as china developed economically, pressure on the baiji river dolphin grew significantly. industrial and residential waste flowed into the yangtze. the riverbed was dredged and reinforced with concrete in many locations. ship traffic multiplied, boats grew in size, and fishermen employed wider and more lethal nets. noise pollution caused the nearly blind animal to collide with propellers. stocks of the dolphin's prey declined drastically in the late 20th century, with some fish populations declining to one thousandth of their pre-industrial levels. in the 1950s, the population was estimated at 6,000 animals, but declined rapidly over the subsequent five decades. only a few hundred were left by 1970. then the number dropped down to 400 by the 1980s and then to 13 in 1997 when a full-fledged search was conducted. on december 13, 2006, the baiji ("lipotes vexillifer") was declared "functionally extinct", after a 45-day search by leading experts in the field failed to find a single specimen. the last verified sighting was in september 2004.