Underwater behavioral study of Ganges river dolphins by using echolocation clicks recorded by 6-hydrophone array system

Targeting the 9 Ganges river dolphins inhabiting the Narora area in Ganges river system, the first recordings using an compact acoustic observation system, which consists of a high frequency 6-hydorophone cross array connected with a PC on land-base by a wireless LAN system have been carried out during 17 to 21 February 2008. The system also uses the prototype of a GPRS modem-based real-time data transmission system to share the processed data with the world over the internet. The array structure consists of a horizontal cross array formed by 2 arms each of 1.6 meter length with a hydrophone on each corner and at the intersection forming two equi-spaced 3-hydrophone linear SBL system for omni-directional horizontal coverage, and one additional hydrophone which is fixed 0.8 meter below the center hydrophone of the cross array also forming an equi-spaced triangular SBL system for vertical measurement. The clicks of the dolphins received on each hydrophone were sampled at a rate of 500 kHz per channel. Total recording was done for approximately 40 hours due to power supply interruptions. However, the GPRS modem-based real-time data transmission system worked well during the recordings. The results of the basic analysis of the data show that the dolphins are found swimming up and down the river. The results are in accordance with the visual observation results. Underwater behavior of the dolphins approaching near to the array, which could not otherwise have been observed by visual survey, has been mainly analyzed in this paper. Based on ICI during this period of approaching to the array, we estimated ldquowhat this dolphin is seeingrdquo or interested in. We invoked the idea of lag time that the dolphin requires to perform echo-location process and to emit the next click to the target [7]. Defining the lag time as DeltaP (pre-click time) and assuming that DeltaP is constant, DeltaP and distance D to the target where the dolphin is looking at is calculated by using ICI and range between the dolphin and the array, which is measured by the array. Utilizing available on-axis clicks from the data of the preliminary experiments in Budhabalanga river, the value of DeltaP is calculated. Constant DeltaP is estimated as 14.3 milliseconds. To determine D of the Narora data, we applied it when the dolphin was approaching to the array within 20 meters. The results show that the dolphin does not look at any object beyond 20 meters range. On the other hand, when ICI reduced below the constant DeltaP (14.3 milliseconds) , it is estimated that the dolphin is in a different stage of the echo-location process.