Furuno GP-7000F Operator's Manual page 113

Fish school density
If two schools appear with the same color at different depth, the one in deeper
water is denser because the ultrasonic wave attenuates as it propagates and the
fish school in deep water tends to be displayed in a weaker color.
Less Reddish
(Sparse echo)
Fish echo
Plankton
A plankton layer, a likely place to find fish, is displayed in green or blue dots. It
usually descends in the day and rises at night.
Current rip
When two ocean currents meet with different speeds, directions and water
temperatures, a current rip develops. A current rip's on-screen appearance is as
shown below.
Difference in
Reddish
signal strength
(Dense echo)
Weak Echo
Fish school density
Plankton
Fish
School
Plankton
Current
rip
Current rip
10. VIDEO SOUNDER OPERATION
Strong
Echo
10-27