To read a fish finder, remember one thing about the screen: it is not a picture of what is under the boat now. It is a history. The transducer sends a pulse of sound down in a cone, listens for the echo, and draws one thin vertical column of what it heard on the right-hand edge of the screen; the columns scroll left as new ones arrive. So a fish that sits still under an anchored boat draws a flat line, a fish that swims through the cone as the boat moves draws an arch, and a rock ledge draws a step. Once that is clear, the rest follows: what a hard bottom looks like against mud, what a bait school looks like, why the thermocline shows as a band, and how the newer down and side imaging turn the arches into pictures. Below: the reading table, the cone and why it matters, imaging, and the settings that make any unit readable.
The screen is not a window. It is a strip chart of the last minute, and every mark on it is a sound that came back.
What the marks mean on 2D sonar
| What you see | What it is | What to do |
|---|---|---|
| A clean arch, thick in the middle | A fish that crossed the whole cone under the boat; the arch grows as it enters the cone, peaks when it is closest and fades as it leaves | Bigger, thicker arches are bigger fish; a long arch at trolling speed is a fish that swam with the boat |
| A half arch or a slash | A fish that clipped the edge of the cone, or one that moved while in it | Still a fish; the cone missed most of it |
| A flat line at mid-depth over an anchored boat | A fish holding still under you | Drop a bait to it |
| A cloud or ball of dots, often with arches around or under it | A school of baitfish, and the predators on it | Fish the edges and the bottom of the school |
| A thick, bright bottom band with a second, fainter band under it | Hard bottom: rock, gravel, shell; the strong echo bounces twice | Structure fish hold on; note the spot |
| A thin, dull bottom line with no second echo | Soft bottom: mud, silt | Catfish and carp water; bass and walleye pass through |
| A band of clutter across the screen at one depth in summer | The thermocline, where warm surface water meets cold deep water and the density change reflects sound | Fish sit just above it in summer; set the bait there |
| Vertical streaks or a line rising from the bottom | Standing timber, weed stalks, a bridge piling | Cover; fish are inside it |
| Scattered dots near the surface | Small bait, plankton, or air bubbles from the prop | Ignore, or lower the sensitivity |
| A ragged, broken bottom line at speed | The transducer losing the bottom from turbulence | Slow down or re-angle the transducer |
The arch is the part beginners doubt, and it is only geometry. The cone is wide at the bottom and narrow at the top; a fish enters it at the edge, far from the transducer, so the echo is weak and the depth reads deeper; it passes under the boat, close and strong; and leaves at the far edge, weak again. Drawn column by column, that is an arch. Wired2Fish’s guide makes the same point with a warning: the “fish” icons the unit draws in beginner mode are the software guessing at arches, and turning them off and reading the raw arches teaches more in a day than the icons do in a season.
The cone, and how much bottom you see
Lowrance’s own figures for a standard 83/200 kHz transducer are a cone of about 20° at 200 kHz (12° on the dual-search models) and 35–50° at 83 kHz. The rule of thumb that follows: at 200 kHz the circle of bottom you see is about a third of the depth across, at 83 kHz about the depth. In 30 ft (9 m) of water the high-frequency beam looks at a circle 10 ft (3 m) wide. A fish 15 ft to the side of the boat does not exist on the screen. High frequency gives detail and target separation; low frequency gives coverage and depth. The fish finders hub has the frequencies and what CHIRP adds, which is a sweep of frequencies per pulse that separates a fish from the bottom and one fish from another.
Down imaging and side imaging
Imaging sonar uses a very high frequency, 455 or 800 kHz and Humminbird’s MEGA at over a megahertz, in a beam that is thin fore-and-aft and wide side-to-side, so each column of the screen is a slice, and the slices stack into something like a photograph. Sport Fishing’s explainer gives the shapes: down imaging covers about 60° straight under the boat, side imaging reaches out to several hundred feet each side. On imaging screens:
- Fish are dots or short dashes, not arches, because the beam is too thin for a fish to draw an arch through. On side imaging a fish off the bottom casts a shadow, and the length of the shadow, like a person’s shadow in low sun, tells you how far off the bottom the fish is; a fish on the bottom has none.
- Structure is drawn as it is: a brush pile looks like a brush pile, a bridge piling like a piling, a drop-off like a cliff seen from above. Side imaging is for finding structure fast; 2D sonar and down imaging for seeing the fish on it once found.
- Hard bottom is bright, soft bottom is dark, the same rule as 2D, because a hard bottom returns more sound.
- The water column on side imaging is the dark band next to the center line; anything in it is above the bottom.
Forward-facing sonar, which points the beam ahead of the boat and shows fish moving in real time, is a different tool again and has its own page: forward facing sonar.
Settings that make a unit readable
- Turn off fish ID icons and read arches.
- Sensitivity (gain) high enough to show bait and the thermocline, low enough that the screen is not snow; start at auto, then raise it until clutter appears and back off a step.
- Range on manual, set to a little deeper than the bottom, so the bottom sits in the lower third and fish above it have room.
- Scroll speed matched to boat speed: fast when idling around structure, slow when anchored so a holding fish draws a line rather than a smear.
- Frequency: 200 kHz or the high CHIRP band for detail in under 100 ft; 83 kHz or low CHIRP for coverage or depth.
- Color palette: one with a clear hard-bottom color (the yellow-and-red palettes on most units) and learn it, rather than switching.
- Transducer angle: level with the boat at rest and at speed; a tilted transducer reads a false bottom and loses fish.
Related
The fish finders hub explains the sonar types and the brand ladders; the Garmin, Humminbird and Lowrance pages sort each maker’s units; the kayak fish finder page covers the small screens. For what the arches usually are, the bass hub and catfish hub have the depths and structure each fish holds on by season.