4-3 Fish detection and displayWhere to find fish
Underwater features like reefs, wre cks and
rocky outcrops attract fish. Us e the 83/200
kHz frequency display to find the se features,
then look for fish by passing over the featu re
slowly several times using the Zoom display
(see section 5-2 Sonar Zoom display). If th ere
is a current, the fish will often be foun d
downstream of the feature.
When fishing with the FISH 4432/4433 with
the Fish symbols Off, a weak fuzzy band
may appear between the bot tom trace and
surface. This might indicate a therm ocline - a
rapid change in water temperature, such as the
edge of a warm or cold current. The tempera ture
difference can form a barrier whi ch the fish may
not swim through. In fresh water, fish ofte n
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 provi des the
best information as every echo is di splayed,
whether it is surface clutter, a thermoc line or
a fish.
The picture in section 4-1 Interpret ing 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-shap ed
ultrasonic pulse is displayed as a fis h arch. The
83 kHz frequency uses a wider cone than t he
200 kHz frequency. This makes the fish ar ches
easier to see.
A fish arch occurs when a fish enters the we ak
edge of the sonar cone, generating a weak
echo that is displayed as the first pixel o f
the fish arch. As the fish moves closer to th e
middle of the cone, the distance betwe en the
transducer and the fish reduces and the e cho
is displayed at progressively shallow er depths,
producing the start of an arch. Whe n the fish
passes directly beneath the middl e of the cone,
the echo becomes strongest and thickes t. As
the fish passes out of the middle of the con e
the reverse happens with a progressivel y
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 wa ter
give the best fish arch returns.
• Range is important. It will be much easie r
to see fish arches when using zoom mode
to concentrate on a particular sectio n
of water, rather than just displaying
everything from the surface to th e bottom.
Zooming increases screen resolutio n and is
necessary for good fish arches.
FISH 4432/4433 Installation and Operation Manual21 NAVMAN