Using acoustic broadband to count fish in 'high-def'

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WHOI's low-frequency broadband acoustic system being deployed

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It will be like going from black-and-white television to high definition color TV - that’s how researchers at America’s Woods Hole Oceanographic Institution (WHOI) have envisioned an upcoming leap forward in undersea acoustic imaging. Tim Stanton and Andone Lavery have developed and tested two broadband acoustic systems that leave conventional single-frequency systems eating their dust... or water droplets, or whatever. Developed over 20 years, the new technology could revolutionize oceanography, and also has huge commercial and military potential.

Underwater acoustic imaging systems, also known as echosounders, work much the same way as a dolphin or whale’s clicks and pops - sound waves are sent out, they bounce off objects in the water, and the amount of time they take to echo back determines how far away those objects are. In the case of the man-made technology, that data is then displayed visually on a screen. By analyzing properties such as the strength and timing of the signal, scientists can determine what caused the echo.

With the current single-frequency systems, however, there can be a lot of ambiguity when it comes to interpreting the data. Different types of echoes, for instance, could indicate different numbers of fish, sizes of fish, species of fish, or that the fish were oriented differently in the water. Varying interpretation of these echoes, says Stanton, can change fish population estimates by orders of magnitude.

Things get even trickier when it comes to near-microscopic zooplankton. According to Lavery, the scattered echoes sent back from turbulent water are very similar to those sent back from accumulations of zooplankton, making the two almost indistinguishable over a single frequency. That’s where broadband comes in.

Stanton and Lavery’s new instruments measure sound-scattering over a continuous range of frequencies, not just one or a few. How is that better?

  • Very low frequencies, missed by most echosounders, resonate with the air in a fish’s swim bladder - researchers can now discriminate between fish and other marine organisms, and identify both sizes and densities of fish
  • By observing the graphed shape of the acoustic spectrum, researchers can tell the difference between turbulence (downward slope) and zooplankton (upward slope) - the frequency at which the shape flattens out indicates the size of the zooplankton
  • The additional information allows for the use of noise-reducing processing algorithms, which increase the distance at which organisms are detectable, and which increase resolution, allowing closely-spaced organisms to be distinguishable from one another

One of the two new systems spans the lower frequencies, and was designed for the detection of fish. The other, that spans the higher frequencies, was designed to discriminate zooplankton from turbulence. Both systems are modified versions of commercial echosounders, designed by marine tech firm EdgeTech for studying the seafloor.

The WHOI researchers plan to build other systems designed to detect larger fish and smaller zooplankton, that can be mounted on ships, autonomous underwater vehicles, and moorings. The technology should also be useful to the commercial fisheries industry, for determining fish stocks, and to the Navy, for observing how fish interfere with their undersea operations.

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