Furuno GP-3700F, Operator'S Manual

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11. ECHO SOUNDER
11-32
Echoes from school of fish
Fish volume
The size and density of a school of fish are indicators of the
quantity of fish.
Size of school of fish
Usually the size of fish echoes on the screen is proportional to the actual size of the
school of fish. However, if two fish echoes appear at different depths with the same
size, the school of fish at the shallower depth is larger because the ultrasonic beam
widens as it propagates and a school of fish in deep water is displayed larger.
Density of 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.
Large school
Small
school
Size of school of fish
Sounding time for a deep school of fish
Sounding time for a shallow school of fish Sounding time for a shallow school of fish
School depth and sounding time
Fish echo
Weak echo
Less reddish
(Sparse echo) Reddish
(Dense echo)
Strong echo
Difference in signal strength