Raymarine DSM250, Owner'S Handbook Manual

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6-2 DSM250 Digital Sounder Module
The strength of echoes is indicated by different colors (or shades
of gray in monochrome displays). You can use this information to
determine the size of fish and the bottom structure. Other objects
in the water, such as debris and air bubbles, also return echoes;
these echoes are generally weaker than the fish or bottom echoes
and produce background noise or
clutter on the display.
The sounder provides controls to reduce the background noise and
to adjust the way in which echoes of different strengths are dis-
played. The color displays also let you select the background
color and remove the display colors for weaker echoes.
The sounder provides a White Line feature which helps to distin-
guish between the bottom and fish (or other objects such as weed)
that are close to the bottom.
Fish Indications
When the sounder detects a fish, it displays an arch-shaped mark.
In general, a larger arch indicates a larger fish, though this rule is
not always true. Let’s say that there are two fish of the same size:
one is close to the surface, the other is near the bottom. The fish
which is close to the surface has a larger arch because, being
closer, more sound waves get returned to the module.
To display the depth of the fish, switch on the
TARGET DEPTH ID
from the
SONAR SET UP
menu, as described in Section 4.4, Sonar
Set Up Parameters .
The sound waves are actually reflected by the swim bladder near
the center of the fish, not by the body of the fish. The size of this
swim bladder is different for different kinds of fish and this can
affect the size of fish arches on the image. A fish with a large
swim bladder produces a large fish arch, while a fish with a small
one produces a small arch. Several arches together show a school
of fish.
Figure 6-1 illustrates some typical fish indications.