CalCOFI Reports, Vol. 30, 1989 - California Cooperative Oceanic ...

SMITH ET AL.: ZOOPLANKTON PATERN ANALYSIS
CalCOFl Rep., Vol. 30,1989
33O30'
33000'
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San Clemente Is.
32O30'
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Figure 1, Track of R/V New Horizon during ZB1 cruise. Data were collected during the entire cruise. See figure 8.
this report we analyze the information content of
the echo amplitudes, rather than their frequencies.
The speed of sound in water is approximately
1500 meters per second. Therefore a sound oscillation
of 300,000 cycles per second (Hz) has a wavelength
of 5 mm; in general, insonified objects much
larger than 5 mm scatter sound more efficiently
than objects equal to or smaller than 5 mm (Holliday
1980). The proportion of the sound transmitted
through, reflected from, and scattered around the
object is influenced by small contrasts between the
compressibility and density of water and these features
of the object. For example, an organism with
a bony skeleton, scaly integument, and air bladder
returns much more sound than an organism which
is primarily protoplasm. Similarly, organisms that
are aggregated into patches or layers return more
scattered sound per unit of local volume than the
same organisms would if distributed evenly
throughout a larger volume.
The probability that the acoustic pulse will encounter
a target increases with depth because the
widths of each acoustic beam increase with depth
(figure 2). The probability of detection of a given
target decreases with depth because the sensitivity
of the receiver is fixed but the sound available for
reflecting or scattering is decreasing as the inverse
square of the range. In addition, there is a frequency-specific
attenuation of sound in water, with
more sound Iost as a function of range in the higher
frequencies. To use the ADCP as an acoustic sampling
system, we must be able to define the conditions
of encounter and the probability of detection.
Targets smaller than 240 m cannot be resolved at
the deepest depths because of the transducer beam
geometry.
The ADCP used in the April cruise operated at
307.2 kHz. The transducer installation on the R/V
New Horizon was 4 m deep (the draft of the ship),
and the outgoing pulse was set at 4-m length (2.7
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