Simrad GB40, Manual

Page: 93 / 156

of water, rather than just displaying everything from the
surface to the bottom. Zooming in increases the amount of
screen space the sonar data can be represented in, and is
necessary for good fish arches.
•
It is difficult to get fish arches in shallow water as the
transducer sonar beam is very narrow near the surface and fish
do not stay within the beam long enough to display an arch.
Several fish in shallow water tend to display as randomly
stacked areas of color.
•
Wave motion may result in distorted fish arches.
Identifying Species
The swim bladders inside different fish species vary in size and
shape. The size of the swim bladder can affect the strength of the
echo. With experience, it's possible to recognize the type of fish
from the strength of the echoes that they're producing.
When fishing amidst a school of fish and catching them, note the
type of fish and the strength of the echo that they're returning on
the A-scope screen. When you see that particular echo pattern
again it's likely to be the same type of fish.
Shadows
'Shadows' are likely to be created around areas such as hollows in
the bottom or next to individual rocks and rocky ledges. The weak
echoes returned from any fish in these areas are often obscured by
the stronger echoes that are returned from the whole area.
The 200 kHz frequency is the best defence against shadows. The
high frequency and narrow sonar cone tends to reduce the effect
of shadows. Shadows are more likely to occur with the 50 kHz
frequency.
Bottom Detail
Soft bottoms such as mud, weed, or sand tend to appear as a
narrow band on your
Echosounder pane.
Hard bottoms such as coral or rock tend to appear as a wide band
on your
Echosounder pane.
A rocky ledge reflects echoes from two different depths and is
likely to produce a double trace on your
Echosounder pane.
When you're at anchor, all the echoes are returned from the same
area of bottom. This produces a flat, uniform trace on your
Echosounder pane.
92 | Echosounder