length or equivalent spherical radius (a mathematical tool expressing the equivalent radius of a non-cylindrical object as that of a cylindrical object exhibiting
identical acoustic properties) to the sound frequencies suitable for their detection
(Fig. 8.15). Note that the smaller the body, the higher the frequency needed to
detect the animals. In fish, the impedance mismatch between the body and the
swim-bladder or other air bodies, has a very similar structure to the biological
pyramid. The swim-bladder in fish is the dominant scattering organ. As illustration, the swim-bladder produces about 80 % of the scatter in perch (a fish that
strongly utilizes the entire water column) and about 22 % in northern hog sucker (a
fish that tends to stay near the bottom) (Sun et al. 1985). But even if no air-bodies
are present in an animal, it can still be detected based on its other scattering
characteristics. For acoustical models of fish, the scatter produced by the swimbladder must be added to that produced by the fluid-filled body (i.e., the muscles,
bones and blood vessels) and then solved for different aspects of the fish relative to
the incident sound wave (Nakken and Olsen 1977; Clay and Horne 1994).
Fig. 8.14 a A downward-looking ADCP deployed from a small vessel. The instrument subtracts
vessel speed from current speed; b typical sensor configuration on an ADCP; c profile of a
downward-looking ADPC transect across a gently flowing water body. The vertical (y) axis is
depth. Distance travelled by the instrument is shown on the x-axis, horizontally increasing from
left to right. Distance is expressed by the number of adjacent, processed pings. Each ping is
mathematically ‘cut’ into several sectors of pre-determined length within which current speed is
calculated from the Doppler shift of sound bouncing off scatterers. Thus, each ping appears as a
stack of differently colored cells (strength of Doppler shift gives current strength, which is coded
along a color ramp visible to the right of the graph) that in sum make up a patchwork of cells
along the entire transect that fully characterize current speed
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B. Riegl and H. Guarin
identical acoustic properties) to the sound frequencies suitable for their detection
(Fig. 8.15). Note that the smaller the body, the higher the frequency needed to
detect the animals. In fish, the impedance mismatch between the body and the
swim-bladder or other air bodies, has a very similar structure to the biological
pyramid. The swim-bladder in fish is the dominant scattering organ. As illustration, the swim-bladder produces about 80 % of the scatter in perch (a fish that
strongly utilizes the entire water column) and about 22 % in northern hog sucker (a
fish that tends to stay near the bottom) (Sun et al. 1985). But even if no air-bodies
are present in an animal, it can still be detected based on its other scattering
characteristics. For acoustical models of fish, the scatter produced by the swimbladder must be added to that produced by the fluid-filled body (i.e., the muscles,
bones and blood vessels) and then solved for different aspects of the fish relative to
the incident sound wave (Nakken and Olsen 1977; Clay and Horne 1994).
Fig. 8.14 a A downward-looking ADCP deployed from a small vessel. The instrument subtracts
vessel speed from current speed; b typical sensor configuration on an ADCP; c profile of a
downward-looking ADPC transect across a gently flowing water body. The vertical (y) axis is
depth. Distance travelled by the instrument is shown on the x-axis, horizontally increasing from
left to right. Distance is expressed by the number of adjacent, processed pings. Each ping is
mathematically ‘cut’ into several sectors of pre-determined length within which current speed is
calculated from the Doppler shift of sound bouncing off scatterers. Thus, each ping appears as a
stack of differently colored cells (strength of Doppler shift gives current strength, which is coded
along a color ramp visible to the right of the graph) that in sum make up a patchwork of cells
along the entire transect that fully characterize current speed
216
B. Riegl and H. Guarin