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16
rSOUNDER MODEq HOW TO READ IMAGES
(16)-2 Reflection from a fish
school
Å Reading the size of a fish school
In general, reflections from a fish school form a light-colored downturn
between the sea level and bottom on the screen.
Information provided by the screen image of a fish school includes the size and
density of the shoal. Comparison, as a routine matter, between the screen image
and the actual fishery will permit you to read the image exactly.
The image size of s a fish school is in proportion to the actual size.
When the system detects two same-sized fish school at different depths, however,
the fish school located at a deeper position will be displayed as a smaller-sized
image on the screen. This is because the ultrasonic radiates at a cone-shaped
beam angle from the transducer and therefore the deeper the object position is, the
longer the time for which it is radiated with the beam. The relationship between the
image size and the depth of an object depends on the image feed rate and the ship
speed.
Larger fish school
Smaller fish school
H
Radiated with the
beam for a shorter time
Radiated with the
beam for a longer time
315°T 10.2KT 58°F
FT
71.4
GPS 48°21.417N 125°26.643W
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40
60
80
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100
The higher the density of a fish school is, the closer to red the color of the
image is (i.e., the higher the strength of signals reflected from the object is).
When two images having the same density (same signal strength) are displayed at
different depths on the same screen, however, the density of the two fish schools
are different; the deeper one is actually denser than the shallower one. This is
because the deeper the object position is, the more the pulses reflected from the
object delay.
ı Reading the density of a fish
school
Lower-density fish school
Higher-density fish school
H
Decrease of transmitted
pulse is small
Decrease of transmitted
pulse is large
315°T 10.2KT 58°F
FT
71.4
GPS 48°21.417N 125°26.643W
20
40
60
80
0
100