4-3
Bottom nature
The nature of the bottom is known from the intensity and length of the bottom tail. Generally,
when observing the bottom nature, the lower sounding frequency is used, the pulselength is set
to long, and the gain setting is not disturbed. In the hard and craggy bottom, the bottom appears
more reddish and with a long tail. However, the bottom with a sediment may give a short tail if a
low frequency sounding is used.
Rock base
Mud & sand
Bottom nature
4.4 Fish Schools
Fish quantity
Fish quantity can be estimated to a certain extent from fish echoes on the screen if fish school
size and fish school density are kept in mind.
Fish school size
Usually the size of fish echoes on the screen is proportional to the actual size of the fish school.
However, if two fish echoes appear at different depths with the same size, the fish school at
shallower depth is smaller because the ultrasonic beam widens as it propagates and a fish
school in deep water is displayed larger.
Size of
fish school
Small
school
Large
school
School depth
and sounding
time
Deep fish school sounding time
Shallow fish school sounding time
Fish school size Difference of fish school sounding times
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