BEIJING (Los Angeles Times) -- After a fruitless six-week expedition up and down the Yangtze River, a six-nation team of scientists has concluded that the baiji, a freshwater dolphin that was one of the world's oldest species, is almost certainly extinct.

The researchers said they believed that it was the first time that a large mammal species had been wiped out primarily by human-led destruction of its habitat, and they warned that another species, the Yangtze finless porpoise, appeared headed for the same fate.

The dolphins were killed off by overfishing, habitat loss, and collisions with ships, according to the expedition, led by China's Agriculture Ministry and organized by the Baiji.org Foundation and China's Wuhan Institute for Hydrobiology. The last two factors, especially, reflect the increasing industrial development along the river. "The Yangtze is not a natural habitat any more for cetaceans," said August Pfluger, a Swiss economist and environmental activist who helped organize the expedition. "The natural habitat that the baiji would use, it's not there anymore."

Large parts of the river have been turned into concrete channels, and sandbars that were an important part of the dolphin's habitat have largely been destroyed, he said.

The beloved baiji, known as "the goddess of the Yangtze," was believed to date back 20 million years, and was once a common sight as it cavorted in the river. As late as the 1980s, scientists counted 400.

The expedition team said that it was possible, if unlikely, that it had missed one or two animals, but that the species could be considered "functionally extinct." Any remaining dolphins would have no chance of survival, the researchers said.

Pollution did not appear to be a culprit, the scientists said. "The river is highly polluted," Pfluger said in a telephone interview, "but if people say that pollution killed the dolphins, that's just not true... .It's not a toxically polluted river."

The Yangtze, the longest river in Asia and third-longest in the world, is a vital source of water and electricity. Its basin is home to 400 million people, about one in every three Chinese. But China's economic growth has put unprecedented pressure on the environment.

Using two research vessels, the expedition surveyed 2,200 miles of the river from near the Three Gorges to its mouth at Shanghai, China's largest city.

The researchers counted 400 finless porpoises, a dramatic decline from about 5,000 in the 1980s. Wang Ding, deputy director of the Wuhan Institute of Hydrobiology, was quoted as saying on the Baiji.com Web site: "If we do not act soon, they will become a second baiji."