Fish Indications - Raytheon HSB L760 Owner's Handbook Manual

4-2

Fish Indications

When the Fishfinder 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 Fishfinder.
Switch on a fish depth indication (Fish ID) to display the depth of the fish.
The sound waves are actually reflected by an air sac near the center of the fish,
not by the body of the fish. The size of this air sac is different for different kinds
of fish and this can affect the size of fish arches on the image. A fish with a
large air sac produces a large fish arch, while a fish with a small sac produces a
small arch. Several arches together show a school of fish. Figure Figure 4-1
illustrates some typical fish indications.
Figure 4-1:
Effect of Frequency and Boat Speed
The return from a fish has a different appearance depending on the frequency
used: 50kHz produces a wide beam, so a fish spotted by this frequency gives a
broad arch; the 200 kHz beam is narrower so the same fish produces a
narrower arch.
The shape of the arch is also affected by the speed of the boat. If the boat is
moving slowly, the arches tend to be longer; if the boat is moving more
quickly, the arches are short and peaked. If the boat is moving very fast, a fish
may be indicated by a single vertical line.
An arch-shaped image indicates a fish.
The same fish appears differently when shown by the
50 kHz or 200 kHz transducer beams.
50 kHz
200 kHz
A shallow arch is generated when the the boat is
moving slowly.
This indicates a group of small fish.
A fish moving under a stopped boat shows as a line.
When the boat is moving very fast, a fish appears as
two parallel lines.
Fish Indications
HSB Series L755 and L760 Fishfinder
D5017-2