Furuno MFD12 Operator's Manual page 170

Density of a school of fish
If two schools appear with the same color at different depths, the one in deeper water
is denser because the ultrasonic wave attenuates as it propagates and the school of
fish in deep water tends to be displayed in a weaker color.
Less Reddish
(Sparse echo)
Fish Echo
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 move up at night.
School of
fish
Current rip
When two ocean currents of different speeds directions and water temperatures meet,
a current rip occurs. A current rip appears on the screen as shown in the illustration.
Current rip
Difference in
Reddish
signal strength
(Dense echo)
Weak Echo
Plankton
7. Fish Finder
Strong
Echo
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