Radio Script
1999

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Tools of Ecosystem Management
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STORY: In the deep, dark undersea world, sound is the best tool for locating prey and avoiding predators. Sea mammals like whales and dolphins, as well as many fish, emit and receive sounds over great distances and depths to communicate and survive. For the better part of this century, humans have used sonar to locate schools of fish, submarines and sunken ships, but according to Dr. Gary Thomas at the Prince William Sound Science Center in Cordova, Alaska, human echolocation technology is rudimentary compared to that of sea life.

THOMAS: "They have this tremendous central processor, their brain, that has developed this ability to process all of this sound information, and we're just infants as far as understanding how to apply sound underwater to effectively measure and detect things."

Thomas is leading an effort to improve fisheries management by refining the use of acoustical techniques to monitor the Prince William Sound ecosystem. The practice involves dragging a tethered underwater transducer, which emits and receives sound waves, behind a boat. The transducer sends a signal down into the sea, where it bounces off marine life and is picked up again. Colleen Sullivan, a marine biologist with the company which makes the sensing equipment, explains.

SULLIVAN: "People are probably familiar with ultrasound. It's the same kind of system, only at a different frequency. You know people use those to see what kind of baby they're going to have. Well, we put the same kind of instrument in the water and look at differences in density and see how many fish there are."

Sullivan says the different densities of various life forms return specific echoes, signaling things like big fish or small fish, but don't differentiate species, so you have to know what you're looking at. By precisely altering the frequency sent out, researchers can target and find everything from tiny near-surface zooplankton to pollock hundreds of meters deep. Sounds fairly simple, but seeing a fish and quantifying an entire population in a bay, for example, goes beyond the limits of an ordinary fish finder. Enter Prince William Sound Science Center electrical engineer Jay Kirsch, who with special computer programs sorts and quantifies the backscatter of reflected sound to estimate the size of a school of fish.

KIRSCH: "We actually have a technique called echo-integration that allows us to take the voltages from the transducer and be able to work with them and convert them into a biomass density value. So we go from voltage, and then we convert that to decibels, and then we convert that to kilograms per cubic meter."

Kirsch says the program yields precise and regular population size estimates when conducted repeatedly on a given school of fish, but that the accuracy of those estimates is still in question and needs to be tested in a controlled environment. In a similar way, sound waves can also be used to identify available fish food like zooplankton, and even track underwater currents and upwellings. Science center oceanographer Sheri Vaughn uses what's called a Doppler current profiler to measure water movement.

VAUGHN: "It's sort of like the radar gun the police use, you know. They send out a signal, it reflects off of something moving, and by the frequency shift, they can tell how fast that moving object is traveling. The objects are like little particles in the water column."

Vaughn says the unit uses four transducers to emit a frequency in two planes, registering feedback from these tiny suspended particles. On the receiving end, the transducers filter out any errant hits on fish or plankton to give an accurate 3-dimensional depiction of water movement. When integrated with information on an area's fish populations and plankton, the dynamic forces of an ecosystem take shape on a computer screen, patterns emerge, and fisheries management is less guesswork.

OUTRO: From the Sea Grant Program at the University of Alaska Fairbanks, this is Arctic Science Journeys. I'm Dan Bross.