9-4 Fish detection and display
Where to nd sh
Underwater features like reefs, wrecks and rocky
outcrops attract sh. Use the 50 kHz or 50/200 kHz
frequency window to nd these features, then look for
sh by passing over the feature slowly several times
using the Zoom window. If there is a current, the sh
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 sh may not swim through. In
fresh water, sh often collect around a thermocline.
Fish arches
In good conditions, a sh passing through the cone-
shaped ultrasonic pulse is shown as a sh arch. The
50 kHz frequency uses a wider cone than the 200 kHz
frequency. This makes the sh arches easier to see.
A sh arch starts when a sh enters the weak edge of
the echosounder cone, generating a weak echo that is
shown as the start of the sh arch. As the sh moves
closer to the middle of the cone, the distance between
the transducer and the sh reduces and the echo is
shown at progressively shallower depths, producing a
rising shape. When the sh passes directly beneath the
middle of the cone, the echo becomes strongest and
thickest. As the sh passes out of the middle of the cone
the reverse happens with a progressively weaker and
deeper echo.
There are many reasons why sh arches may not be
seen. For example:
Poor transducer installation (see Transom Transducers •
Installation Guide).
If the boat is anchored then sh 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 sh arch
returns.
Range is important. It will be much easier to see sh •
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 sh arches.
It is difcult to get sh arches in shallow water as the •
transducer echosounder beam is very narrow near
the surface and sh do not stay within the beam
long enough to show an arch. Several sh in shallow
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