Fish Detection And Display - NorthStar EXPLORER 443D Installation And Operation Manual

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 Explorer 443d/443df 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 Northstar'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:
Northstar Explorer 443d/443df Installation and Operation Manual
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.
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