Fish Detection And Display - Simrad NX40 Installation & Operation Manual

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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. If there is a current, the fish
will often be found downstream of the feature.
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 arches
In good conditions, a fish passing through the cone-
shaped ultrasonic pulse is shown 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 echosounder cone, generating a weak echo that is
shown 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
shown 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 echosounder 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 showing
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 echosounder beam is very narrow near
the surface and fish do not stay within the beam
long enough to show an arch. Several fish in shallow
Simrad NX40/45 Installation and Operation Manual