How to Read a Fish Finder

Reading a fish finder can initially seem complex, but with a bit of practice, it becomes intuitive. At its core, a fish finder displays underwater information using sonar waves, translating them into images. On the screen, fish often appear as arches or dots, depending on the type of sonar.

Over time, as one becomes familiar with the device and observes how fish respond in real-world situations, distinguishing between fish sizes, species, or even distinguishing fish from underwater structures becomes easier.

Our comprehensive guide is designed to help you proficiently read and interpret these sonar images.

How to Read Fish Finder Screen

Reading a fish finder screen can be a bit challenging for beginners, but with some understanding and practice, it becomes intuitive.

fish finder screen displaying 2D sonar and down imaging.
Fish Finder screen displaying 2D sonar and down imaging.

Here’s a step-by-step guide to help you interpret the data on a fish finder screen:

  1. Identify the Surface: At the top of the screen, you’ll often see a thick line or clutter. This represents the water’s surface and might show interference from waves, boat movement, or floating debris.
  2. Locate the Bottom: As you move down the display, you’ll eventually see another distinct line, which represents the bottom of the water body. The space between the surface and this line is the water column.
  3. Recognize Fish Arches: On traditional sonar, fish often appear as arches. The reason for this shape is the movement of the boat over the fish. The fish enters the sonar beam, reaches its center (peak of the arch), and then exits the beam. On DownScan or SideScan, fish appear as dots or streaks.
  4. Distinguish Fish Size: The color or thickness of the arch (or dot) can give clues about the size of the fish. A thicker, more intense color usually indicates a larger fish or a fish directly in the center of the sonar beam.
  5. Identify Schools of Fish: Multiple dots or arches close together can indicate a school of fish.
  6. Interpret Bottom Structure: The thickness and color of the bottom line can tell you about the bottom’s composition. A hard bottom, like rock, will return a strong signal and appear thick and bright, while a soft bottom, like mud, will be thinner and less intense.
  7. Look for Vegetation: Tall, thin lines or columns rising from the bottom often represent underwater plants.
  8. Understand Color Palettes: Different color palettes can be used to represent the intensity of returns. Commonly, warmer colors (like red or yellow) indicate stronger returns, while cooler colors (like blue or green) indicate weaker returns.
  9. Use the A-Scope: The bar on the right side of the screen, called the A-scope, shows real-time data beneath the transducer. Fish finders work by using a transducer to send signals down to the water bottom. Anything interrupting this signal will appear on the A-scope.
  10. Additional Information on Screen: On the display, additional information such as depth, water temperature at the surface, and frequency of the transducer is shown. Water depth and temperature are crucial for finding fish as certain fish species prefer specific depths and temperatures.

How to Read Fish Finder Sonar


CHIRP (Compressed High-Intensity Radiated Pulse) Sonar is an advanced technology that has revolutionized fish finding. Unlike traditional sonar that sends out a single frequency pulse, CHIRP sends a continuous sweep of frequencies, ranging from low to high. This range provides more detailed information about underwater objects, including fish, structures, and bottom compositions.

CHIRP Sonar explained with labels
CHIRP Sonar explained with labels

Here’s how to read CHIRP sonar:

1. Understand the Basics:

  • Screen Orientation: Typically, the right side of the screen shows the most recent data, while the left side displays older data.
  • Depth Scale: This scale, often found on the side of the screen, indicates how deep the water is.

2. Identify Fish:

  • Arches: On CHIRP sonar, fish often appear as arches. The size and intensity of the arch can give clues about the fish’s size and position relative to the transducer.
  • Horizontal lines: In some instances, especially if the boat is stationary, fish appear as lines rather than arches.

3. Analyze Color and Intensity:

  • Most CHIRP sonars use a color palette to indicate signal strength. Brighter or warmer colors (like reds and yellows) usually represent stronger returns, while cooler colors (like blues and greens) indicate weaker returns. This can help distinguish between larger fish, smaller fish, and other underwater objects.

4. Recognize Structure and Bottom Composition:

  • Hard vs. Soft Bottom: CHIRP sonar can help differentiate between hard and soft bottoms. A hard bottom will return a strong, bright signal, while a soft bottom appears as a thinner, less intense line.
  • Structure Identification: Rocks, trees, wrecks, and other structures are easily identifiable due to their distinct shapes and the strong return signals they produce.

5. Thermoclines:

  • A thermocline is a layer in the water where the temperature changes rapidly. On CHIRP sonar, it might appear as a fuzzy, horizontal line. Fish often congregate above the thermocline where the water has more oxygen.

