Acoustic Survey of Irrawaddy Dolphins in Chilika

January 2006


A. Motivation

The Irrawaddy dolphin being the only known marine mammal in Chilika lagoon, it is also on top of the food chain. The overall health of the lagoon's ecosystem is indicated by the status of these dolphins. Thus, if the numbers of these dolphins are growing then it can be said that the lagoon's ecosystem is thriving. In view of the on-going efforts of Chilika Development Authority (CDA) to improve the ecosystem, use of advanced and automated technology that can assist in the survey of the dolphins will be a major contribution.
It is proposed to bring new acoustic technology for observing the behaviour and ecosystem of the Irrawaddy dolphins.

B. Survey Methods

Visual observations have traditionally been used for survey of endangered marine mammals including small cetaceans. This is a tedious process of observation that depends on visibility and physical proximity to the animals. In addition, being a manual method, it is prone to inaccuracies. It is all the more challenging for surveying animals that spend most of their time underwater. Small cetaceans such as dolphins and porpoises emit sonar pulses that they use as an echolocation tool for finding prey and for navigation. These sound pulses appear as “clicks” of specified signal duration and bandwidth to an observer. These clicks can be observed with specially designed underwater acoustic devices called hydrophones. The clicks are usually emitted in a confined beam. It is proposed to use modern acoustic-based methods for survey of endangered cetaceans. These passive observation methods can also be automated that allow day-night observation in turbid waters without causing any disturbance to the animals.

C. Acoustic observation of a single Chinese River Dolphin

Acoustic observations have previously been carried out on certain dolphins. We report here results of some of these studies that are relevant for the survey of Irrawaddy dolphins. The Chinese river dolphin “baiji” uses its sonar for finding prey. Its sonar clicks have been observed at a hydrophone and have been recorded in an ox-bow lake in China. This animal frequently dives for up to 3 minutes in search of prey. A typical animation sequence of its underwater movement is shown in figure 1. It also shows the recording system using a single hydrophone lowered from a boat. The sonar beam of the dolphin is forward looking and its clicks are picked up by the hydrophone whenever it is facing the boat.


Figure 1   

Passive observation of the sonar clicks can greatly help in understanding underwater behaviour of these dolphins. A typical series of clicks recorded for the Chinese river dolphin “baiji” is shown in the figure 2 below. The click trains appear in bursts due to the nature of its underwater movement.


Figure 2   

Since the recorded clicks with this simple system included the directly received click and its surface reflected echo, we could estimate the time delay between them as a measure of its dive depth. A plot of baiji’s time delay for an observation period of 24 minutes revealed its underwater dive behaviour as shown in the figure 3 below.


Figure 3   

This work demonstrates the possibilities of underwater observation using hydrophone technology.


D. Multi-animal Acoustic Observation System

A more sophisticated hydrophone array system for precise monitoring of individual animals in a group of dolphins has recently been proposed. The system shown in figure 4 consists of an array of hydrophones vertically separated by 1.6 metres. The central portion has three hydrophones. The hydrophone signals are amplified and digitally recorded in a PC-based recording and track analysis system.


Figure 4   

The five hydrophones have the capability to accurately resolve several cetaceans at the same time. Such a system has been designed and tested on bottlenose dolphins and finless porpoises in Japan. The system has now been modified to be deployed horizontally for shallow water operations in Chilika.  

 
A picture of the hydrophone array being assembled for trials is shown in figure 5, with one of the hydrophones clearly indicated.
Figure 5   

Results of tracking several dolphins (in different colours) are shown in figure 6. The dolphin click trains are seen in lower left. On the right hand side plots we can clearly see the individual dolphins resolved in depth as well as in the horizontal plane.


Figure 6   

However, due to likely operational constraints of deploying a horizontal array in shallow water in Chilika, depth information of dolphins is difficult to obtain. The dolphins can be resolved in the horizontal plane.

E. First Trials in Chilika in January 2006

This will be the first time that Irrawaddy dolphins in Chilika will be acoustically observed and surveyed using these technologies. The Japanese team consists of:

  1. Professor Tamaki Ura, University of Tokyo, Japan
  2. Dr. Tomonari Akamatsu
  3. Ms. Harumi Sugimatsu
  4. Mr. Junichi Kojima
  5. Mr. Hideyuki Takahashi
  6. Mr. Takashi Sakamaki
  7. Mr. Tomoki Inoue
    In addition, the following experts will also participate:
  8. Professor R. Bahl, IIT Delhi
  9. Dr. Sandeep Behera, WWF_India, New Delhi


The team will bring two survey technologies:
  1. It is proposed to use the 5-hydrophone system for initiating underwater observation of Irrawaddy dolphins in Chilika. It is expected that movements of several dolphins in a group can be independently monitored.
  2. A miniature acoustic data logger 12cm in length and 2cm in diameter will also be tested. The system is expected to provide not only the migration timing but also the rough number of animals that pass by the data logger system.


These systems will be brought from Japan by the 7-member team led by Professor Tamaki Ura of University of Tokyo. At this time, we would like to introduce the operation of these systems and discuss the application of this new methodology for the conservation of isolated freshwater Irrawaddy dolphins in Chilika lake. The trials will be jointly conducted with the assistance of Chilika Development Authority (CDA), wildlife wing of Orissa Forest Department and WWF-India. The team will share all data regarding the behaviour and movement pattern of the Irrawaddy dolphins with the CDA, wildlife wing of State Forest Department and WWF. Another advantage of this technique is that it may be possible to acoustically recognize other dolphin species that may be present in the vicinity. These trials will also help the team to carry out any further improvements in the methodology.

F. Future Plans

The Chilika experience will be a major first step for introducing and testing a day-night automated acoustic survey system for different species of endangered marine mammals. More ambitious surveys, including the endangered Ganges River dolphin, would be planned jointly with the participating agencies.


Address:URA Laboratory,
	Underwater Technology Research Center,
	Institute of Industrial Science, The University of Tokyo,
	4-6-1, Komaba, Meguro, Tokyo, JAPAN 153-8505
Tamaki URA
E-mail:ura@iis.u-tokyo.ac.jp
Tel: +81-3-5452-6487