Issue: May 2011
NAUTICAL KNOW-HOW: Glancing Sideways

Side-scan sonar is now available in the recreational marketplace. Learning how to read the images requires patience and know-how, but the effort will be highly rewarded.

by John Page Williams

Once the province only of marine scientists, underwater archaeologists and maritime law enforcement agencies, reliable side-imaging sonar is now readily available to recreational boaters from several marine electronics manufacturers. Various companies have tinkered with recreational-grade side-imaging sonar for at least fifteen years, but truly useful units have appeared only in the last five or so, first from Humminbird and more recently from Lowrance. 

As the terminology suggests, side-imaging units scan the bottom out to the sides of the boat and beyond--more than 200 feet out with some units--and then produce a remarkably clear and detailed picture of what lies below. In developing side-scan sonar, companies had to figure out how to cut through material that collects in the upper layers of any waterway: dust, algae, silt and the like. What Humminbird and Lowrance have been able to do is combine high-frequency sonar signals with very sophisticated digital software that filters out the "noise" of the silt and plankton. By doing this and continuously adjusting both signal output and gain (listening capability) it gives us understandable pictures of what the bottom looks like all around us. Both manufacturers also now offer simultaneous high-frequency down-looking views that provide extraordinary detail of what is directly below, whether baitfish, predator fish, wrecks or even restoration oyster reefs.

Recreational fishermen are buying the devices as fast as the companies can make them. But as with conventional down-looking sonar, learning to read them well takes time on the water and good understanding of how they "see." Consider the following story. 

Lovers of the Chesapeake know the late William W. Warner primarily for his classic book Beautiful Swimmers. But in a later book, Distant Water, Warner details his experiences as a visitor aboard Russian, Eastern European, Spanish, and American factory trawlers. In a conversation with the fishing captain of a Spanish trawler, Warner asks the captain to teach him to read the various electronic fishfinders crowding the helm. "First," says the captain, gesturing to a bookshelf of fishing logbooks behind him, "read these. Then you’ll be able to understand how these machines can help you catch fish."

That fishing captain had it right. Side-imaging sonar--and indeed, conventional down-looking sonar--is bewildering, until you spend some time with it in familiar water and begin to understand both. 

The following notes come from a season’s experience aboard my 17-foot Whaler, First Light, running a Lowrance HDS-8 display with not only side-imaging, but also both high- and low-frequency down-view sounders. It also has an LGC-4000 16-channel GPS receiver, and a Navionics Platinum+ chart card. I have not spent any time with an equivalent Humminbird system, but I have fish-savvy friends running them, and they are enthusiastic about the value of that brand as well. My transducers are transom-mounted, positioned precisely to kiss the water without causing undue drag. Getting them right requires reading the directions, placing the brackets--and then fine-tuning them after trial runs.

How It Sees   

Unlike the rounded, cone-shaped coverage of conventional down-looking sonar, both the Humminbird and Lowrance systems sweep a super-thin slice of water from side to side, in a 180-degree arc. The Humminbird system uses four transducer crystals and the Lowrance six, resulting in views straight down, down at an angle and out to the side. In the display unit, sophisticated software processes the signals, synthesizes them and displays the results on the LCD screen. Both brands allow splitting the screen for a high-frequency side view, an 800 kHz (kilohertz) high-frequency down view and a 200 kHz "conventional" down view. Lowrance also offers a four-panel display that includes the GPS chartplotter.

Note that the three sonar views "see" roughly the same water, but their scales are not exactly equivalent. You’ll also find that the high-frequency sonar software in both brands has the capacity to "shade" what it can’t see, like the underside of timber or rocks, producing an image that our eyes perceive as three-dimensional.

All of this visual material can be confusing. The first step in cutting through the confusion is spending time on the water in a place you know reasonably well--running the chart plotter and the conventional 200 kHz sounder together, so you become familiar with specific bottom features, including lumps and bottom hardness. (A detailed chart card like the Navionics Platinum+ shades changes in depth, so our eyes "see" three dimensions there too.) 

While on the same spot, switch to the three- or four-screen view and carefully note what the two high-frequency views display in relation to what you see on the conventional view. Gradually, you’ll begin to connect the conventional color images with the down scan, which will display much less "noise" while providing a highly detailed image. Meanwhile, the side-scan image will show you where features like bait schools are in relation to the boat.

