A quality fish finder transforms guesswork into informed decisions — instead of randomly casting across a lake, you can see exactly where the fish are, how deep they are, and what the bottom structure looks like. Modern units from Humminbird, Garmin, and Lowrance are more accessible than ever, and understanding what the display shows dramatically improves fishing success.
How Sonar Works
A transducer mounted to your hull or trolling motor sends sound pulses straight down into the water. Those pulses travel at a known speed (~4,800 feet per second in water), bounce off objects (bottom, fish, structure, suspended particles), and return to the transducer. The unit calculates depth from the travel time and displays a color-coded image of what’s below.
The sonar cone expands as depth increases — at 20 feet, a standard 200kHz cone is roughly 10 feet in diameter; at 40 feet, roughly 20 feet. Everything within that cone at any given moment appears on screen. This cone width is important for understanding arch shapes and coverage.
Reading the Display
The Scrolling Screen
The display scrolls right-to-left. The right edge of the screen is directly below you right now. Move left and you’re seeing what was below you moments/seconds ago. When you’re stationary, everything stacks on the same vertical position.
Colors and Signal Strength
All sonar returns are colorized by signal strength (how hard/dense the target is):
| Color (typical) | Signal | Meaning |
|---|---|---|
| Red/Orange | Strongest | Hard bottom, large fish, dense school, rocks |
| Yellow | Strong | Firm bottom, medium fish, dense baitfish |
| Green | Moderate | Soft bottom, smaller fish, weeds |
| Blue | Weakest | Very soft mud, tiny baitfish, plankton |
Hard bottom (rock, gravel, shell): produces a thick, bright, distinct bottom line with a strong second echo below it. Fish near hard bottom are highly productive targets.
Soft bottom (mud, silt): produces a thin, fuzzy, diffuse bottom line with no second echo.
Identifying Key Features
Fish Arches
A fish creates an arch because:
- The sonar cone reaches the fish at an angle → the range is longer → it appears deeper on screen
- The fish passes through the center of the cone → shortest range → appears shallowest
- The fish exits the cone → range increases → it appears deeper again
This creates the arch shape. Key details:
- Full arch = fish moved through the full cone (moving fish or you’re moving)
- Half arch = fish at the edge of the cone
- Thick arch = dense, large target (bigger fish or two fish close together)
- Color of arch = target strength (bright red/orange = stronger return)
When stationary: Fish show as a horizontal line or blob — the cone sits over them and they paint across the screen as it scrolls.
The Thermocline
A thermocline is a layer where water temperature changes rapidly with depth. In summer, it appears as a hazy, horizontal band across the screen — lighter than the bottom but clearly differentiated from the water above. Fish rarely cross the thermocline from below in summer (low oxygen below). Most summer fish are found above the thermocline or right at it.
Structure
Submerged points, humps, rock piles, and drop-offs appear as sudden changes in the bottom depth line. A hump rises then falls back; a drop-off shows as a sudden plunge in the bottom line. These are prime fish-holding locations — concentrate your fishing on these transitions.
Submerged timber shows as vertical lines rising from the bottom; brush piles as irregular hard returns rising above the bottom.
Practical Settings
Sensitivity / Gain
Increase sensitivity until you see a clear layer of baitfish (tiny returns scattered at a depth) or subtle structure. Too low = missing fish; too high = screen fills with clutter. A good rule: increase until you see false noise, then back off 20%.
Frequency
- 200kHz: Better detail in shallow water (under 60 feet); narrower cone
- 50–83kHz: Better depth penetration (60+ feet); wider cone coverage
- CHIRP: Uses a range of frequencies automatically; best of both in most situations
Zoom
Bottom zoom (lock) keeps the bottom in the lower portion of the screen and zooms in on the bottom zone — useful when fish are holding tight to bottom and you want to see the separation between fish and bottom clearly.
Finding Fish: A Practical Workflow
- Run the area at trolling speed while watching the sonar — note every change in bottom depth, composition change, or structural feature
- Mark waypoints on anything promising (a point, a hump, a hard-bottom spot, an obvious group of fish arches)
- Slow down over marked spots — watch the sonar for fish positions and exact depth
- Match your lure depth to the fish depth — if arches are at 18 feet over a 25-foot hump, fish a drop shot or jig at 18 feet
- Look for fish near structure transitions — the corner of a drop-off is more productive than the middle of flat; the tip of a point is more productive than the flat behind it