Furuno MFD12 Operator's Manual page 169

7. Fish Finder
Color difference
Second bottom
echo
Bottom characteristic
The characteristic of the bottom is known from the intensity and length of the bottom
tail. To find the bottom characteristic, use a long pulse length and normal gain. For the
hard and rough bottom, the bottom echo is reddish-brown with a long tail. For the mud
or sand bottom, the bottom echo has less red and with a short tail. A bottom with a lot
of small particles can give a long tail if the low frequency is used.
Fish quantity
The size and density of a school of fish are indicators of the quantity of fish.
Size of a 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 smaller because the ultrasonic beam
widens as it propagates and a school of fish in deep water is displayed larger.
Size of
school of
fish
7-26
Rugged
bottom
Rock base
Mud & sand
Sounding time for a shallow school of fish
School depth
Large
and sounding
school
time
Small
school
Sounding time for a deep school of fish