Researchers at Woods Hole Oceanographic Institution (WHOI) have developed two advanced broadband acoustic systems that they believe could represent the acoustic equivalent of the leap from black-and-white television to high-definition color TV. For oceanographers, this could mean a major upgrade in their ability to count and classify fish and to pinpoint tiny zooplankton amid seas of turbulence.
Lead authors Tim Stanton and Andone Lavery in the Department of Applied Ocean Physics and Engineering have already tested the two systems off the east coast of the U.S., with highly promising results. They and their colleagues describe the groundbreaking work in back-to-back papers recently published in the International Council for Exploration of the Sea (ICES) Journal of Marine Science.
The technology is the culmination of efforts spanning two decades. Stanton explains, "Components of these advances separately have been achieved by previous investigators, but this is the first of its kind with all of the technologies in one package."
Because sound quickly travels large distances in water, oceanographers have long recognized the power of acoustic measurements to rapidly survey what lies beneath the ocean surface. When a sound wave hits an object, such as a fish or a shrimp, it scatters. Acoustic scientists analyze the frequency, strength, and timing of the scattered signal to determine what caused the echo.
Most acoustic instruments use sound waves that contain only one or a few frequencies. But, interpretation of these echoes is not straightforward. A single frequency sensor used to study two different patches of ocean will probably measure two different echo levels. Those different echo levels might mean the two patches contain different numbers of fish, different sizes of fish, different species of fish, that the fish were oriented differently in the water, or some combination of all of these factors. Stanton emphasizes that these ambiguities can change acoustic estimates of the numbers of fish by orders of magnitude.