6. Noise and Interference:

  • If you see random lines or clutter on the screen, it could be due to interference from other electronic devices, turbulent water, or even schools of very tiny baitfish. Adjusting the sensitivity or filtering settings can help reduce unwanted noise.

Down Imaging Sonar

Down Imaging provides a detailed, vertical, picture-like view of underwater structures, terrain, and fish directly beneath a boat. Reading Down Imaging (DI) sonar effectively can significantly enhance your fishing experience.

down imaging sonar
Down Imaging Sonar with fallen tree visible at the bottom

Here’s a guide on how to interpret down imaging sonar:

1. Understand the Display:

  • Down imaging provides a vertical view of the water column and bottom terrain directly beneath your boat.
  • The most recent data appears on the right side of the screen and moves to the left as new data is collected.

2. Identify Fish:

  • Fish usually appear as small dots or blobs.
  • The shadows trailing these dots can help confirm their presence, as they indicate the fish interrupted the DI signal.

3. Analyze Color Intensity:

  • On color DI units, the intensity of the color can indicate the strength of the return. Warmer colors (like red or yellow) often signify strong returns from larger objects or fish, while cooler colors (like blue or green) indicate weaker returns.

4. Recognize Structure and Terrain:

  • Harder structures, like rocks or submerged trees, will appear more intense because they return strong signals.
  • Softer structures or terrains, such as mud or sand, will be less intense due to weaker returns.

5. Estimate Depth:

  • Use the depth scale (usually on the side of the screen) to gauge the depth of objects and fish. This can help you adjust your fishing depth accordingly.

6. Recognize Thermoclines:

  • A thermocline can be visible on DI as a thin, horizontal line.

7. Adjust Sensitivity:

  • If the screen appears too cluttered or too sparse, adjust the sensitivity settings. Increasing sensitivity will show more details (but also possibly more clutter), while decreasing it will show fewer details but a cleaner image.

Side Imaging Sonar

Side Imaging (SI) is a technology that provides anglers with a wide, almost aerial view of the underwater environment to the sides of their boat. Reading Side Imaging (SI) sonar can initially seem challenging due to its unique perspective and presentation, but with understanding and practice, it can become an invaluable tool for anglers.

Side imaging explained
Side Imaging explained with labels

Here’s a step-by-step guide on how to read side imaging sonar:

1. Understand the Display:

  • The screen is split into two main sections, representing the areas to the left and right of your boat.
  • The centerline typically represents the path of your boat. As you move forward, the underwater imagery scrolls from top to bottom.

2. Recognize Fish:

  • Fish often appear as bright white dots or streaks because they reflect the sonar signal effectively.
  • Shadows cast by the fish can give clues about their position relative to the bottom. A long shadow indicates a fish that’s higher in the water column.

3. Identify Structure and Terrain:

  • Rocks, logs, underwater plants, and other structures will show up with varying degrees of brightness based on their composition and how well they reflect the SI signal.
  • Drop-offs, channels, and humps can be identified by changes in the terrain’s darkness. Darker areas represent deeper water, while brighter areas indicate shallower regions or hard bottoms.

4. Look for Shadows:

  • Almost all objects will cast a shadow on SI, depending on their height and orientation to the sonar beam. Shadows can help differentiate between suspended objects and those resting on the bottom.

5. Estimate Distance and Depth:

  • Marks closer to the centerline of the display are nearer to the boat, while those further out are more distant.
  • Some SI units provide depth markers, scales, or overlays to assist in gauging distances and depths.

6. Adjust Sensitivity and Range:

  • Depending on water clarity and depth, you might need to adjust the SI’s sensitivity to get a clearer picture. Increasing sensitivity can help detect smaller objects, while decreasing it can reduce screen clutter in some conditions.
  • Adjusting the range will determine how far out to the sides the SI scans. In deeper water or when searching for distant structures, you might want to increase the range.

7. Differentiate Between Fish and Debris:

  • While fish typically appear as bright dots or streaks, bubbles, debris, or schools of tiny baitfish might look similar. Experience and observing the behavior of these marks (like how they move or their consistency) can help differentiate between them.

8. Use Color Palettes:

  • Many modern SI units offer different color palettes. While many anglers prefer grayscale for its simplicity, other palettes can provide better contrast or clarity in certain conditions. Experiment to find what works best for you.

