Side-scan sonar provides a sideways view of structural elements out to either side of the boat, providing the ability to quickly cover more water in search of fish-holding structure.
February 20, 2025
By Thomas Allen
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My early experience with fishing electronics was with a colorless 5-inch screen that beeped every time a fish passed beneath the transducer, and then indicated the fish with an assortment of different-sized fish icons. Valuable information included depth, water temperature, and speed—all critical pieces of the puzzle.
Since those early days, many fishing sonar units now offer side-scanning, providing a sideways view out to either side of the boat, allowing you to search a larger area. Generically termed side-scan sonar, Humminbird calls it Side Imaging , Garmin’s term is Side-Vü , and to Lowrance it's known as SideScan . All three are similar in perspective, application, and settings.
Understanding what you’re looking at and how to apply it is the first step. Once mastered, the technology will become one of your most-used tools. Most often I use Side Imaging from the driver’s seat while idling by structure. Rarely will I switch the sonar to the bow through the use of Humminbird’s One-Boat Network, but it’s an option when needed.
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There are several areas of note in this image. First, the school of fish to the upper left is obvious, but notice the shadows the fish create—that perspective adds to depth perception. The soft low area in the middle could be an old roadbed or creek channel; the darker color indicates a softer bottom. The bright areas indicate a hard bottom, which is where the fish are staged. Build Recognition As with any fishing technology, seat time is the best way to build familiarity. Mostly, I use side-scan sonar while scouting and only occasionally when I have rod and reel in hand, such as when I’m contour trolling for muskies or open-water trolling for stripers.
Start by scanning recognizable features, such as boat ramps, rock jetties, spillway openings, and submerged trees. This helps you establish a recognizable perspective that you can apply everywhere you fish. Over time, you’ll understand how to interpret the sonar images.
It’s also necessary to understand that what appears at the top of the screen, as it begins to scroll downward, represents what is directly behind the boat.
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The scrolling screen creates a short visual history that remains visible for several seconds before cycling off the screen. As the boat moves along, the speed at which the image passes by, known as chart speed, should move at a comparable rate. Making adjustments to chart speed affects how fast the information is being provided—you have to determine what makes the most sense. Some anglers don’t change the chart speed ever and still effectively use the technology. Experiment and apply accordingly.
Generally, idling a big motor moves the boat from 3.5 mph to 5 mph, and that’s probably the most productive range of speeds to run side-imaging. Setting the chart speed to 5, 6, or 7 accommodates average idle speeds with quality image returns.
If you increase chart speed, you’re essentially double-printing the image, which means more detail, as long as you’re not moving the boat too fast. Inconsistent speeds and quick direction changes distort the image. I rarely run chart speed below a 5 setting.
This image makes a case for focusing on the value of hard bottom. Here the bright (hard-bottom) area shows some texture, possibly rocks and boulders, and the fish suspended above the adjacent channel indicate their position. Boat Positioning Putting the boat in the best position for precise casting is important, but the transducer’s perspective comes first—how far the unit looks out to either side, which is called “range.” You can select to view one side over the other if you wish, or view both sides, which is the norm. Under most bass-fishing scenarios, I prefer to set the range at 65 to 75 feet, depending on the depth. When scanning deeper water, extend the range to effectively view the bottom.
Tip: Always keep the depth readout on the graph, as that serves as a reference measurement to gauge how far certain objects are from the boat or from each other.
The line down the middle of the screen is the boat’s path, the dark blank space either side of the center line is the water column’s vertical depth directly beneath the boat to bottom. Then moving out to the side, elements begin to become apparent. The view takes getting used to, but it essentially represents 90 degrees from directly below the boat out to either side at the preset distance.
Hard bottom and hard objects produce a brighter return while soft bottom shows darker or more subdued. Sonar shadows are important to understand, too. Scanning a boulder, tree, or even a fish will cast a shadow. These shadows help identify an object but can also indicate if it’s on the bottom or suspended. For example, a rock or tree trunk will show the shadow all the way to its base. In the case of a fish or horizontal limb, the shadow will appear beyond the target and better indicate the shape of the object you’re scanning. There are cases, especially with larger fish, when a shadow allows you to identify a fish’s fins, body shape, and even the head or tail. Smaller fish generally appear as bright blobs or dots.
