basic sonar: how to read the fishfinder
Understand how sonar sees underwater, what each mark on the screen means, and how to adjust the unit to find bottom, structure and fish.
tap the numbers
- Surface: noise from waves, bubbles and the boat itself. Adjust surface clutter rejection if it gets in the way.
- Thermocline: a diffuse band where the water changes temperature. Many fish sit just above it or in it in hot weather.
- Baitfish school: a cloud of small dots. Predators are usually nearby, below or on the edges.
- Fish arch: the fish enters and leaves the cone and the trace becomes an arch. A thick, hot-colored arch = a bigger fish or one closer to the center.
- Submerged timber / tree: lines rising from the bottom. Structure where predators hide; mark the spot.
- Hard bottom: a thick, bright band, sometimes with a second return below. Rock, gravel, packed sand.
- Soft bottom: a thin, dark band. Mud, silt, vegetation. No second return.
- Right edge = now: this is what's under the boat at this moment. Everything to the left has already passed.
how sonar works
The transducer, mounted on the boat, emits sound pulses downward and measures how long the echo takes to come back. Since sound travels at about 1,500 m/s in water, the unit converts that time into depth. Everything that returns an echo shows on the screen: bottom, stone, branch, bait, fish, bubble.
Sound leaves in the shape of a cone. The deeper it goes, the larger the circle it covers on the bottom. A 20-degree cone covers a circle about a third of the depth (at 10 m, about 3.5 m in diameter). A 60-degree cone covers roughly the depth itself (at 10 m, about 11 m). What falls outside the cone does not show.
tips
- 50 kHz: wide cone (usually 40 to 45 degrees) and long range. Use: sea, reefs and waters tens to hundreds of meters deep.
- 83 kHz: quite wide cone (close to 60 degrees on several models). Use: shallow to medium water, when you want to see more area around the boat.
- 200 kHz: narrow cone (about 20 degrees), more definition and less noise. Use: rivers, reservoirs and shallow coast; it is the best for separating fish from the bottom.
- CHIRP: instead of a single tone, each pulse sweeps a band of frequencies. Result: close targets are separated, fish stuck to the bottom are more visible and there is less noise. If your unit has CHIRP, use it as the default.
- Angles vary by model. Check them in your transducer's manual.
what the screen shows
On traditional (2D) sonar, the screen scrolls from right to left. The right edge is what is below the boat now; the rest is what has already passed.
tips
- Bottom: the strongest continuous band, usually at the bottom of the screen.
- Fish arch: the fish enters at the edge of the cone, farther from the transducer, gets closer in the center and moves away as it leaves. The distance decreases and then increases, and the drawing becomes an arch. A full arch only appears with the boat or the fish moving. A stopped boat and a stopped fish give a straight line. A half arch is a fish that only crossed the edge of the cone.
- Size: the thickness of the arch relates to the size of the fish. The length of the arch depends on how long the fish stayed in the cone, not on its size.
- School: a dense spot or cloud. Small bait forms balls and clouds; the predator usually appears as larger arches at the edge or below them.
- Thermocline: a horizontal band of dots at the same depth, unrelated to the bottom. It marks the sharp temperature change in deep reservoirs and lakes in summer. Many fish stay above it or at its level; below it, oxygen may be lacking.
- Vegetation: vertical, diffuse marks rising from the bottom, less dense than the bottom, sometimes tilted by the current.
- Timber and rock: a branch appears as irregular streaks coming off the bottom; rock appears as a hard, bright protrusion.
- Hard vs. soft bottom: with the same sensitivity, a hard bottom (rock, gravel, compacted sand) returns a strong echo, with a thicker, brighter line and often a second reflection (a second, weaker bottom at twice the depth). Soft bottom (mud, silt) absorbs sound: a thin line, weak color and almost never a second reflection.
- Blind zone: a fish stuck to the bottom, at the edge of the cone, can hide inside the bottom line. Use bottom zoom.
- Dots near the surface are usually engine bubbles, a wake or plankton.
settings that matter
Start on automatic. Then change one setting at a time and watch the effect on the screen.
tips
- Sensitivity (gain): raise it until a few specks appear in the water column, then lower it by one or two steps. Too low erases small fish and thins the bottom; too high fills the screen with noise.
- Range (depth): on automatic, the unit follows the bottom. On manual, keep the bottom in the lower third of the screen.
