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The Echosounder

The basic tool in fisheries acoustics is the scientific echosounder. This instrument sends out sounds down into the water column and receives echoes from objects in the water. Echoes, reflected from the fish school and the seabed, are displayed on the echogram. The intensity of the school echo is determined by echo integration and measured as the Nautical Area Scattering Co-efficient (NASC, symbol sA).

Illustration of how an echosounder works

How Does it Work?

  • This instrument produces an electrical signal, which is converted by a transducer to an acoustic signal called a pulse or ping
  • The transducer is mounted in the water, on a platform (like the hull of a ship), and so the ping is sent vertically downwards into the water, in a beam very much like the beam of light from a torch
  • When fish are encountered by the sound, part of the acoustic energy is reflected back as an echo
  • The echo is then received by the transducer and converted back to an electrical signal
  • The distance or range to the fish is obtained by timing the interval between sending and receiving the signal (the speed of sound in water is approximately 1,500 m s-1 and, therefore, distance = speed x time)
  • The total energy in the echo signals is measured by a component called an echo integrator. The more energy or intensity there is in the echo signal, the more fish (or the bigger the fish) there are
  • An echosounder will transmit sound continuously, once every second. As the ship moves through the water, a 2D picture is built up of echoes; this is called an echogram.
 

The Echogram

An echogram

  • The thick dark continuous line at about 100 metres depth is the seabed
  • The intensity of the signal (echo) is given by the colour of each pixel according to the scale on the right hand side: solid objects are strong targets and make strong dark red echoes (such as the seabed, with an echo intensity >-45 dB)
  • Fish schools are also strong targets: in this image there is one extremely large school in the middle of the picture (40 metres high and 140 metres wide). This has a very high echo intensity (dark red: >-45 dB). Three smaller schools are evident around the larger one
  • This school was identified by trawl catches as 100% herring (Clupea harengus)
  • The blue green cloud centred around 30 metres is a layer of plankton, which by virtue of their small size (and lack of air) are weaker targets (blue/green: -65 to -75 dB)