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Fish Arches at Cabela's

Fish Arches

Author: Lowrance

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.
Difference in pixel displays.
To the left 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.
Fish Arches
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 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.