Northstar
Explorer 660
Installation and Operation Manual
36
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 660 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 660 uses Northstar’s SBN technology
to analyse sonar echoes and identify which are
likely to be fish. The Explorer 660 can be set up
to display a fish symbol and the depth over these
echoes (see section 15-3, Fish symbols). While
SBN is very sophisticated it is not foolproof - there
will be times when the Explorer 660 will not be
able to differentiate between fish and large air
bubbles or rubbish.
Depending on the strength of a fish signal, the
Explorer 660 can display a small, medium or large
symbol (see section 15-3, 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.
Fun fish symbol
Fish arch
Fun depth
Normal fish symbol
