6.5 feet (2 meters) in length or 220 pounds (100 kilograms) in weight.
Hogan calls the giant fish "the real-life Loch Ness monsters and Bigfoots of the aquatic world." (See photos of the "monster" fishes.)
Hogan's mission has brought him to the Yangtze River, home to the Chinese sturgeon and Chinese paddlefish—ancient leviathans that were once plentiful in the world's third-longest river but are now on the brink of extinction (see China map).
The Chinese paddlefish, which can grow to be 23 feet (7 meters) long and weigh 1,100 pounds (500 kilograms), may be the largest freshwater fish in the world.
But no one has seen one in the Yangtze since 2003.
"Everywhere around the world these large fish are in big trouble," said Hogan, who is a National Geographic Emerging Explorer.
"In many places they're now so rare that the opportunity for documentation and study may soon be lost."
River Titans, Half-Ton Stingrays
The troubled story of the world's giant freshwater fish underlines the environmental crisis facing many rivers and lakes, Hogan said.
The world's fresh waters are experiencing declines in biodiversity far greater than those seen in the oceans or on land, he pointed out.
Although rivers and lakes only make up about 0.01 percent of Earth's water, about 8 percent of all species and 40 percent of global fish diversity are found there.
But more than a fifth of the world's known freshwater fish species have become extinct or gravely threatened in recent years, Hogan said.
"The conservation of freshwater biodiversity traditionally receives low priority, partially because people think of fish as food, not as wild animals," he said.
Focusing the conservation effort on the largest fish makes sense to Ian Harrison, a fish biologist at the American Museum of Natural History in New York.
"In terms of helping people to understand the special importance or uniqueness of a region, it is usually easiest to identify some flagship species that capture people's attention," Harrison said.
Among these unusual animals are the arapaima, a 15-foot (4.5-meter) titan found in the Amazon River in Brazil, and the giant freshwater stingray, which may grow to more than a half ton and lives in the Mekong River in southeast Asia (see a map of the Mekong River).
"But the project isn't just about finding the world's largest fish," Hogan said.
"It's about healthy aquatic habitats, clean drinking water, living rivers. These are things that we all rely on every day."
Devin Bartley is a fisheries specialist with the United Nation's Food and Agriculture Organization in Rome, Italy.
"Megafish are good indicators of the health of a river," he said, "because they are often the first to be overfished or suffer from habitat degradation."
The threats facing large-bodied fish are vast, including pollution, increased boat traffic, habitat fragmentation from dams, climate change, and invasive species.
Many of the huge fish take years to mature, making them particularly vulnerable to environmental threats.
And in contrast to the giant animals found in the seas, river giants often live close to dense human populations.
On the Yangtze, which sees the most cargo ship traffic of any river in the world, some ten Chinese sturgeon are killed or seriously injured by boat propellers every year.
"Compounding the problem is the [inadequate] system of governance for many inland waters," Bartley said.
"Many of the world's large rivers cross one or more national borders, but fish or pollution don't respect these borders, so actions in one part of the river affect fish elsewhere."
Much of Hogan's work has focused on Southeast Asia's Mekong River, where overfishing represents the most serious concern.
The river is home to the critically endangered Mekong giant catfish, which can tip the scales at more than 660 pounds (300 kilograms).
Fishers along the Mekong once hauled in hundreds of giant catfish every year. Last year, only one was caught.
Race Against Time
Nancy Knowlton is director of the Center for Marine Biology and Conservation at the University of California, San Diego. She said one of the most immediate threats facing megafishes is that they're so poorly understood.
"In the ocean we have already lost most of the apex predators and other large fish, but at least their plight has received a lot of attention," Knowlton said.
"These freshwater giants are even more threatened because of their much narrower ranges and because we know so little about them, making them much harder to protect.
"Megafish in the ocean are known to play crucial ecological roles, but we have no idea of what the ecological consequences might be for rivers and lakes that are missing these giants," she added.
Hogan said more attention is now being focused on the plight of large freshwater fish, but he recognizes there are still challenges ahead.
"Due to the precarious state of populations of large freshwater fish, this new project is a race against the clock," he said.
"We must identify and protect these aquatic giants before they are gone forever."