PE EIE[R-Rg RESEARCH ON - HJ Andrews Experimental Forest

targets are insonified several times . The tru e
number of fish is determined by countin g
only the peak echo amplitudes from eac h
series of returns from a single fish (corresponding
to the location of the fish nearest
the acoustic axis) . Counting errors are directl y
proportional to the concentration of fish .
When several fish are observed within th e
counting stratum simultaneously, it is difficult
to keep track of the various targets .
Under these conditions an alternative counting
technique can be used . Instead of counting
every fish target, the number of targets
within the stratum can be noted for randomly
selected pulses, and a mean number of target s
per pulse determined for the transect . This
number can be directly compared with th e
sampling volume of the cone to determine th e
mean density of fish along the transect .
Determination of the Sampling Volum e
The sampling volume of the sounder con e
can be approximated from the directivity pat -
tern. However, the volume is also dependen t
on the size and depth of the fish targets, th e
transmitter power and receiver gain of th e
sounder and the minimum threshold fo r
counting. The sampling volume can be directly
determined from the number of times a
target remains within the sounder cone as th e
boat passes over the fish at a known speed .
The width of the path of a fish through th e
sound cone is determined from the formula
w =
boat speed (meters/sec) times duration in cone (pulses)
pulse rate of sounder (pulses/sec )
where duration is the average number of time s
an individual target is sounded upon . It can b e
shown mathematically that the average length
of parallel chords through a circle is 7r/4 time s
the diameter. Thus the diameter of th e
sounder cone at the depth of the fish target s
is 4/7r times the average path width . The sampling
volume of the cone on a single pulse i s
then the cross-sectional area of the cone (irr 2 )
at the mean depth times the depth interval .
The volume of water surveyed along a transect
is the diameter of the sampling cone a t
the mean depth, times the depth interva l
times the transect length .
Integration Procedure
Since echo counting from the oscilloscop e
is time consuming and its application limited
to lower density situations, basic data processing
is generally done primarily by echo integration.
Determination of integrated voltages
from the data collected on magnetic tape is
done by use of a special integration system
utilizing a small general-purpose computer .
This system, called the Digital Data Acquisition
and Processing System (DDAPS), integrates
voltages from fish targets within several
depth intervals simultaneously and calculate s
the fish abundance using input calibration an d
target strength data (Moose, Green, an d
Ehrenberg 1971) . A block diagram of the
system is shown in figure 3 . An example of
the DDAPS output is shown in figure 4 .
Tap e
playe r
Teletyp e
printer
Digita l
squarin g
circui t
Figure 3 . Block diagram of digital data acquisitio n
Upper limit of
depth interval
system (DDAPS) .
Integrated
(voltage) 2
Rectifie r
and
filte r
PDP- 8
Computer
Fish density
(N/m 3 )
Numbe r of
digital sample s
6 3485 +0 .344950E-04 818 1
12 6489 +13 .642376E-04 828 2
18 38175 +0 .377970E-03 818 1
24 24162 +0 .239227E-03 715 4
30 3394 +0 .336040E-04 5 0
36 0 +0 .000000E+00 0
42 0 +0 .000000E+00 0
48 0 +0 .000000E+00
54 0 +0 .000000E+00
Figure 4 . Example of DDAPS output .
Analog t o
digita l
converte r
Typically the average target strength is no t
known precisely before processing . The density
outputs of DDAPS are thus relative rathe r
than absolute. Conversion to absolute density
can be made either from measurement of tar -
get strengths or by direct comparison of
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