Dial It In The fish schooled on the top right are suspended between 30 and 40 feet, and nearly directly below the boat. It’s important to keep an eye on what depth the fish seem to prefer—a first step to building a pattern. To determine a target’s position, viewing it from all sides improves precision. Drive the boat around the target a couple times and drop a few waypoints in key locations and you’ll be able to position the boat to make a precise cast. That’s when it all starts to come together. Get familiar with your settings and make adjustments as needed to return the most discernible image. A few important settings:
Grid (Range) Lines: Turning on the grid (distance or range) lines on-screen can help you determine how far something is from the boat’s position. As depth increases and objects also become deeper, however, the true horizontal distance to an object becomes less and less than indicated by gridline distance. This is because of the steeper down-and-outward angle from the transducer beam to the object in deeper water—a longer distance relative to that indicated on the horizontal plane. This all takes a bit of geometry (a “deeper” topic for another day), but let’s just say in shallow- to middepths, I prefer to keep grid lines turned off, but in depths of 30 feet or more, I generally turn them on to better estimate the true horizontal distance to an object.
Color Palettes: Several screen colors are available. Some colors work best in sunlight, others better in low light. Choose one that creates enough contrast between the structure and the water column. I’ve even seen some color combinations work better on certain lakes or rivers. Experiment to produce the best return.
Sensitivity and Contrast: If there’s an abundance of “clutter” on-screen, try increasing contrast and reducing sensitivity. Tiny spots suspended up from the bottom are likely fish, and increasing sensitivity creates a more robust return that might give you a better idea of fish size and species. It’s likely these two settings will need to be adjusted by body of water and perhaps time of year. For example, springtime often brings high and dirty water, and during late summer in the North Country an algae bloom typically occurs on many lakes, which usually clutters the top 5 to 8 feet of the water column. Or in the case of clear water, increasing sensitivity and contrast should make the fish and structure pop on screen, which can help in identification.
When scanning brush or trees, turn up sensitivity slightly to help determine if fish are in the structure, as fish body density will produce a brighter return.
Frequency: Just like with 2D sonar, frequency adjustments are worth making in side-scan imaging. With Humminbird, Mega Side Imaging (1,200-kHz imaging frequency) produces a very high-resolution image, but the distance it’s able to view is more restricted than what 455 kHz can see. Mega is a big improvement, but it works best in warmer, denser water, while a lower frequency might be best in colder water. Swapping between several frequencies can help maximize information.
Depending on the time of year, and water clarity, density, and temperature, 455 and 800 kHz will come in handy. Play with sensitivity and contrast to maximize the return. Using 455 kHz for longer ranges, say from 150 out to 300 feet, is a great choice for clear perspectives.
To start, choose a frequency based on depth and range. Try Mega Imaging for shallower depths like 6 to 20 feet; 800 kHz for middepths like 20 to 30 feet; and 455 kHz for depths of 40 feet and deeper. If you want to look deeper, extend the range until adequate bottom images are indicated. Each frequency will require additional settings adjustments based on the lake and water conditions.
Comparative screening is running multiple views at the same time to verify what you’re looking at. Utilizing mapping, down-scanning and side-scanning together, among other technologies, allows inquisitive anglers to garner multiple perspectives at the same time to put themselves in the best position to catch more fish. On the Water The longer I’ve fished, the more time I invest in sonar scouting before ever casting a line. It helps narrow my focus and find the most complex structure and cover in a given system. Being able to idle the boat along a weed break, creek and river channels, and other structural and cover elements like isolated trees and brushpiles will help you make more precise casts when it’s time to fish.
Scan, scan, and scan some more. When I see something of interest, I drop a waypoint, make a quick turn, and run by it again to pinpoint its exact location—sometimes a 360-degree evaluation provides enough detail to start fishing. I’ve found it most productive to find several spots and examine them closely before fishing. On larger southern lakes such as Guntersville or Pickwick , those bass have become conditioned to getting scanned and have been known to contract a case of lockjaw for a short period. Drop a waypoint and return later, they’ll be close by.
Scouting is one of the most important disciplines an angler can master. The more you learn about what you’re fishing and why, the more accurate your casts will be and the more fish you’ll contact. That level of information is not possible without the right technology.
