One of the most common questions that we receive is
"How do I get fish arches to show on my screen?" It's really pretty simple to
do, but it does require attention to detail, not only in the way you make the adjustments
to the unit, but to the whole sonar installation.
It also helps to see the Why Fish Arch section below. This
explains how arches are created on your sonar's screen.
Screen Resolution
The number of vertical pixels that the screen is capable of showing is called Screen
Resolution. The more vertical pixels on a sonar's screen, the easier it will be for it to
show fish arches. This plays an important role in a sonar unit's capability to show fish
arches. The chart below lists the pixel sizes and area they represent down to 50 feet for
two different screens.
|
PIXEL HEIGHT |
|
PIXEL HEIGHT |
| 100 VERTICAL PIXEL SCREEN |
|
240 VERTICAL PIXEL SCREEN |
|
|
|
|
RANGE |
|
PIXEL HEIGHT |
|
RANGE |
|
PIXEL HEIGHT |
| 0-10 feet |
|
1.2 inches |
|
0-10 feet |
|
0.5 inches |
| 0-20 feet |
|
2.4 inches |
|
0-20 feet |
|
1.0 inches |
| 0-30 feet |
|
3.6 inches |
|
0-30 feet |
|
1.5 inches |
| 0-40 feet |
|
4.8 inches |
|
0-40 feet |
|
2.0 inches |
| 0-50 feet |
|
6.0 inches |
|
0-50 feet |
|
2.5 inches |
As you can see, one pixel represents a larger volume of water with the unit in the 0-100
foot range than it does with the unit in the 0-10 foot range. For example, if a sonar has
100 pixels vertically, with a range of 0 - 100 feet, each pixel is equal to a depth of 12
inches. A fish would have to be pretty large to show up as an arch at this range. However,
if you zoom the range to a 30-foot zoom (for example from 80 to 110 feet), each pixel is
now equal to 3.6 inches. Now the same fish will probably be seen as an arch on the screen
due to the zoom effect. The size of the arch depends on the size of the fish - a small
fish will show as a small arch, a larger fish will make a larger arch, and so on. Using a
sonar unit with a small number of vertical pixels in very shallow water, a fish directly
off the bottom will appear as a straight line separate from the bottom. This is because of
the limited number of dots at that depth. If you are in deep water (where the fish signal
is displayed over a larger distance of boat travel), zooming the display into a 20 or 30
foot window around the bottom shows fish arches near the bottom or structure. This is
because you have reduced the pixel size in a larger cone.
| 100 pixels |
|
240 pixels |
 |
|
 |
On the right above is a section of a 240 vertical pixel screen. On the left is a simulated
version of the same screen with only 100 vertical pixels. As you can see, the screen on
the right has much better definition than does the one on the left. You can see fish
arches much easier on the 240 pixel screen.
Chart Speed
The scrolling or chart speed can also affect the type of arch displayed on the screen. The
faster the chart speed, the more pixels are turned on as the fish passes through the cone.
This will help display a better fish arch. (However, the chart speed can be turned up too
high. This stretches the arch out. Experiment with the chart speed until you find the
setting that works best for you.)
Transducer Installation
If you still don't get good fish arches on the screen, it could be the transducer's
mounting is incorrect. If the transducer is mounted on the transom, adjust it until its
face is pointing straight down when the boat is in the water. If it is angled, the arch
won't appear on the screen properly. If the arch slopes up but not down, then the front of
the transducer is too high and needs to be lowered. If only the back half of the arch is
printed, the nose of the transducer is angled too low and needs to be raised.
Fish Arch Review
Sensitivity
Automatic operation with ASP™ (Advanced Signal Processing) turned on
should give you the proper sensitivity settings, but if necessary the sensitivity may be
increased.
Target Depth
The depth of the fish can determine if the fish will arch on the screen. If the fish is in
shallow water, the fish is not in the cone angle very long, making it difficult to show an
arch. Typically, the deeper the fish, the easier it is to show an arch.
Boat Speed
The boat's engine should be in gear at an idle or just above. Experiment with your boat to
find the best throttle location for good arches. Usually, a slow trolling speed works
best.
Chart Speed
Use at least 3/4 chart speed or higher.
Zoom Size
If you see markings that are possible fish, but they do not arch, zoom in on them. Using
the zoom function lets you effectively increase the screen's resolution.
Final notes on fish arches
Very small fish probably will not arch at all. Because of water conditions such as heavy
surface clutter or thermoclines, the sensitivity sometimes cannot be turned up enough to
get fish arches. For the best results, turn the sensitivity up as high as possible without
getting too much noise on the screen. In medium to deep water, this method should work to
display fish arches.
A school of fish will appear as many different formations or shapes, depending on how much
of the school is within the transducer's cone. In shallow water, several fish close
together appear like blocks that have been stacked in no apparent order. In deep water,
each fish will arch according to its size.
Why Fish Arch
 The reason fish show as an arch is because of the relationship between
the fish and the cone angle of the transducer as the boat passes over the fish. As the
leading edge of the cone strikes the fish, a pixel is turned on the display. As the boat
passes over the fish, the distance to it decreases. This turns each pixel on at a
shallower depth on the display. When the center of the cone is directly over the fish, the
first half of the arch is formed. This is also the shortest distance to the fish. Since
the fish is closer to the boat, the signal is stronger and the arch is thicker. As the
boat moves away from the fish, the distance increases and the the pixels appear at
progressively deeper depths until the cone passes the fish.
If the fish doesn't pass directly through the center of the cone, the arch won't be as
well defined. Since the fish isn't in the cone very long, there aren't as many echoes to
display, and the ones that do show are weaker. This is one of the reasons it's difficult
to show fish arches in shallow water. The cone angle is too narrow for the signal to arch.
Remember, there must be movement between the boat and the fish to develop an arch.
Usually, this means trolling at a slow speed with the main engine. If you are anchored or
stopped, fish signals won't arch. Instead, they'll show as horizontal lines as they swim
in and out of the cone.
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