NEW DELHI (Reuters) - Japanese technology to track and monitor the behavior of India's endangered Ganges River Dolphins using underwater acoustics will play a vital role in efforts to conserve the freshwater mammals, the WWF-India said. The Gangetic cetaceans are one of only four species of dolphins in the world which inhabit rivers and lakes and are much less common than their marine counterparts, numbering only around 2,000 in India, according to the conservation group.Over the last 25 years, their numbers have halved -- trapped in fishing nets, hunted for oil, dead from pollution or the construction of dams along the Ganges river which stretches from the northern Himalayas to India's east.WWF-India said researchers from the University of Tokyo have developed an underwater acoustic device or hydrophone which measures the individual sonar pulses or the clicking sounds emitted by the dolphins which are as unique as fingerprints.>From this, they can locate the dolphin from up to one kilometer (0.6 miles) away and track its movements. "The problem in studying the dolphins is that they are under the water and it is very difficult to monitor or identify them," Sandeep Behara, from WWF-India's Freshwater and Wetlands Programme told a news conference late on Thursday."Now we can identify their individual clicks and get an idea of populations, analyze their behavioral and migration patterns and feeding habits ... this will help in conserving the animals," he said.The dolphins are blind and emit sonar pulses in order to navigate and find their prey along the Ganges and Brahmaputra river system, which stretches across 6,000 km (3,730 miles) and spills into the Bay of Bengal.Tamaki Ura, head of the Underwater Technology Research Center at Tokyo University, said the hydrophones would not harm the mammals."The hydrophones are totally passive sonar instruments which do not make any sound and do not impact the dolphins," said Ura. WWF-India said the findings of such research will broaden knowledge of conservationists about the river dolphins and help focus initiatives aimed at saving the threatened mammals.