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For targeting 13 Ganges river dolphins which includes calves from Karnavas to Narora, the world's first long-term real-time monitoring using the integrated observation system that can record the high frequency bio-sonar clicks from the Ganges river dolphins to estimate their 3-D location had been conducted continuously for nearly 6 months from 21st November 2008 to 28th April 2009. The dolphins' realtime underwater 2-D movements around the observation spot could be monitored anywhere in the world over the internet with an appropriate viewer program through the host server during the monitoring period. From the analysis of the data of early monitoring period and the visual survey by WWF India, the dolphins' behavior around the observation spot is classified into two types, i.e. staying and moving types. Based on the data, it is estimated that the area 40?20 meters downstream from the observation spot is utilized by the dolphins as a calf-rearing place. Calves' behavior can be also classified into staying type while the adults are moving type. It is estimated that the calves mostly stay at the area and adults periodically come to take care of them during the calf-rearing period. It suggests that adults do not always swim together with their calves but keep them at the selected area for a definite period of their calf-rearing. Based on three parameters of clicks number, average XY position of the dolphins and its standard deviation during the entire monitoring period, it is estimated that the migration of the dolphins from this site started from 24th March 2009, when the click number decreased drastically. This shows that the dolphins' behavior changes from staying type to moving type. It also indicates that the calves staying around the area also have started migration. However, even after changing the mode, they sometimes returned to the area utilized as a selected calf-rearing place. It is, thus, estimated that the area has been not only utilized as a selected - calf-rearing place for a definite period of their calf-rearing, but also has a role as one of the sustainable feeding source and safety shelter for the dolphins during their migration period.