This is the third installment in a 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 freshwater fishes.
It's known as the "mother species" and the "panda under the water."
Yet the nickname that most aptly describes the Chinese sturgeon, a giant fish that's been around since the age of the dinosaurs, may be "living fossil."
Now conservationists are locked in a race against time to save this ancient river titan from extinction.
Adult sturgeons, which can measure up to 4 meters (13 feet) in length and weigh 1,000 pounds (450 kilograms), migrate from the East China Sea into the Yangtze River to spawn.
But the Yangtze's deteriorating environment and increased shipping traffic have taken their toll on the mammoth fish.
Thirty years ago there were 2,000 spawning Chinese sturgeons in the Yangtze River every year. Now that number is down to several hundred.
There may be only a thousand of the animals left in the river, said Wei Qiwei, a lead researcher at the Yangtze River Fisheries Research Institute in Jingzhou
Wei and other scientists hope to reverse the trend by breeding sturgeons in captivity and putting them back into the river before the species disappears the wild.
"The Chinese sturgeon is very precious to us," Wei said. "I don't want it to disappear on my watch."
Wei's institute operates a breeding base tucked into a sleepy farming community outside Jingzhou.
A slogan on a wall greeting visitors reads, "Love the Chinese Sturgeon, Our National Treasure."
Inside, a hatchery facility the size of two football fields contains rows and rows of tanks holding sturgeons in varying stages of development, from larvae to one-year-old fish.
Sexually mature sturgeons taken from the wild are kept at the hatchery to provide eggs for breeding.
"The short-term goal is to preserve the fish in captivity, but the long-term goal is to preserve the fish in the river as part of the ecosystem," Wei said.
To do that, scientists want to keep some of the newly hatched sturgeons in captivity until they are sexually mature before releasing them into the wild.
But the reproductive capacity of the fish is poor; it takes more than ten years for the Chinese sturgeon to begin spawning.
It will take at least another five years for the oldest fish at the hatchery, which are kept in holding tanks outside, to reach sexual maturity, the scientists estimate.
"The critical issue for us is to make brood stock [from the fish taken from the wild] and then to release them again," said Zeng Lingbing, director of the institute's Fish Pathology Laboratory.
"But we have not come full circle yet, so we don't know if this will be possible."
Fisheries biologist Zeb Hogan heads the National Geographic Society's Megafishes Project, a three-year program to document the world's largest freshwater fishes.
(National Geographic News is a division of the National Geographic Society.)
Hogan recently visited the hatchery and said Wei's breeding program could help offset the many threats now facing the Chinese sturgeon in its native waters.
"This breeding program is like an insurance policy to make sure this ancient fish does not disappear," said Hogan, standing waist deep in the green waters of a holding tank with a seven-foot (two-meter) sturgeon in his grip.
The fish can grow twice that size, but no sturgeons that big have been seen in the Yangtze in the past 20 years, Wei explained.
The Chinese sturgeon moves from seawater to fresh water to spawn. It has the longest migration of any sturgeon in the world and once migrated more than 2,000 miles (3,500 kilometers) up the Yangtze.
That was before the Ghezouba Dam was built on the Yangtze River in the early 1980s, cutting off the sturgeon's migratory path, just as it did for the critically endangered Chinese paddlefish.
(Read: "China's Giant River Fish, the World's Largest, Feared Extinct" [July 26, 2007].)
All of the sturgeon's original spawning grounds were located upstream from the dam, Wei explained.
"There used to be spawning grounds totaling more than 600 kilometers [375 miles] in the river," Wei said. "Now there is less than 30 kilometers [19 miles]."
Increasing boat traffic on the Yangtze is a major threat to the sturgeon, which frequently swims near the surface. Every year, about ten Chinese sturgeons are killed by boat propellers.
The sturgeon is also highly sensitive to increased noise on the river caused by growing traffic.
In addition, Wei speculates that worsening water contamination from industrial runoff and other sources may be causing sturgeons to change their sex.
(Read: "Animals' Sexual Changes Linked to Waste, Chemicals" [March 1, 2004].)
"After 1995 the ratio of male to female has totally changed," he said. "It used to be one to one, but now there may be up to ten females for every one male."
In addition to restocking the river with fish, Wei is also trying to find ways to create artificial spawning grounds in the Yangtze River.
"Habitat restoration is another way to save the species," he said.
Hogan, who is a National Geographic Emerging Explorer, says there is no umbrella solution for saving megafishes like the Chinese sturgeon.
"If we look at rivers around the world, we see all kinds of problems—habitat fragmentation from dams, pollution, invasive species, overfishing," he said.
"There are a lot of threats to large-bodied species of fish, and we have to look at each river separately to find the best way to save these amazing creatures."