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4-3 Fish detection and display
Where to find fish
Underwater features like reefs, wrecks and rocky
outcrops attract fish. Use the 50 kHz or 50/200 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 4500/4600 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 con-
verted 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 differenti-
ate between
large air bub-
bles, rubbish
containing air,
fishing floats
etc. and gen-
uine fish.
The follow-
ing picture
shows the
Sonar display
with the Fish
symbol: On +
depth:
Fish symbols Off
For experienced users this always provides the
best information as every echo is displayed, wheth-
er 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 50 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 progres-
sively 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 progres-
sively 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 in-
creases screen resolution 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 blocks of pixels.
• Wave motion may result in distorted
fish arches.
FISH 4500/4600 Installation and Operation Manual 19
NAVMAN