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7. FISH FINDER OPERATIONS
7-25
Plankton
A plankton layer appears as a large amount of
green or blue dots, and is a possible location for
fish. A plankton layer moves down in the day and
up at night.
Current rip
When two ocean currents of different speeds di-
rections and water temperatures meet, a current
rip occurs. A current rip appears on the screen as
shown in the right illustration.
Surface noise
When the sea is rough or the ship moves over a
wake, surface noise can appear at the top of the
screen.
Air bubbles in the water
When the sea is rough or the ship makes a quick
turn, there can be blank spots in the bottom echo
(see the right illustration). These blank spots are
caused by air bubbles which stop the movement
of the sound wave. These air bubbles can occur
with the low frequency ultrasonic waves.
Unstable bottom echo
The bottom echoes can have a saw-tooth ap-
pearance. This occurs in heavy weather because
pitching and rolling changes the direction of the
ultrasonic pulses and the vertical motion of the
ship causes the distance to the bottom to change.
Plankton
School of fish
Current rip
Surface noise
Ultrasonic wave
blocked by air bubbles
Unstable bottom echo