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Whether you run a center console in search of stripers or bass boat looking for largemouth, side-scanning sonar will help you catch more fish. Side-scanners look off to either side of the boat to help you spot structure, schools of fish, and even individual fish hundreds of feet away. And while side-scan may not be as new or detailed as real-time imagers (which allow you to point a fish-finding beam in a specific direction to see a video-like view of what’s beneath the water’s surface), it doesn’t require manipulation of a trolling motor to aim its beam, covers far wider swaths of water, and has significantly more range.
What is Side-Scan Sonar?
Side-scan sonars utilize multiple transducer crystals to send fish-finding beams 180-degrees to the port and starboard sides of a boat simultaneously. These beams take constant snapshots of what they “see” on either side of the boat and the unit’s computer brain stitches them together one after the next to build a moving, constantly updated visual representation of what’s off to either side at any given time.
The below image shows you the picture; the white line in the middle of the screen represents the boat’s path, and the dark section running down the center is what the sonar beams see in the water column before hitting bottom. Then, the orange areas to the left and right show what they see after reaching bottom and spreading out in either direction.
In the dark section in this screen shot you can see some fish are below the boat; the sonar beam has picked these up prior to hitting the bottom.
Side-scan comes in several different flavors from several different manufacturers, each with its own strengths and weaknesses. The critical factor to keep in mind is frequency. The higher a sonar’s frequency, the more sensitive it is and the better detail it can provide. However, the higher the frequency is, the more limited the range becomes. As a point of reference, traditional down-looking fishfinders with beams that ping at around 240 kHz generally have a range of 600’ to 1000’ depending on conditions. They don’t have very good detail, and provide low-resolution images. Most manufacturers’ side-scan units utilize something closer to 450 kHz and have a range of a few hundred feet, but with much better detail. Some go as high as 1000 or more kHz and may only be able to see 150’ to 200’ (again, depending on conditions), but these have shockingly good detail.
Detail levels at very high frequencies are utterly spectacular—you can make out every branch on a tree and a school of relatively small fish hovering around those branches. Image courtesy of Humminbird.
Some of the more advanced side-scanning systems give you the ability to vary the frequency depending on the situation, conditions, and your priorities. They can be set to a lower frequency like 450 kHz for viewing broad swaths of the water when searching for fish or structure, then set to higher frequencies to gain more detail when searching for large fish among bait, or when probing structure for fish.
Finding Fish with Side-Scan
One of the biggest advantages side-scan gives an angler is the ability to do more catching and less blind casting thanks to the ability to spot fish hiding among structure over broad areas. In a grove of standing timber, you no longer have to cast to every tree to figure out where the fish are concentrated. Instead, you can drive through the grove until you see fish around a particular tree, just like in that image above, and then focus on it.
Another big advantage you can harness with side-scan is the ability to find schooling fish in open water, a particularly effective tactic when predators chase bait to the surface and may be visible to the naked eye for brief periods, but then disappear for long stretches of time. Doing so is fairly straightforward: simply set range as far as the depth and conditions allow, and cruise through likely areas as fast as your system can handle. Just how fast that is will vary from boat to boat depending on the hull bottom and transducer installation. Some boats can only go a few mph before interference from turbulence and air bubbles ruins the view, and other boats can travel at planing speeds while retaining a clear view.
Although lower-cost systems and/or small screen sizes won’t allow for it, screen-splitting is another critical element in finding fish. Having a traditional down-looking sonar visible at the same time as the side-scanner allows you a better view of what lies directly underneath the boat, since the fan-shaped beams emitted by the side-scanner will miss some portion of the water directly under the transducer (just how much will vary by depth). And having a chartplotter visible at the same time as the side-scanner will allow you to look for charted structure that often attracts fish, like channel edges, and also helps you keep track of your exact position when you spot something that looks like it’s worth further investigation.
When you’re trying to find fish, screen-splitting (with systems that allow for it) is often more effective than dedicating the entire screen to the side-scanner.
Also remember that on most systems you can save waypoints over structure you’ve spotted off to the sides (with or without the chartplotter screen up in a split) simply by tapping the screen or moving a cursor and pressing a button. This is a very simple tactic that can boost your catch by a wide margin, but it’s often overlooked. All but the simplest side-scan systems track GPS coordinates as they take their rapid-fire snapshots off to the sides, so you can put a mark on a return of interest hundreds of feet away, then press “Go To” on the chart plotter, drive over to the mark, and take a cast or investigate it more closely.
Advanced Tactics for Fishing with Side-Scan
There are a number of more advanced techniques and tactics that will take some time and experience to master, but eventually will help you put more fish in the boat. The most critical is learning how to modulate contrast and sensitivity. Contrast and sensitivity go hand-in-hand when it comes to maximizing detail at different ranges, and especially when it comes to maximizing range and still being able to spot fish. There’s no one-size-fit-all piece of advice as to how to set them, however, because the best combination will change depending on water conditions, boat speed, the size and type of fish you’re seeking, and depth. As a matter of practice, however, you should plan to raise and lower each setting until gaining the best view on-screen in any given situation (in different depth ranges, when water turbidity changes, etc.) As the situation changes, you’ll have to constantly re-adjust.
Sharpies who are very experienced will incessantly dial contrast and sensitivity up and down, but most people newer to side-scan will more or less leave these settings alone until they have a season or two under the belt. Then when they’re comfortable with the basic operation of their unit, they’ll begin experimenting. It does take practice, but try modulating these settings and you’ll soon discover that you’re able to boost the range and maintain a detail level that you previously saw only at shorter distances.
One of the tricks that’s most difficult to master is learning to judge the depth of returns off to the sides of the boat. Fish (or any other items) protruding above the bottom cast a visible shadow, and you can use this to make a determination as to their depth. Notice how the fish in this screen shot show up under the boat (green circle) and their shadows appear on the bottom far off to the right (blue circle).
The distance a shadow is from the return gives you a clue as to its depth. Speckled-sea-trout – The author used side-scan to spot a school of fish 100’ away in 20’ of water, began casting, and reeled up dinner.
The farther a fish is from its shadow on the screen, the higher in the water column it is. Fish that are sitting on or very close to bottom will often cast a shadow that appears to be connected to the return itself, as will structure sitting on the bottom. But the higher off the bottom something is the more space there will be between the mark and that shadow.
What makes judging the target’s exact depth so difficult is that how much distance appears between the return and the shadow will change with overall depth, the scale of the screen, and how far the return is from the boat. Thanks to all these variables there’s no easy formula to use to nail down any specific return’s exact depth. However, over time a user can get proficient at judging whether a fish is sitting deep, at mid-depth, or near the surface, with a glance at the return and the shadow it casts.
Judging bottom composition is another technique that seems like it should be simple but in reality can be tough to do with confidence. On a standard fishfinder the strongest returns would show up as red, while a side-finder will show stronger returns as brighter versus darker. Hard bottom and structure like oyster shell or a rocky outcropping will be brighter than a soft mud bottom or weedy bottom. But again, there are complicating factors. Remember how we talked about constantly tweaking sensitivity and contrast to get the best views? This will also affect the brightness of the returns. So, you have to judge the brightness of one item or swath of bottom versus the brightness of the other things you’re seeing on screen at any given time.
Yes, we know this is a lot to keep track of. But side-scanners have become so popular that you’ll see them offered on most of the best fishing boat brands being built today. They’ve made serious inroads on everything from aluminum bass boats to offshore fishing boats. You’ll be hard pressed to find a pro who fishes without side-scanning abilities regardless of the venue or the target species. And the reason is simple: these things help you catch more fish.
