A very large male Sperm Whale drifts towards to Odyssey.
Photo: Chris Johnson
December 22, 2000
How We Find and Track Sperm Whales
This is Roger Payne on board the Research Vessel Odyssey. One of the most unique aspects of this vessel is the acoustic system it carries which enables us to locate the direction to sperm whales and thereby follow them 24 hours a day.
To find and track sperm whales we use a hydrophone array (two underwater microphones housed in a 30 foot, oil-filled tube and towed behind the ship from a 100 meter-long cable).
Alone, this acoustic array gives us very good stereo sound from the sea surrounding the boat. Many sounds, like the clicks of sperm whales can be heard through headphones from distances of 5 or even 10 miles away (distances we have determined through our experience of the past few months).
What the array hears is sent to a computerized analysis program called "Rainbow Click" (designed by Douglas Gillespie and Jonathan Gordon of the International Fund for Animal Welfare and lent to us for this expedition). One of the most useful features of Rainbow Click is that it dissects whale sounds out of ocean noise and displays them cleanly on a computer screen, using specific colors to indicate the sounds from specific animals (all pink sounds are from one whale, all green sounds from another, etc.).
The array is deployed at all times day and night and what it hears is constantly broadcast through loudspeakers into the pilot house. With the Odyssey pulling the array taut behind it, Rainbow Click can not tell which hydrophone got hit by which whale sound first, and how long it was before the second hydrophone heard the same sound. (It can also detect and discards the ocean noise.)
If the rearmost hydrophone heard a whale first the whale must have been somewhere behind the boat. If the forward?most hydrophone heard it first, then somewhere ahead. If both hydrophones heard a sound at the same time the whale was off to one side (though Rainbow Click can't tell us which side). The computer uses the delay between the two hydrophones to calculate what angle the sound source must have been at in order to have produced that delay. The helmsperson can figure out whether the whale is to the left or right by turning the boat to either side and noting whether the sound source goes more ahead of the boat (indicating that the whale was on that side and we have turned correctly towards it) or more behind it (indicating that the whale was on the other side of the boat and we turned away from it).
Finally we can also make a rough estimate of how far away the whale is. We do this by noting how loud the sound is-information that Rainbow Click also displays for each sound.
With a lot of experience, and a lot of interpretation, this system allows the helmsperson to deduce a great deal about what is going on around the ship-what groups are hanging out together, what groups are diving at the moment and which coming up. In this way, even though they are invisible below the surface we can often track several sperm whales in a given area simultaneously and see roughly where each group is. We have found that when their clicks cease, the whales are often on their way to the surface (and presumably no longer hunting for food). The helmsperson relays that message throughout the boat along with the direction in which the whales will probably appear. Those on watch rededicate themselves to scanning the sea for blows, while the biopsy dart person makes their way out along the long side boom to the chair on the end, and the science team take up their stations and prepare to collect data.
I am proud to say that the crew has become so proficient in using this system that if we can hear but 2 or 3 clicks of a sperm whale we can usually get the boat headed in the right direction, improve our signal and eventually find the individual we first heard. And when we have done so, we usually find the entire herd with which it is traveling.
Log by Roger Payne