INVENTIVE COLLABORATION—Engineer Mark Johnson (right) and biologist Peter Tyack work together to learn about whale behavior, using Johnson's D-tag to record whale movements, depth, and sounds on dives. Back in the lab, D-tag data tell the story of the whales' dives, from their swimming behavior to the kinds of vocalizations they use while foraging. (Photo by Tom Kleindinst, WHOI)
ALONG FOR THE RIDE—D-tags were placed on deep-diving pilot whales in waters off Tenerife, Canary Islands, in a collaborative project with the University of La Laguna, Canary Islands, to study the whales' behavior during dives and in response to ferries and whale-watching boats. (Photo courtesy of the University of La Laguna, Canary Islands)
By Mark P. Johnson, Research Engineer
Applied Ocean Physics & Engineering Dept.
Woods Hole Oceanographic Institution
The challenge of designing a device to learn what marine mammals do
on dives is the stuff of dreams for an electronics engineer.
In the
spring of 1999, the time was right to build the digital acoustic
recording tag, or D-tag—an instrument to record the movements of whales
and the sounds they make and hear in the ocean. Miniature cell phones,
MP3 players, and PDAs had created a demand for small, lightweight,
dense memory components and batteries. In many ways, the tag is just
like an MP3 player, PDA, and home medical monitor all rolled into one,
and then sealed against seawater and the pressure in the deep ocean.
Helped
by electronics engineers Tom Hurst and Jim Partan at WHOI, and a young
summer student studying mechanical engineering, Alex Shorter, we put
together the first D-tag in record time. Driving us was an opportunity
in the summer of 1999 to use the tag on endangered North Atlantic right
whales, as part of an effort to understand why they are hit by ships
all too often. (See Whither the North Atlantic Right Whale?)
A small, chock-filled package
The D-tag is actually a miniature computer with its own microprocessor,
memory, and software. It records sound using one or two hydrophones
(underwater microphones) with better-than-CD quality—not only the
sounds made by the tagged whale but also sounds from other whales,
noises from boats, and all of the sonars and sound sources in the area.
The tag also contains a digital compass, a pressure sensor (the
underwater equivalent of an altimeter) to measure the depths of the
whales' dives, and an orientation sensor that measures the animal’s
pitch and roll. The pitch sensor records a whale’s body undulations
fast enough for us to count each beat of its tail fluke.
Think
of the displays in the cockpit of a small plane: The tag sensors are
measuring similar things but under water. Everything gets stored in
digital form. The tag has as much as 6 gigabytes of memory, enough to
record continuously for a full day.
Putting it on, getting it back
To keep out the saltwater and survive harsh treatment from socializing
animals, the tag has a plastic skeleton and is sealed inside a thick
urethane bag. To keep the weight and size down, the tag does not have a
pressure housing (the aluminium bottle normally used to protect
electronics from high pressure in the deep ocean). Instead, we spent a
lot of time at the WHOI pressure-testing facility choosing electronic
components that would withstand pressures of up to 3,000 pounds per
square inch—that’s 200 times atmospheric pressure at sea level. As a
result, the entire electronics unit measures about 4 by 1.5 by 1 inches
and weighs about 5 ounces—no problem for even a small whale to carry.
To
allow us to retrieve the tag after it comes off the whale, it is
equipped with flotation so that it rides atop the surface like a buoy,
and a tiny radio beacon, so we can find it by tracking its radio signal.
Of
course, the tag is no use at all if it doesn’t stick to the whale, and
so we have spent a lot of time studying suction cups. For three years,
we tested every suction cup we could find to figure out which would
hold best. Finally, we decided that we had to build the cups ourselves
to get the right mix of strength and softness—o be tenacious and yet
not hurt the whale. Using a mold built by the WHOI shop, we now make
cups out of medical-grade silicone that work incredibly well.
New heights (and depths) for D-tag
That first caffeine-powered field season in 1999—working with northern
right whales in the Bay of Fundy with the International Fund for Animal
Welfare—was just the start. Since then, the tag has gone on more than
30 field expeditions all over the world.
We
have worked with D-tags on sperm whales in the Gulf of Mexico and
Italy, on manatees in Belize, on narwhals in northern Canada, on beaked
and pilot whales off the Canary Islands, and on humpbacks off Australia
and Cape Cod. Colleagues have taken the tags to Antarctic islands to
study fur seals and to California and Canada to work on blue whales and
gray whales.
D-tags have gone on the deepest
dives ever recorded on a marine mammal and have discovered the sounds
made by two of the world’s most mysterious whales: Cuvier’s and
Blainville’s whales are little-known mid-sized beaked whales whose only
claim to public attention is their occasional mass strandings
associated with sonar use during naval maneuvers. Many marine
mammalogists have never seen these whales alive! They are very shy and
usually live way out in the big blue. They are so inconspicuous at the
surface that you can sail right by them unless the sea is flat and you
know what to look for.
We have learned that
these whales are incredible divers. Using D-tags, we have recorded
dives 85 minutes long with depths of up to 1,900 meters. Amazingly, the
tag is sensitive enough to hear echoes from objects in the water,
insonified (lit up—but with sound) by the click sounds made by the
beaked whales.
An increasingly noisy ocean
Marine mammals remain one of the least understood groups of animals in
the world. D-tags allow us to explore the world the way marine mammals
do: with sound. (See Run Deep, but Not Silent)
Meanwhile,
human noise in the oceans is increasing by the decade as more and
faster ships are made, as oil exploration moves into deep water, and as
more navy ships with high-power sonars patrol for submarines. There are
ample signs that these noises can disrupt marine mammals, even causing
mass strandings and death. But without a more complete understanding of
how whales use and sense sound, we cannot begin to figure out which
noises are problematic and at what levels. (See How to See What Whales Hear)
My
hope is that this device will eventually help us learn how to be better
neighbors under water.
