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October 03, 2012

Side-Imaging Sonar

Can side-viewing sonar help anglers see and catch more fish?

Side-scan, or side-imaging, sonar has always intrigued me. Perhaps that’s because I’m a scuba diver, and I’ve experienced the underwater world in three dimensions. So when I try to wrap my mind around what side-scan “sees,” I picture myself diving beneath the boat.

I imagine how a sonar ping, focused along a sharp edge — like a slice through the water column on either side of the boat — would return a picture of structure and fish. Humminbird marketing director Mark Gibson puts it this way: “Say you’ve got a handful of BBs. Using traditional sonar is like slamming those BBs on a concrete floor. Imagine how many of those will bounce back up — lots. Side imaging is like throwing BBs down a driveway. Imagine how few would come back; maybe only one or two return toward you from hitting a rock.

“Side imaging is all about how sensitive the receiver is, how it filters out noise.”

Structural Integrity

That issue of returns speaks to why Humminbird and Navico (Lowrance/Simrad) emphasize finding structure, more so than fish, with their ­recreational side-scan units. Humminbird offers a range of multifunction displays with side imaging included ($1,049.99 to $2,799.99); Navico offers its StructureScan as a black-box add-on ($599) that pairs with many Lowrance and Simrad MFDs. However, that doesn’t mean anglers won’t see fish, particularly large schools of baitfish.

“Where StructureScan really shines and makes me look like a champ is finding bait,” says Capt. Kevin Beach (504‑451‑3886, tmgfc.­com), a Venice, Louisiana, offshore guide who uses a Simrad NSE display with the original StructureScan module that first debuted in 2009. “From now until October, we’re fishing almost exclusively threadfin herring [for tuna]. They like the sand bottom, and they’re not ­generally structure oriented.”

Beach also can see cobia milling about the legs of offshore rigs with StructureScan. But because recreational side-scan is really limited to depths shallower than 300 feet, its uses offshore are more specific to bait, shallow-structure and upper-water-column species.

Florida captain Sam Heaton, Humminbird field promotions manager, uses side-scan offshore and inshore. “I use it inshore anytime I look around bridge pilings or docks or under boats. Structure has a shadow [that’s adjacent to it on the bottom]. You read that shadow to determine how high the structure is. Anything suspended doesn’t have as big a shadow on the bottom — that’s the way you tell fish from structure.”

Individual fish show as white streaks, as long as the sonar beam passes them lengthwise. If the fish is facing the beam, it shows as a dot. In addition, a hard background of rock appears white on screen; fish against that background just blend in.

The same distance and direction issues found with traditional sonar translate similarly with side-scan. “If you see a target on screen and it says it’s 40 feet to the left of the boat, you don’t know if that’s 40 feet out on the surface or 40 feet straight under the port side of the boat,” Gibson says.