4-3 Fish detection and display
Where to find fish
Underwater features like reefs, wrecks and
rocky outcrops attract fish. Use the 83/200
kHz frequency display to find these features,
then look for fish by passing over the feature
slowly several times using the Zoom display
(see section 5-2 Sonar Zoom display). If there
is a current, the fish will often be found
downstream of the feature.
When fishing with the FISH 4432/4433 with
the Fish symbols
Off
, a weak fuzzy band
may appear between the bottom trace and
surface. This might indicate a thermocline - a
rapid change in water temperature, such as the
edge of a warm or cold current. The temperature
difference can form a barrier which the fish may
not swim through. In fresh water, fish often
collect around a thermocline.
Fish symbols
The fish symbol can be customized or switched off
altogether so that the echoes are not converted to
fish symbols on the display. See section 3-2 Setup
> Sonar. The differences between Fish symbol
On
and
Off
are:
Fish symbols
On
Using Navman’s SBN-
II
sonar technology the
fishfinder analyses all echoes and eliminates most
false signals and clutter so that remaining targets
are most likely fish. Depending on the strength
of the remaining echoes, they are displayed as
either small, medium or large fish symbols - with
or without depth. While the SBN-
II
processing is
very sophisticated it is not foolproof - there will
be times when
the fishfinder
will not be able
to differentiate
between large air
bubbles, rubbish
containing air,
fishing floats etc.
and genuine fish.
The picture shows
the Sonar display
with the Fish
symbol:
symbol
+ depth
:
Fish symbols
Off
For experienced users this always provides the
best information as every echo is displayed,
whether it is surface clutter, a thermocline or
a fish.
The picture in section 4-1 Interpreting the
display, shows the Sonar display with the Fish
symbols
Off
. The fish appear as arches.
Fish arches
In good conditions and with Fish symbols
Off
, a fish passing through the cone-shaped
ultrasonic pulse is displayed as a fish arch. The
83 kHz frequency uses a wider cone than the
200 kHz frequency. This makes the fish arches
easier to see.
A fish arch occurs when a fish enters the weak
edge of the sonar cone, generating a weak
echo that is displayed as the first pixel of
the fish arch. As the fish moves closer to the
middle of the cone, the distance between the
transducer and the fish reduces and the echo
is displayed at progressively shallower depths,
producing the start of an arch. When the fish
passes directly beneath the middle of the cone,
the echo becomes strongest and thickest. As
the fish passes out of the middle of the cone
the reverse happens with a progressively
weaker and deeper echo.
There are many reasons why fish arches may
not be seen. For example:
•
Poor transducer installation (see
Transom
Transducers Installation Guide
).
•
If the boat is anchored then fish will tend
to show on the display as horizontal lines
as they swim into and out of the transducer
sonar beam. Slow speeds in deeper water
give the best fish arch returns.
•
Range is important. It will be much easier
to see fish arches when using zoom mode
to concentrate on a particular section
of water, rather than just displaying
everything from the surface to the bottom.
Zooming increases screen resolution and is
necessary for good fish arches.
FISH 4432/4433
Installation and Operation Manual
21
NAVMAN
