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in-depth: satellite tagging tech
 
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In-depth: Satellite Tagging Technology

Tag, You're It! Tracking the Gray Whale Journey
by Robin Marks

How would you like to get an email from a whale? "Hi, it's whale 5032 here! So far today, I swam another 20 miles (it's still early). I'm still going south, but veering a bit to the east - maybe because the water's a little cooler that way. More later!"

close-up of tag attached to surfacing whale
Each tag transmits information to satellites about time, date, latitude, longitude, dive depth and more. (Photo credit: Matt Ferraro)
Click to enlarge

Some researchers working with whales that have been tagged with radio transmitters receive many emails with this kind of information each day. The whale itself isn't sending the email, of course (and the messages don't really say "Hi" or "More later"). The message is being sent by a satellite tag attached to the whale. Each time the whale surfaces for air, the tag sends a signal to GPS satellites orbiting the earth. Using the changing latitude and longitude data, whale researchers can plot the course of an individual animal along its migration route.

Blair Mott holds antenna
By turning on the tags only when the animal surfaces, saltwater sensors enable the transmitter batteries to last for several years.
Click to enlarge

Tags on even a small number of animals can provide a great deal of information. Scientists can get a closer view of whale behavior and can see whether they alter their route to avoid man-made disruptions, like sonar and whale-watching expeditions. Tags on a variety of marine mammals, especially seals, can also send back information about the oceanic environment, such as surface water temperature and the density of chlorophyll. These kinds of data would be difficult to collect with such frequency and over so much area in any other way. Because different animals follow different paths through the ocean, combining data gathered by tagging many different species gives researchers a broad view of ocean conditions. Ultimately, this information can give scientists clues about changes that may affect entire ecosystems as well as the global climate.

Tiny Transmitters Send to a Global System

Scientists have been tagging animals to follow their movements since the 1970s, but since the mid-1990s, advances in technology have made the tags much smaller and more powerful. Each tag contains a tiny radio transmitter, a battery and an antenna. The signal coming from the transmitter is too weak to travel through water, so to increase battery life, the tag has a saltwater sensor that turns it on only when the animal surfaces. This makes marine mammals ideal for studying conditions over long distances and periods of time because the demand on the batteries is so little per day that the transmitters can keep running for several years.

Hey, Where Ya Goin'?

Thanks to satellite tracking and the Web, you can follow the paths of tagged marine animals as they make their migrations. The Tagging of Pacific Pelagics program tags fish, mammals and reptiles, including blue whales, sea lions, sharks, turtles and sunfish. See a map of close to real-time data for all the animals, follow one species or even see the path of just one animal.

Tagging of Pacific Pelagics Web Site (at las.pfeg.noaa.gov)

Timing the signal to the right satellite that can then relay the data to researchers is a bit of a game of chance. Each time a tagged whale (or other marine mammal) surfaces, the transmitter sends a signal skyward. It also stores the data for the next four to six hours so that information from each surfacing within that time period can be sent with each transmission. In order for that signal to be received, the satellite that the signal is "talking to" must be overhead. The number of passes a satellite makes over the animal each day depends on where the animal is. In any given 24-hour period, there are a limited number of chances for the whale and the satellite to line up, sometimes 10, sometimes one, and some days there aren't any chances at all. In addition, at least three data points are needed in order to get an accurate location for the whale. Transmission begins as soon as the whale surfaces and happens quickly, so several hours' worth of data are often sent, providing plenty of information.

Developing an Attachment

The task of getting a tag securely affixed to a whale has its own set of challenges. Taggers go out to sea in small boats and wait for the whales to surface. They attach a dart to the tag and load the package into a compound crossbow. When the whale comes up to breathe, the tagger aims the dart at the top of the head. Ideally, it will be inserted just behind the blowhole, about 5 inches into the blubber layer. As far as researchers can tell, the whales have enough blubber to absorb the transmitter without feeling pain.

Dr. Jef Goodyear prepares to tag a whale
To avoid the risk of injury to the whales, Jean-Michel's team uses a barbed tag designed to penetrate only the skin and blubber layers.
Click to enlarge

The initial attachment of the tag is only the first obstacle to overcome. Once the dart has been embedded, scientists are at the mercy of the whale's behavior and mannerisms. Any number of events can unseat the transmitter: Calves basking on their mother's backs can knock tags off, and a male whale may lose his while wrangling with another male over a female during mating season. And all bottom-feeding whales, like grays, grind their bodies around in the sand of the ocean floor, tilting and twisting and potentially dislodging the tag. Why? Some believe this behavior allows the whales to slough off dead surface skin. In addition, some whales may have an immune response, working their tags out of their bodies in much the same way we humans lose slivers stuck in our fingers.

In the end, many of the tags put on whales stop transmitting within a few days, but about 20 percent keep working for many months or even a few years. So if 40 whales are tagged, researchers might expect to get long-term data from about eight of them. That may sound like a small number, but even those few provide a tremendous amount of insight that can promote the recovery of endangered species and protect the habitats of those that are flourishing.

Sources:

toppcensus.org: Tagging of Pacific Pelagics

whale.wheelock.edu: "How Satellite Tags Work," WhaleNet

argosinc.com: Service Argos, Inc., a Global Data Telemetry and Geo-Positioning Services Company for North America

currents.ucsc.edu: "High-Tech Tags on Marine Animals Yield Valuable Data for Biologists and Oceanographers," U.C. Santa Cruz, Currents Online