FishTV Keeps a Wide Eye On Ocean Ecosystems
Sonar-based system helps researchers tune in to habits of sea creatures
BOSTON — The calm, cold waters of Vancouver Island's Saanich Inlet have become a test site for sensors that could help revolutionize the study of marine life.
Using a unique sonar system known as FishTV, as well as lasers and digital cameras, researchers are developing the electronic equivalent of a one-way mirror. The system allows scientists to watch their briny subjects in a natural setting without disturbing them.
Tracking the behavior of creatures ranging from plankton to the fish that feed on them is vital if scientists hope to understand how the ocean's ecosystems work. For the most part, those ecosystems remain shrouded in mystery, says Peter Franks, a marine biologist at the Scripps Institution of Oceanography in La Jolla, Calif.
Particularly for the various forms of plankton that serve as food for larger species of marine life, he says, "it's remarkable how little we know about them - what, why, and how they do what they do."
Compared with their land-based colleagues, marine ecologists have a poorly stocked tool kit for answering these basic questions.
"An ecologist on land can sit on a hill or hide in a blind, pull out a pair of binoculars, and do his research. Oceanographers don't have that option," says Jules Jaffe, an oceanographer at Scripps. "We're badly in need of tools for studying ocean ecology."
Working with colleagues at Scripps, the Woods Hole (Mass.) Oceanographic Institution, and at Hebrew University of Jerusalem, Dr. Jaffe has been working to provide some of those tools.
At the heart of the package is FishTV, a shoe-box size array of sonar microphones and speakers that can monitor a 4 cubic-meter (5.2 cubic-yard) volume of water. Eight speakers emit extremely rapid pulses of sound, which bounce off fish or other organisms and are picked up by eight microphones.
The return signal carries information about the organism's size and distance. By sampling the signal 100,000 times a second, computers can build a high-tech "flip book" movie of an organism's movements. The result is a three-dimensional record of whatever passes through the TV's view.
As the frequency of its sound signals are raised, the sonar can detect ever smaller organisms - down to a few millimeters long. When coupled with a digital underwater camera, the sonar signals can be correlated with specific creatures. Ideally, resear- chers would like to be able to identify different organisms directly from their sonar signal, "but that's not going to be easy," Jaffe says. "The acoustic signal varies as the fish changes its orientation."
Although FishTV can spot fish, it is configured to study various forms of plankton and the predators that feed on them. The FishTV has yielded "very exciting results," says Dr. Franks. Detailed tests, he says, can now be applied to theories about relationships between marine organisms or to explanations for their behavior.
For example, he says, if plantlike phytoplankton provide the food for animal-like zooplankton, then one would expect to see the tiny zooplankton grazers wherever their fodder was found. That would suggest that their behavior was shaped by nutritional needs. But another explanation holds that zooplankton are captive to the moving mass of water they inhabit. If so, then zooplankton should exhibit a clustering, or "patchiness," when drifting from one group of phytoplankton to another. This patchiness has been demonstrated experimentally, especially with larger organisms like fish, but not to the small scales of the Scripps research.
"We're confirming some ideas in a very strong way," Franks says, "but we're also coming up with a whole new set of questions, or ideas for a series of measurements we should have made to answer them. The more data we get, the less we know."
This summer, the team added a new device to the package designed to get phytoplankton and zooplankton to "glow." As a result of the biochemical processes going on inside them, each gives off light at a different wavelength. The idea is to use their faint phosphorescence to map their distribution and track their activities.
During last month's experiments in Saanich Inlet, the imaging system was used to study the shrimp-like Euphausia pacifica. The system used a low-power laser to stimulate phosphorescence. A camera with lenses "tuned" to different wavelengths of light recorded the "patches" of plankton.
Nobody knows what E. pacifica does during the day, Jaffe says, "but they eat phytoplankton at night, and we got some good pictures" of their foraging.
Not surprisingly, the system had bugs. At one point, Jaffe says, the team was getting discouraged with all the problems. Someone suggested that the researchers just put it in the water anyway. "It worked perfectly," he says.