What do Fish Look Like on Fish Finder

On a fish finder, fish typically appear as arches or lines on traditional sonar due to the boat’s movement over them. In down imaging, fish manifest as small dots or blobs, often with shadows behind them, indicating their position in the water column. In side imaging, fish appear as bright white dots or streaks, with shadows that can provide clues about their height off the bottom.

Below is a more detailed explanation of what fish look like on different sonar types.

Traditional Sonar

On traditional or CHIRP sonar, fish are typically represented in the following ways:

fish schools and fish arches on traditional sonar
fish schools and fish arches on traditional sonar
  • Arches: The most common representation of fish on traditional sonar is arches. These arches form due to the boat’s movement over stationary fish. As the sonar beam first makes contact with the fish, it begins to draw the arch, which peaks when the fish is directly under the transducer and then diminishes as the boat moves past the fish. The size and color intensity of the arch can give clues about the size and position of the fish relative to the transducer.
  • Half Arches or Check Marks: If you see half arches or shapes resembling check marks, it could indicate that the fish are on the edge of the sonar beam or that the transducer might not be properly aligned.
  • Lines: If the boat is stationary or moving very slowly, fish appear as straight horizontal lines rather than arches because they remain in the sonar beam for a more extended period.
  • Dots or Small Blobs: On some traditional sonar displays, especially when the fish are not directly beneath the transducer or if the fish are very small, they may appear as dots or small blobs.
  • Dense Clouds: Fish schools typically appear as dense clouds or clusters of dots or arches, often showing a stronger color intensity in the center, indicating a denser concentration of fish. The shape and density of these clusters can provide clues about the size and type of the baitfish present.
  • Fish Size: Traditional sonar uses color palettes to indicate the strength of the return. For example, a strong return from a big fish or a fish directly in the center of the sonar beam appears in warm colors like red or yellow, while weaker returns might be shown in cool colors like blue or green.
  • Position in the Water Column: By looking at where the arches or dots appear on the sonar screen, you can determine the depth at which the fish are located. This is useful for setting your fishing depth accordingly.

Down Imaging 

On down imaging (DI), fish typically appear differently than on traditional sonar. Here’s what fish look like on down imaging:

Identifying fish on down imaging
Identifying fish on down imaging
  • Dots or Small Blobs: The most common representation of fish on DI is as small dots or tiny blobs. These dots can be isolated or clustered together, depending on whether the fish are solitary or schooling.
  • Shadows: Behind many of these dots or blobs, you might see a faint shadow. The shadow is created because the fish interrupts the DI signal, preventing it from reaching directly below the fish. This shadow can help confirm the presence of fish, especially when it’s consistent with the direction and angle of the DI beam.
  • Location in the Water Column: Just like traditional sonar, the vertical position of the dots on the screen indicates the depth of the fish. You can determine how deep the fish are by looking at their position relative to the water’s surface and bottom.
  • Baitfish vs Game Fish: Baitfish often appear as tightly packed clusters of dots or blobs, resembling a cloud. Game fish will typically appear as distinct, solid marks, separated from each other.
  • Fish Near Structures: On DI, it’s easier to distinguish between fish and structures. Fish near the bottom or close to other structures like weeds, rocks, or timber can be identified by the slight separation or gap between the fish dot and the structure. This clarity is one of the advantages of DI.
  • Size and Shape: While traditional sonar might depict larger fish as bigger arches, on DI, the size of the dot isn’t always a direct indicator of fish size. However, larger targets (like big fish or tightly grouped schools of baitfish) might appear as slightly larger or elongated blobs.

Side Imaging

Here’s how fish appear on side imaging:

group of fish on side imaging
Group of fish on side imaging
  • White Dots or Streaks: Fish typically appear as bright white dots or streaks on the screen. The reason they’re bright is that they reflect the sonar signal well, making them stand out against the darker background.
  • Shadows: Just like in down imaging, fish on side imaging often cast shadows. These shadows are created when the fish interrupts the side imaging beam. The distance between the fish dot and its shadow can indicate the fish’s height off the bottom. A longer shadow can mean the fish is suspended higher in the water column.
  • Clusters or Schools: Individual fish will be seen as singular dots or streaks. In contrast, schools of fish can appear as clusters of dots or more substantial bright areas, especially if the school is dense.
  • Fish Near Structures: One advantage of side imaging is its ability to distinguish fish from structures clearly. Fish near logs, rocks, or other underwater features can be identified by the white dots near or around these structures. The shadows cast by both the fish and the structures can provide additional context.
  • Orientation: The orientation of the fish to the boat and the SI beam can impact how they appear. Fish that are oriented perpendicular to the boat’s path might appear as longer streaks, while those parallel to the boat’s path might show up as dots.
  • Depth and Distance: The position of the dots or streaks on the screen indicates their relative location. Marks closer to the centerline of the display are closer to the boat, while those further out are farther away. Some side imaging systems also offer depth markers or overlays to help gauge the depth of detected objects.
  • Brightness and Size: Larger fish or schools of fish can produce brighter and slightly larger marks on the screen. However, it’s essential to note that the brightness can also be affected by the fish’s composition (like its air bladder) and orientation to the sonar beams.
  • Background and Clutter: The background on side imaging is usually darker, helping fish and other objects stand out. However, debris, bubbles, or other suspended particles can sometimes produce clutter or noise, which might require adjustments to the SI settings for clearer images.

