How to Read a Fish Finder Screen: Arches, Structure and Bottom Hardness

To read a fish finder, start with the bottom line, then look for arches and clusters above it. An arch is a fish passing through the sonar cone; a thick, bright bottom line with a second echo below it points to hard bottom, while a thin, faint one often means soft mud or weeds. Set the sensitivity so the background is mostly clean, use 200 kHz for detail in shallow water and a lower frequency for wider coverage, and then compare what you see with the depth and structure you expect. This guide is for anglers who own a sonar unit and want to turn the screen into a map of where fish are holding.

Quick Answer

• The top is the surface, the bottom line is the lake or sea floor, and everything between is water column. Time scrolls from right to left.
• A complete arch is a fish passing through the cone. A half arch or a short line can still be a fish near the edge of the cone.
• A fish suspended a foot or two above the bottom line is easier to separate than one sitting on it.
• Hard bottom usually returns a stronger, brighter bottom signal and often a second echo; soft bottom returns a weaker, more spread-out signal.
• Use 200 kHz for detail and shallow water, and 83 kHz or 50 kHz when you want wider coverage or deeper water.

What You Need

Reading the screen is about the settings, the mounting and the way you move the boat as much as about the symbols.

• Transducer: mounted on the transom or trolling motor so it points straight down and sits below the waterline. If it is tilted or in turbulent water, the picture will show noise and gaps.
• Frequency: most units offer 200 kHz and a lower frequency such as 83 kHz. Cone angles are narrower at 200 kHz (about 10 to 20 degrees) and wider at 83 kHz (about 20 to 40 degrees), depending on the transducer.
• Display setting: set sensitivity or gain so the background is mostly clean with a few specks of noise. Too high and the screen fills with clutter; too low and small fish vanish.
• Speed: run at a slow idle, around 2 to 4 mph. At higher speeds, arches stretch into lines or disappear.
• Power: check that the battery is charged and the connections are clean. Low voltage can cause false readings.
• Safety: keep your eyes up while you are underway. Look at the screen only during stops or at idle, and wear a life jacket on small boats.

Understand the Screen

Step by Step: Read a Spot

1. Set the range. Use automatic range at first. Once you know the depth, set the range to about 5 to 10 feet below the bottom so the image fills the screen.
2. Adjust sensitivity. Raise it until you see the bottom clearly, then lower it until the water column background is mostly clean.
3. Find the bottom line. Note its thickness and brightness. A thick, bright line with a second line below it suggests rock, gravel or hard clay; a thinner, weaker line suggests soft mud or sand.
4. Scan the water column. Look for balls of bait, arches and clusters between the surface and the bottom. Fish often sit just above structure.
5. Look for structure. Humps, ledges and drop-offs show as a change in the bottom line. A steady slope appears as a diagonal line. Brush or trees appear as irregular shapes rising from the bottom.
6. Mark waypoints. Press the mark key when you see fish or structure so you can return to the spot. Then make a second pass at a slower speed from another direction.
7. Confirm with a lure. Drop a jig or bait to the depth where you saw fish. If your lure appears on the screen as a line, you can see how close it is to the fish.

Bottom Hardness and Frequency

Different bottoms reflect sound differently. A hard bottom such as rock, packed sand or clay returns much of the signal and often creates a second echo. A soft bottom such as mud or weeds absorbs sound and often looks wider and less intense. Sources disagree on whether the line looks thin or thick, so rely on color intensity and the second echo rather than only the thickness.

If a thick gray band sits under the bottom, lower the gain and compare it at 200 kHz. A wide cone or high gain may spread the return, and the band may narrow when you reduce the sensitivity.

Choose Frequency and Cone Width

As a rough guide, a 200 kHz cone is about 10 feet wide at a depth of 20 feet and about 20 feet wide at 40 feet. In deeper water, the cone covers more bottom, so fish at the edge of the cone may not be directly below you.

Common Mistakes

• Running too fast. Arches stretch out or vanish. Slow down to a slow idle.
• Gain too high. The screen fills with clutter and hides fish. Lower it until the background is clean.
• Reading every arch as a fish. Jagged lines are often noise. Smooth curves are more likely fish.
• Ignoring the transducer. If it is angled, too high or in turbulence, the picture is unreliable. Check the mount.
• Fishing the wrong depth. Fish at 12 feet will not take bait at 25 feet. Match your lure depth to the arches.
• Not marking spots. If you see fish or structure, mark it so you can return.

Questions

Why do fish show as arches?

The sonar cone is wider at depth than at the surface. As the boat passes over a fish, the fish enters the edge of the cone, passes through the center, then leaves the edge. The distance to the fish shrinks and then grows, which draws an arch.

How can I tell fish from noise?

Fish are smooth, curved arches or short bars that show up consistently. Noise looks jagged, random and unrelated to depth. Lower the sensitivity if clutter hides the picture.

What does a second line under the bottom mean?

It is a repeat of the bottom return, usually at roughly twice the depth. It appears over hard bottom, in shallow water, and when the gain is high.

Which frequency should I use?

Use 200 kHz in shallow water for detail and 83 kHz for wider coverage at mid-depths. Many units can run both at once.

Do I need to read the unit’s manual?

Yes, at least once. Menus, color palettes and frequency options differ between brands, and the manual explains how each setting changes the picture.

Update History

• October 15, 2026 – Initial publication; details checked against current manufacturer guidance and angling references.
• Conditions, regulations and tackle specifications vary by water and region; check local rules before you fish.

Sources

• Fishfinder, general reference entry
• Master Fishing Mag, how to read a fish finder visual guide
• Apex Angler Pro, guide to reading a fish finder
• Knots.fish, how to read a fish finder
• Ohio Game Fishing forum discussion of fish finder frequency and cone angles