World Ocean Census - Extract 8 - Using sound to see underwater

12 February 2010

Eighth amazing extract from The World Ocean Census: A global survey of marine life: Expanding the use of technology - Using sound to "see" underwater.

To achieve the Census’s goal of quantitative estimates of species distributions and densities in many different realms and habitats, specimens must be observed and collected and data must be recorded about the environments in which they live. This is accomplished by direct sampling and by taking acoustic, chemical and optical measurements. The vastness of the ocean realms has prompted development of various innovative technologies to reach these goals.

Side-scan sonar is a type of acoustic technology that scientists use to “see” in the ocean; it is well-established as a technique for mapping the ocean floor and tracking schools of fish. Pulses of sound are projected by a ship or a device towed by a ship. These sound waves bounce off objects – whether they are living things or physical features of the ocean floor – and are reflected back to the ship, where instruments convert them into images. The use of sonar is not confined to humans: biologists have learned that many animal species, including dolphins and bats, use echolocation – a type of natural sonar – to locate prey or to navigate.

Census scientists are using more sophisticated sonar systems called standard and multi-frequency echo sounders, or multibeam sonar, to estimate the size of plankton and fish populations. Echo sounders are also being used for species identification. This acoustic technology increases the success of imaging animals underwater because fish species respond differently to different sound frequencies, and they produce different reflected acoustic signals. Multibeam sonar has met with remarkable success and holds great promise for the future. In one Census study off the coast of New Jersey, for example, Census scientists found a school of 20 million herring that was roughly the size of Manhattan Island!

Text and images reprinted with permission from World Ocean Census: A Global Survey of Marine Life by Darlene Trew Crist, Gail Scowcroft and James M. Harding Jr., Firefly Books, 2009.

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