Humans have achieved a dubious first by killing off every member of a species of cetaceans, the group that includes whales, dolphins and porpoises, a scientific paper says.

An intensive six-week hunt for the Yangtze River dolphin, or baiji, in late 2006, covering the complete historical range of the dolphin in the main Yangtze channel, "failed to find any evidence that the species survives," the researchers reported Wednesday in Biology Letters.

"We are forced to conclude that the baiji is now likely to be extinct," probably because fishermen caught them accidentally along with other species. Pollution and collisions with ships may also have contributed to the dolphin's extinction.

"This represents the first global extinction of a large vertebrate for over 50 years, only the fourth disappearance of an entire mammal family since AD 1500, and the first cetacean species to be driven to extinction by human activity."

The dolphin, 20 million years old, was one of the world's oldest animals. It was called the "goddess of the Yangtze" in China.

While the result was expected and had been telegraphed by reports from the survey last year, the publication confirms the extinction.

The baiji — Lipotes vexillifer — has been recognized as endangered for many years. From as many as 400 in the 1980s, its numbers fell to 13 in a 1997 survey.

The population of the Yangtze finless porpoise is also falling, to fewer than 400, the researchers reported.

The researchers travelled nearly 3,500 kilometres on two research boats from near the Three Gorges Dam to the river delta and back, using high-performance optical instruments and underwater microphones in the hunt.

The research was done under the direction of the Institute for Hydrobiology Wuhan and the Swiss-based baiji.org Foundation.

The baiji joins the Caribbean monk seal on the list of animals made extinct in recent years. The last sighting of a monk seal was in 1952.