Long duration real-time observation of Irrawaddy dolphins in Chilika lagoon

The authors have developed a passive acoustic observation system to monitor Irrawaddy dolphins (Orcaella brevirostris) in Chilika lagoon. The system is designed to monitor the detailed underwater behavior of dolphins using a Short Base Line (SBL) system all through the day and night. The underwater sounds are recorded at a sampling frequency of 500 kHz with 16 bits resolution. This system was deployed in Chilika lagoon from 29th January to 3rd February and on 18th February in 2007. Experiments were carried out all through the night. By replacing the batteries and hard disks, the observation system was made to keep running all the time. Consequently, the authors succeeded in obtaining click data from Irrawaddy dolphins for a continuous duration of about 21-hour consecutively for each trial and about 1TB of acoustic data was obtained in total. The data contained many high frequency signals, which are considered to be the snapping noise made by shrimp, which are typically found in warm shallow water areas such as Chilika lagoon. A de-noise calculation is developed and applied to the obtained data. A clear picture of trajectories of dolphins in the turbid environment of the Chilika lagoon can be observed that has never before been seen. In this paper, detailed ideas and descriptions of the set up and deployment of the system are presented. Furthermore, the de-noise calculation and the observed dolphin trajectories will be discussed.