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Backscatter Variability

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What is it?

Anyone who has used acoustics to map, count, or size aquatic organisms is very familiar with variability in echo amplitudes. Backscattered sound from fish or zooplankton is a complex function of physical factors that influence the speed of sound through water, and biological factors associated with the orientation and reflective properties of the target. The table below lists the primary physical and biological sources of backscatter amplitude and variability.

Potential Sources of Backscatter Amplitude and Variability

Physical

  • Sonar Carrier Frequency: determines acoustic wavelength
  • Water Temperature: influences speed and attenuation of sound
  • Water Salinity: influences speed and attenuation of sound

Biological

  • Swimbladder presence: major source of reflected sound
  • Organism behavior: tilt, roll, and activity influence the amount and direction of reflected sound
  • Organism morphology: size and shape influences the amount of reflected sound
  • Organism physiology: gut fullness and gonad development influences the shape of a swimbladder

The presence of a swimbladder is the primary biological factor influencing the amount and variability of backscattered sound from a fish. Swimbladder size and the angle relative to the longitudinal or sagittal axis of the fish body will determine the amount of sound reflected back to a transducer. A 'typical' fish anatomy is pictured below in the radiograph of brook trout (Salvelinus fontinalis).

Image - Brook Trout radiograph
see other radiographs in the radiograph gallery

Notice that even though the fish are approximately the same length and size, that there is tremendous variability in the size, shape, and angles of the swimbladders. Over acoustic frequencies used by fisheries scientists (12 kHz -- 420 kHz), the surface of the swimbladder closest to the acoustic wave acts as a reflective wall due to the density difference between gas in the swimbladder and the flesh around it. The amount of food in the stomach influences the volume and shape of the swimbladder. Swimbladder volume does not affect the magnitude of a returned echo except at low frequencies. At frequencies around 1 kHz the swimbladder resonates like the chamber of a wind or brass musical instrument and will reflect even more sound than at higher frequencies.

If you look closely below the swimbladders you will see small light spots in the contents of the brook trout stomachs. These fish were originally used in feeding rate experiments where small beads were placed in the food. The beads are counted to index the rate of feeding.

Like people, fish come in different sizes and shapes. Some species of fish even differ in anatomy. In the radiograph to the right, lavnun (Mirogrex terraesanctae or Acanthobrama terraescanctae) in Lake Kinneret (better known as the Biblical fish from the Sea of Galilee) have a dual-chambered swimbladder. The anterior chamber aids in hearing and helps to maintain buoyancy while the posterior chamber is the primary buoyancy control organ. Image - Lavnun radiograph

To illustrate the variability in backscattered sound, target strengths from a tethered lavnun are plotted as a function of ping number.

Plot - TS vs ping number

The variation in target strength can range from 20 to 30 decibels between successive pings. A range of 3 decibels is a change in the sound level by a factor of two. The distribution of target strengths in the lower panel shows a normal distribution of echo amplitudes with a single peak at -49 dB.

 

Relevant Publications
Horne, J.K. 2000. Acoustic approaches to remote species identification: a
review. Fisheries Oceanography (in press).
Horne, J.K., P.D. Walline, and J.M. Jech. 2000. Comparing acoustic model
  predictions to in situ backscatter measurements of fish with dual-chambered swimbladders. Journal of Fish Biology (in press).

©2010 Fisheries Acoustics Research