NorthStar Navigation EXPLORER 657 Fish detection and display Where to find fish, Fish symbols

8-4 Fish detection and display

Where to find fish	A fish arch starts when a fish enters the weak
Underwater features like reefs, wrecks and rocky	edge of the sonar cone, generating a weak echo
	that is displayed as the start of the fish arch. As
outcrops attract fish. Use the 50 kHz or 50/200
	the fish moves closer to the middle of the cone,
kHz frequency display to find these features, then
	the distance between the transducer and the fish
look for fish by passing over the feature slowly
	reduces and the echo is displayed at progressively
several times using the Zoom display (see section
	shallower depths, producing a rising shape. When
9-2). If there is a current, the fish will often be
	the fish passes directly beneath the middle of the
found downstream of the feature.
	cone, the echo becomes strongest and thickest.
When fishing with the 657 with the Fish symbols
	As the fish passes out of the middle of the cone
Off, a weak fuzzy band may appear between
	the reverse happens with a progressively weaker
the bottom trace and surface. This might	and deeper echo.
indicate a thermocline - a rapid change in water
	There are many reasons why fish arches may not
temperature, such as the edge of a warm or
	be seen. For example:
cold current. The temperature difference can
	• Poor transducer installation (see Transom
form a barrier which the fish may not swim
through. In fresh water, fish often collect around	Transducers Installation Guide).
a thermocline.	• If the boat is anchored then fish will tend to
Fish symbols	show on the display as horizontal lines as they
The 657 uses Northstar’s SBN II technology to	swim into and out of the transducer sonar
	beam. Slow speeds in deeper water give the
analyse sonar echoes and identify which are
	best fish arch returns.
likely to be fish. The 657 can be set up to display
	• Range is important. It will be much easier to
a fish symbol and the depth over these echoes
	see fish arches when using zoom mode to
(see section 17-3, Fish symbols). While SBN is
	concentrate on a particular section of water,
very sophisticated it is not foolproof - there
	rather than just displaying everything from
will be times when the 657 will not be able to
	the surface to the bottom. Zooming increases
differentiate between fish and large air bubbles
	screen resolution and is necessary for good
or rubbish.
	fish arches.
Depending on the strength of a fish signal, the
	• It is difficult to get fish arches in shallow water
657 can display a small, medium or large symbol
	as the transducer sonar beam is very narrow
(see section 17-3, Fish filter).
	near the surface and fish do not stay within
To see the maximum amount of information from
	the beam long enough to display an arch.
the echoes, turn Fish symbols off. Fish appear as
	Several fish in shallow water tend to display as
arches on the display.
	randomly stacked areas of color.
Fish arches
	• Wave motion may result in distorted fish
In good conditions, a fish passing through the	arches.
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.

Fun fish symbol Normal fish symbol Fun symbol + depth Fish arch + depth

Northstar Explorer 657 Installation and Operation Manual 37