Operation Manual

8120 Installation and Operation Manual38
9-4 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 window to find
these features, then look for fish by passing
over the feature slowly several times using
the Zoom window (see section 9-2). If there
is a current, the fish will often be found
downstream of the feature.
When fishing with the Instrument 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 Instrument uses NAVMAN’s SBN II
technology to analyse sonar echoes and
identify which are likely to be fish. The
Instrument can be set up to display a fish
symbol and the depth over these echoes (see
section 17-3, Fish symbols). While SBN II is
very sophisticated it is not foolproof - there
will be times when the Instrument will not be
able to differentiate between fish and large
air bubbles or rubbish.
Depending on the strength of a fish signal,
the Instrument can display a small, medium
or large symbol (see section 17-3, Fish filter).
To see the maximum amount of information
from the echoes, turn Fish symbols off. Fish
appear as arches on the window.
Fish arches
In good conditions, 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 starts when a fish enters the
weak edge of the sonar cone, generating a
weak echo that is displayed as the start 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 a rising shape. 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 window 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.