The Yangtze river dolphin, until recently one of the most endangered species on the planet, has been declared officially extinct following an intensive survey of its natural habitat.

The freshwater marine mammal, which could grow to eight feet long and weigh up to a quarter of a tonne, is the first large vertebrate forced to extinction by human activity in 50 years, and only the fourth time an entire evolutionary line of mammals has vanished from the face of the Earth since the year 1500.

Conservationists described the extinction as a "shocking tragedy" yesterday, caused not by active persecution but accidentally and carelessly through a combination of factors including unsustainable fishing and mass shipping.

In the 1950s, the Yangtze river and neighbouring watercourses had a population of thousands of freshwater dolphins, also known as Baiji, but their numbers have declined dramatically since China industrialised and transformed the Yangtze into a crowded artery of mass shipping, fishing and power generation. A survey in 1999 estimated the population of river dolphins was close to just 13 animals.

Historically, the species had been revered and achieved nearly demi-god status among fishermen who recounted tales of dolphins being reincarnations of drowned princesses. But in Mao's Great Leap Forward, the overthrowing of idols saw their protection lifted and they were hunted for food and their skin.

Sam Turvey, a conservation biologist at London Zoo, worked with Chinese government scientists to survey the entire 1,669km stretch of the Yangtze river downstream of the giant Three Gorges Dam to Shanghai, a region which has been the natural habitat for river dolphins.

The team scoured the river four times in two boats during the six-week survey, using high-powered binoculars to spot the dolphins. Sensitive hydrophones were towed behind to listen for their calls.

The researchers hoped that if any dolphins were spotted, they could be taken to a reserve in an oxbow lake which was once part of the Yangtze in the hope of boosting their numbers. But at the end of the survey, they had neither seen nor heard any sign of the dolphins, according to their report in the journal Biology Letters.

"We passed through what were supposed to be Baiji hotspots and every day there was nothing. The hopes of each person on the survey died at different points; everyone had a moment of realisation that we weren't going to find anything," said Dr Turvey.

Around half of all river dolphins were killed as a result of indiscriminate and often illegal fishing practices. Though banned for the past 30 years, some ships still drag long lines of unbaited hooks. "They just drift through the water snagging everything. They slash and entangle and suffocate the dolphins," said Dr Turvey. Low employment in the 1980s and 1990s saw a rise in unskilled fishing.

Dr Turvey said conservation organisations had been quick to call for action to protect the river dolphin, but many were too cautious to take meaningful action.

"The loss of such a unique and charismatic species is a shocking tragedy. The Yangtze river dolphin was a remarkable mammal that separated from all other species over 20m years ago. This extinction represents the disappearance of a complete branch of the evolutionary tree of life," Dr Turvey added.

Gallery: Yangtze river dolphin