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April 10, 2014

Side-Scan Strategy

Fine-tune your view of the underwater world to catch more fish.

Side-scan sonar has the ability to image structure and fish
well to the sides of a boat and in the water column below with far more clarity than traditional sonar. Humminbird’s Bill Carson, who has more than 1,000 hours using side-scan under his belt, explains it.

“Think of a football field under 30 feet of water, and you want to locate structure on the field. With traditional sonar, you can image 10-foot swaths of the bottom with each transit: All you see is what is in the transducer cone at that depth. With side imaging, you ride across the field on the ­50-yard line and see everything from the surface to the bottom, from goal post to goal post, in a single pass with greater clarity and greater target discrimination.”

Leap Ahead

Side-scan-image clarity is a result of higher-frequency sound waves transmitted in thin slices like an MRI. “Most traditional sonar units transmit at 50 and 200 kHz, while side-scan works at 455 and 800 kHz so you get more data back,” says Navico’s Matt Laster. “At 455 kHz the unit offers more range, while 800 kHz provides sharper image quality at ranges under 120 feet.”

The screen images are startling. A school of bait that’s a red blob on traditional sonar shows up as individual baitfish, perhaps with larger game fish in or below the school. It can reach out a few hundred feet to either side, which helps determine how large the school is. Game fish that look like arches on regular sonar look like fish on side-scan. The difference is even more apparent with structure. Large wrecks are identifiable right down to the fish around them, and when it comes to identifying smaller objects and low-lying structure, it’s otherworldly. 

Wrecks Revealed

Once I became accustomed to this different view of the world beneath the surface, I went hunting. I’d been collecting “numbers” for years, but some wrecks avoided detection.

Decades of deterioration and shifting sand had reduced their profile to the point that standard sonar simply didn’t see them. I started going to numbers ­— or the “PA” (position approximate) wreck icons on my chart plotter — for a new look. I’d zoom in the plotter screen, turn on the side-scan, and run an overlapping search grid. Each time I saw something, even at the limits of the range, I’d save it as a waypoint, then go back and scan it close up. The first day out, I found a schooner that went down in the 1800s in 60 feet of water. All that was left was the outline of the hull on the sand and some debris. You wouldn’t see the low relief with traditional sonar, but it’s a great spot for fluke and sea bass.