Reading Different Fish Finder Brands

Fish finders, regardless of the brand, operate on similar principles, but each may have unique features and user interfaces. Here’s a basic guide on how to read the three best fish finder brands in the market.

Garmin Fish Finder

Garmin fish finders often display depth, water temperature, and boat speed at the top of the screen, with the main portion dedicated to sonar returns. 

One of the features offered by Garmin models is Fish Symbols, which display fish as small icons. While helpful for beginners, many seasoned anglers prefer viewing raw sonar data. 

In terms of structure, hard returns, like the bottom or submerged rocks, manifest as a thick, solid line, while softer bottoms might show as a thinner line. 

Garmin fish finders come equipped with CHIRP, DownVu (Down Imaging), and SideVu (Side Imaging) offering detailed sonar images below and to the sides of the boat.

Humminbird Fish Finder

The Humminbird fish finder interface typically shows depth, temperature, and boat speed at the bottom. The main view showcases the sonar return. 

Humminbird is renowned for its Side Imaging and Down Imaging technology, offering a picture-like representation of the area below and to the sides of the boat.

In traditional 2D sonar, moving fish often appear as arches, with the arch’s peak indicating the fish’s depth. Regarding structure, solid objects like rocks will display as dark, thick lines or areas, whereas vegetation may appear as either sparse or dense patches, depending on their thickness.

Lowrance Fish Finder

Lowrance fish finders have a similar layout, with depth, boat speed, and water temperature displayed at the top, while the primary focus remains on sonar views. 

An interesting feature is the Fish ID, which portrays fish as icons, indicating their depth. The color palette in the sonar return is indicative of return strength. Red, for instance, typically represents a strong return, suggesting hard structures or the bottom. In contrast, softer structures or fish might appear in shades of yellow or green. 

Lowrance’s StructureScan is their proprietary version of side and down imaging, offering users a clear, image-like view of the underwater terrain. Lowrance models also feature FishReveal which displays fish as arches in DownScan by overlaying traditional sonar on the DownScan Imaging screen.

How to Identify Different Fish Species on Fish Finder

Identifying specific fish species on a fish finder requires practice, as sonar returns often look similar for different species. However, certain clues and patterns can help you make educated guesses. Here’s how you might differentiate between Crappie, Catfish, and Bass on a fish finder:


Crappie are known to school, especially in open water. On the fish finder, they may appear as tight clusters or groups of dots/arches.

Crappie often suspend in the water column, especially around structures like submerged trees, brush piles, or drop-offs. If you notice a group of arches or dots suspended mid-water or near a structure, it could be a school of Crappie.

It’s also worth noting that the sonar arches for Crappie tend to be smaller and less defined when compared to larger fish species like Bass or Catfish. The best fish finder for Crappie is one that offers high-resolution imaging and allows for detailed views of submerged structures where Crappie often congregate. 


Catfish are often found near the bottom. If you see arches or dots hugging the bottom contour or just above it, you could be looking at Catfish. They often produce thicker, elongated arches on the fish finder due to their longer body shape. Unlike Crappie, Catfish are less likely to be found in tight schools. They may be alone or in small groups.

The best fish finder for Catfish is one that offers a combination of high-resolution down imaging and traditional sonar to effectively locate catfish near the bottom or in structures. 


Bass are known to relate closely to structures like rock piles, submerged trees, weed lines, and ledges. If you see defined arches near such structures on the fish finder screen, it might be Bass. While Bass can be found throughout the water column, they are often seen in mid to bottom depths, especially around cover. 

Bass arches on fish finder will generally be more defined and larger than Crappie but might be similar in size to Catfish. The key is to look at their relation to structures and their depth in the water column.