37Northstar Explorer 650 Installation and Operation Manual
8-4 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 display to find these features, then
look for fish by passing over the feature slowly
several times using the Zoom display (see section
9-2). If there is a current, the fish will often be
found downstream of the feature.
When fishing with the Explorer 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 Explorer uses Northstar’s SBN technology to
analyse sonar echoes and identify which are likely
to be fish. The Explorer can be set up to display
a fish symbol and the depth over these echoes
(see section 17-3, Fish symbols). While SBN is
very sophisticated it is not foolproof - there will
be times when the Explorer will not be able to
differentiate between fish and large air bubbles
or rubbish.
Depending on the strength of a fish signal, the
Explorer 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 display.
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 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.
• 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 areas of color.
• Wave motion may result in distorted fish
arches.
Fish arch
Fun symbol + depthNormal fish symbolFun fish symbol
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