The baiji (Lipotes vexillifer), is a probably extinct species of freshwater dolphin native to the Yangtze river system in China.

This dolphin is listed as "critically endangered: possibly extinct" by the IUCN, has not been seen in 20 years, and several surveys of the Yangtze have failed to find it.

The baiji population declined drastically in decades as China industrialized and made heavy use of the river for fishing, transportation, and hydroelectricity.

[7] The baiji represents the first documented global extinction of an aquatic "megafaunal" vertebrate in over 50 years[8] since the demise of the Japanese sea lion (Zalophus japonicus) and the Caribbean monk seal (Neomonachus tropicalis) in the 1950s.

Swiss economist and CEO of the baiji.org Foundation August Pfluger funded an expedition in which an international team, taken in part from the National Oceanic and Atmospheric Administration and the Fisheries Research Agency in Japan, searched for six weeks for signs of the dolphin.

This had been reduced by several hundred kilometres both upstream and downstream, and was limited to the main channel of the Yangtze, principally the middle reaches between the two large tributary lakes, Dongting and Poyang.

[22] The aquatic adaptations of the baiji and other cetaceans have happened slowly and can be linked to positively selected genes (PSGs) and/or other functional changes.

This information leads scientists to conclude that a bottleneck must have occurred near the end of the last deglaciation, a time that coincided with rapid temperature decrease and a rise in eustatic sea level.

When analyzing the baiji genome, scientists have found that there are four genes that have lost their function due to a frameshift mutation or premature stop codons.

[24] The baiji has been featured in pop culture and media on many occasions due to its status as a Chinese national treasure and as a critically endangered species.

[30] During the Great Leap Forward, when traditional veneration of the baiji was denounced, it was hunted for its flesh and skin, and quickly became scarce.

[31] A range of anthropogenic led causes (e.g. boat collisions, dam construction) which also threaten freshwater cetaceans in other river systems, have been implicated in the decline of the baiji population.

Unlike most historical-era extinctions of large-bodied animals, the baiji was the victim not of active persecution but of incidental mortality resulting from massive-scale human environmental impacts, primarily uncontrolled fishing.

The building of the Three Gorges Dam further reduced the dolphin's habitat and facilitated an increase in ship traffic; these were thought to make it extinct in the wild.

Since the Baiji has a limited distribution endemic to the Yangtze River, the freshwater environment may have a higher pathogen level than marine waters (although systematic environmental studies have yet to be conducted).

The pathogens in these waters could lead to viral infections that can result in epizootics, which has caused the deaths of thousands of marine mammals over the last 20 years.

There have also been captured/killed individuals that have had helminth infestations in the stomach which leads scientists to believe that parasitic infections could be another cause of decline amongst the Baiji.

In 1978, the Chinese Academy of Sciences established the Freshwater Dolphin Research Centre as a branch of the Wuhan Institute of Hydrobiology.

[2] The first Chinese aquatic species protection organisation, the Baiji Dolphin Conservation Foundation of Wuhan (武汉白鱀豚保护基金), was founded in December 1996.

It has raised 1,383,924.35 CNY (about US$100,000) and used the funds for in vitro cell preservation and to maintain the baiji facilities, including the Shishou Sanctuary that was flooded in 1998.

Four were built in the main Yangtze channel where baiji are actively protected and fishing is banned: two national reserves (Shishou City and Xin-Luo) and two provincial (Tongling and Zhenjiang).

Through careful management, both these species not only survived, but did in fact reproduce successfully enough to provide some hope that the Baiji may be able to make a comeback.

August Pfluger, chief executive of the Baiji.org Foundation, said, "The strategy of the Chinese government was a good one, but we didn't have time to put it into action.

The ex-situ projects aimed to raise a large enough population over time so that some, if not all, of the dolphins could be returned to the Yangtze, so the habitat within the river had to be maintained anyway.

The site included an indoor and outdoor holding pool, a water filtration system, food storage and preparation facilities, research labs, and a small museum.

[50][51] Douglas Adams and Mark Carwardine documented their encounters with the endangered animals on their conservation travels for the BBC programme Last Chance to See.

A sexually mature female was captured in late 1995, but died after half a year in 1996 when the Tian-e-Zhou Oxbow Nature Reserve, which had contained only finless porpoises since 1990, was flooded.

Poor water and weather conditions may have prevented sightings,[7] but expedition leaders declared it "functionally extinct" on December 13, 2006, as fewer are likely to be alive than are needed to propagate the species.

[55] "Witness to Extinction: How We Failed To Save The Yangtze River Dolphin", an account of the 2006 baiji survey by Samuel Turvey, the lead author of the Biology Letters paper, was published by Oxford University Press in autumn 2008.

Turvey instead proposed to shift conservation focus to the critically endangered Yangtze finless porpoise, the only freshwater cetaceans left in China.