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DeRuiter used her new tiny tag on captive harbor porpoises in the Fjord & Baelt Center in Kerteminde, Denmark. With the help of Chief Scientist Magnus Wahlberg and the center's porpoise trainers, she tagged porpoises and gathered audio data on how they used sound to forage for fish, including recording behaviors scientists had not seen before in animals that use sonar. (Photo courtesy of Alexander Bahr, MIT WHOI Joint Program) [ Hide caption ]

A Tag Fit for a Porpoise

Grad students surmount big hurdles to build a device for a small marine mammal

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In 2003, Stacy DeRuiter arrived as a graduate student at Woods Hole Oceanographic Institution (WHOI), where a new device developed at WHOI was sparking a revolution in marine mammal research: the D-tag. The cell phone-size digital recording device—affixed temporarily (and non-invasively) to large whales—has given scientists the ability for the first time to track whale movements in the deep sea and record the sounds the animals make and hear.

But DeRuiter was eager to take the next step.

She wanted to use the device to study some of the smallest cetaceans of all, harbor porpoises. She yearned to unveil whether they used sound differently than whales to communicate, navigate, and hunt in shallower coastal waters. She wondered if they were finding it harder to negotiate waters that had become so much “noisier” in recent decades because of increased ship traffic, sonar use, and air guns for oil exploration. She dreamed that a greater understanding of how porpoises use sound could help find ways to balance the needs of people and porpoises in coastal waters.

That next step, however, turned out to be a doozy.

Evolution of new tag

First, DeRuiter consulted with the D-tag’s inventor, WHOI engineer Mark Johnson. Together, they agreed that the D-tag was too big for more diminutive marine mammals and would change the way the animals behaved. Moreover, the D-tags’ electronics were tuned for the sounds made by whales, which are far lower in frequency than those used by porpoises. All the obstacles added up to a problem that seemed too hard for a graduate student to tackle.

She cried. She stayed up nights. DeRuiter described the entire period as one of the most frustrating times of her life.

“I was discouraged, but not out,” she said. “I saw it as a project that would have to wait some more years, until electronics shrank some more and someone came along to build a smaller tag that could be used with porpoises.”

Then DeRuiter asked herself: Why not try to make the tag smaller herself? She enrolled in a class at the Massachusetts Institute of Techonology’s (MIT) media lab called “How to Make Almost Anything”  in an effort to learn the skills required to build her own tag.

“I had just given in to the realization that, given my slow progress as an engineer, it was going to take me 10 years or so to build anything that was remotely functional, and was about to give up,” she said.

Then, at a retreat in 2006 for students in the MIT/WHOI Joint Program in Oceanography, she met fellow student Alexander Bahr and told him about her efforts shrink the D-tag. Bahr, who specializes in navigation systems for autonomous underwater vehicles, was intrigued and offered to lend his electrical engineering talents to the mix. A winning partnership was born.

“The main reason why I was willing to commit myself to this project,” Bahr said, “was that I saw how serious Stacy was about it and that she was willing to take a number of very challenging electrical engineering classes to solve her problem. Over the time we worked on this project, she impressed me several times with her detailed knowledge and that she able was to follow my explanation of the technical details and the problems I encountered along the way. All this was only possible because she acquired an impressive amount of engineering knowledge long before I joined the project.”

With Johnson’s blessing and the blueprints of the original D-tag, DeRuiter and Bahr spent the next few months experimenting to create a porpoise-friendly version with seed funds from the WHOI Ocean Life Institute. In labs at both Woods Hole and MIT, they spent painstaking hours redesigning a 2-centimeter audio processing board that required very little battery power to capture high-frequency porpoise clicks.

Sticking to it to make it stick

Next, the tandem tackled another unanticipated challenge: getting the smaller tag to stick. The original D-tag sticks to the slippery skin of large whales with espresso-saucer-size suction cups. But DeRuiter and Bahr had to devise something smaller and less stiff to work on smaller porpoises.

They tried everything—commercial suction cups, medical suction cups, even the suction cups from the bottom of standard rubber bath mats. When none of these worked, Bahr and DeRuiter modified and shrank the custom design of the original D-tag’s cups, drew up the plans on a three-dimensional printer at MIT, and built a mold.