- Scroll speed: match it to the boat's speed. Compressed arches call for faster scrolling; everything stretched out calls for slower scrolling.
- Zoom: use split screen with bottom lock zoom to see fish lying on the bottom and the texture of the structure.
- Noise rejection: use it at low or medium level to cut interference from the motor, pump and other sonars. At maximum, it erases weak signals, such as small bait.
- Surface filter: reduces bubbles and clutter near the surface. Use the minimum needed.
- Fish ID (fish icon): turn it off while you learn. The unit marks bubbles, branches and grass as fish, and stops marking fish close to the bottom and structure. Learn to read the arches; later, if you like, turn it on only as a backup.
first outing: step by step
Learn in a place you already know. Comparing the screen with what you know about the bottom is the fastest way to understand the unit.
- Turn on the unit with the boat in the water and confirm the transducer is submerged, clean and with no bubble stuck to its face.
- Leave sensitivity and range on automatic and choose the high frequency (200 kHz or high CHIRP) for rivers, reservoirs and shallow coast.
- Turn off Fish ID.
- Move slowly (5 to 10 km/h), in a straight line, over a known place: a channel, a rock, a bank.
- Compare the screen with the place: where the bottom rises, where there is timber, where the depth changes.
- Raise the sensitivity until light noise appears, then lower it slightly. Watch what changes.
- When you see structure or fish, mark a waypoint right away and note the depth.
- Go back over the mark from another direction to confirm what you saw.
- Try the bottom zoom and the scroll speed until the arches are sharp.
- At home, review the screenshots you saved.
tips
- Note what you caught at each mark. Over time you recognize each species' and structure's signature.
installing the transducer
Reading depends more on the installation than on the unit. The transducer needs smooth water flowing over its face, without bubbles and without turbulence. Always follow the manufacturer's manual.
tips
- Transom: the most common. Mount it where the water flows smoothly, away from rivets, hull steps, water intakes and the propeller's wash. On a clockwise-rotating engine, the usual recommendation is the starboard side (right, looking from stern to bow), where the blades move down. With the boat in the water, the face should be parallel to the surface, at the height the manual specifies relative to the hull bottom.
- Through-hull: bonded inside a solid fiberglass hull with epoxy and with no air bubbles. Does not work in aluminum or in a cored (sandwich) hull. It loses a bit of sensitivity, but it doesn't take impacts or get fouled.
- Electric motor: mounted on the bow electric motor's bracket, pointing down. It shows what is under the bow, where you fish. Secure the cable along the shaft with some slack so the motor can turn and retract.
- Leave slack in the cable so the tilting bracket can swing and return without stretching the wire. Coil the excess; do not cut the cable.
- Clean the face with a soft cloth, water and mild soap. Don't scrape it.
common mistakes
- Transducer too high or in turbulent water: the bottom disappears when the boat accelerates.
- Face tilted forward or backward: distorted arches and loss of bottom.
- Bubbles in the glue of a through-hull transducer: weak signal.
common mistakes
Nearly every reading problem comes from settings, installation or interpretation. The unit is rarely the culprit.
common mistakes
- Trusting the fish icon and ignoring the raw screen.
- Thinking a long arch means a big fish.
- Sensitivity too low out of fear of noise.
- Wrong manual range: the bottom goes off the screen.
- Expecting an arch with the boat and fish stopped.
- Circling with a gas engine over the spot you just found: it spooks the fish.
- Forgetting that the screen shows what has already passed: only the right edge is now.
checklist
Check off what's ready. Saved on this device.
sources and references
- technical Simrad — Traditional Sonar: How to Read It Like a ProFish arch, bait, hard and soft bottom, zoom and bottom lock, frequency and CHIRP, the gain rule.
- technical Simrad — Sonar 101: How to Choose a TransducerPower and frequencies of 200, 83 and 50 kHz and the operating depth of each.
- technical Garmin — ECHOMAP UHD2 6/7/9 sv Owner's Manual: Sonar FishfinderSonar chapter of the manual: traditional view (scrolls right to left), ClearVü, SideVü, LiveScope, settings and waypoint on the screen.
- technical Garmin — Transducer Placement GuideVersion 2, Sep./2026. Choosing the transducer location: waterline, propeller rotation direction, interference and hull type.
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