The final key was finding the right material for the cups. After experimentation, they found a soft silicone material that was sufficiently flexible and used it and their mold to cast their own suction cups.

“This was a case where the Joint Program worked at its best,” said Bahr, who hails from Saarbrucken, Germany. “Individually, neither of us could have accomplished these goals, but together, we did something fantastic.”

DeRuiter’s Ph.D. advisor, WHOI biologist Peter Tyack, had his own perspective on the project and the Joint Program: “Students help catalyze the magic between scientists and engineers.”

Ready for their first test

To keep the new tags small, DeRuiter eliminated the VHF radio tracking beacon and flotation equipment. Without these features, there would be no way to retrieve the devices once they fell off porpoises, so applying tags to wild animals in the open ocean was too risky.

What’s more, DeRuiter recognized that affixing a tag to a wild animal would be “like giving the porpoise a big heavy backpack” to carry, she said. “At best, it would have been highly likely to cause large behavior changes in response to the tag. At worst, it might have affected the animal’s health or susceptibility to predators.”

Instead, DeRuiter decided to study captive animals in an enclosed habitat: the Fjord & Baelt Center in Kerteminde, Denmark. There, in 2007, Chief Scientist Magnus Wahlberg welcomed DeRuiter and happily allowed her to use the new D-tags to conduct research on the facility’s three harbor porpoises (one of the animals has since given birth).

Most of this research was conducted during training sessions, when trainers fed the animals dead herring, capelin, and other small schooling fish. Over the course of three months, these new tags showed that they could be used to collect data on how porpoises use sound to locate and range prey.

DeRuiter gathered evidence, for example, that while porpoises searched for prey, they produced periodic echolocation clicks, but then they clicked faster (in the form of a buzz) as they approached each individual fish. The animals continued buzzing even after they secured each fish in their mouths—a behavior that no scientists have witnessed in other animals that rely on sonar to hunt.

While it’s still too early to explain these behaviors, DeRuiter has a number of hypotheses.

“In the wild, the porpoises would be hunting for live fish instead of dead fish, and it’s possible that a fish might escape after the first time they have it in their mouth,” she said. “A long buzz also may be useful when porpoises are hunting schooling prey.”

Even this glimpse into the sound world of porpoises demonstrates the tantalizing potential to use D-tags to open the window wider and learn more about how porpoises operate in their increasingly noisy environment.

Off to France

DeRuiter, who earned her Ph.D. in September 2008, said she will continue to analyze her audio data over the next few years and try to answer these and other questions about how porpoises use sound to forage and capture prey. More immediately, however, she has an even bigger task: studying the potential effects of air gun array operations on whales and other marine mammals as part of a one-year postdoctoral research fellowship at Institut Français de Recherche Pour L’Exploitation de la Mer (IFREMER), a government-sponsored marine research agency near Brest, France.

Though DeRuiter, a native of Grand Rapids, Mich., doesn’t speak much French, she jumped at the opportunity to gain firsthand experience in marine mammal management. The new assignment is certainly close to her heart: two chapters of her doctoral thesis focused on the exposure of sperm whales to air gun array sounds, including an attempt to determine whether they changed their behavior in response to air guns.

So far, DeRuiter has adapted to life in France quickly; after only one week overseas, she joked that she and her husband managed to rent and furnish an apartment and put all the utilities in their names. According to Tyack, given the obstacles she’s already stared down, DeRuiter should have no problems taking on the challenges of research in a new environment.

“Stacy didn’t take no for an answer,” he said. “It’s extremely important for a researcher to develop an intense interest in a scientific problem and to have the kind of stamina she needed to succeed with the porpoise tag. The fact that Stacy was able to keep focus until she was able to overcome all of the hurdles says volumes about the kind of scientist she is.”

Stacey DeRuiter also received funding support from The Richard Vanstone Fund and The Paul McDonald Fye Graduate Fellowship in Oceanography.

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Multimedia

videoVIDEO

Learning How a Porpoise Locates Prey

Mesoplodon buzzes

A Porpoise Uses Echolation to Find a Fish

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