By the way, don’t neglect to look at the water around you and pay attention to what you feel when fishing on the bottom. Visual cues like ranges (lining up two objects, like a navigation marker and a house) and tactile clues like a jig bouncing on oyster shells all add to the information your brain absorbs about a particular location. After living on the Severn River above Annapolis for more than 35 years, I have been amazed at how much new information I have learned about my home waters with this electronics system. 

The following examples are accompanied by screen shots with my notes on what they depict. (And yes, I turned off the GPS/chart panel on purpose, because a couple of these images display hard-won sweet spots.) 

Bait Ball

For openers, note the exceptional detail that the down-scan shows of this school of young menhaden. The densest part of the school shows a lighter color on the high-res down scan and red on the conventional sonar, indicating strong echo reflections. While the conventional system shows a typical blob, the high-res down-scan actually shows individual fish, even though they are only five to six inches long. Note also that the side-scan shows baitfish on each side, indicating that the school is directly beneath the boat.

Pilings on the Naval Academy (Rt. 450) Bridge 

Bridges normally show little detail on conventional sonar, because of the way the cone of sound contacts the whole piling. Note the quality of the high-res down-scan detail on the pilings. The side-scan image shows the pilings on each side of the boat. Note from it that First Light was closer to the piling on the starboard side than the one to port. (The range function was on automatic and somehow picked a very wide 120-degree sweep.)

Naval Academy Seawall

The large rocks placed along the Academy’s seawall at the mouth of Spa Creek form great habitat for eating-size white perch. Here the colors on the conventional sonar and the brightness of the high-res down-scan indicate strong echoes. The side-scan indicates that the rocks extend well off the wall, adding even more fish habitat than is visible at the surface (and indicating danger for any deep-draft vessel that strays too close).

Railroad Bridge

The Baltimore and Annapolis Railroad crossed the Severn on a wooden bridge from 1887 until 1968. Contractors removed most of the abandoned bridge in 1986 because it had become a hazard to navigation. They pulled out as much timber as they could and cut off the rest at least 10 feet below the surface. Even so, a look at any sonar will reveal a tangle of lumber on the old railroad bed, which itself rises about five feet above the surrounding river bottom. Today, it is one of the most productive reefs in the river, with big perch, rockfish to 30 inches and some of the largest oyster toadfish in the Chesapeake. 

Years ago, the late "Perch Professor" of the Severn, Captain Bill Pike, taught me to fish the bridge ruins with grass shrimp. He was a master at "taking marks," lining up visual ranges and double-anchoring to position his skiff over his favorite part of the bridge--which my Navionics chart card revealed last year to be on the base of the old rail bridge’s turnstile. He also understood that current was an important factor both in positioning his skiff precisely and in the fishes’ feeding behavior. 

Today, I still use Captain Pike’s marks to fish the bridge, integrating that information with what I see on the fishfinder and what I feel at the end of my line. True to form, this screen shot shows the bridge rubble, the roadbed and fish (perch) on both the side scan and high-res down-scan screens. The conventional sonar displays them too, but it cannot separate the individual returns nearly as well as the high-res.

The bridge remains extend completely across the Severn, but some places are both more productive and easier to fish without hanging up than others. Fishing with Bill Pike, I’ve seen a distance of as little as two feet make a difference between lots of bites and nothing. If you find one of those places, keep it to yourself. You’ve earned it. The same goes for any other bridge or wreck anywhere in the Bay. These systems can help you find them--and return to them.

Deep Flat with Rockfish

This high-res down-scan shows a deep flat out in the middle of the river where rockfish congregate in the late fall. Look closely at the side-scan and you’ll note that they are directly under the skiff, so they show on both sides. That’s about as fine a scenario as any angler with a light tackle jigging rig could want to see. The pattern on the conventional sonar screen is typical of such fish, so it is also a useful sight to search for. Note the yellow (strong) echoes that indicate that many of the fish are holding belly-to-the-bottom. That’s a good clue for jigging tactics.

If you can’t spring for a big high-frequency down- and side-scan sonar system right now, don’t despair. The Lowrance unit is a $699 add-on to the basic HDS units. Buy it when you can. Lowrance ( also now offers smaller, less-expensive sonar units with StructureScan capability. Humminbird ( also offers smaller units with side- and down-scan imaging. 

Whatever you choose, remember that the only way to get the most out of sonar and GPS is to use it. Hone your skills by spending as much time as you can fishing with, exploring with and learning your sonar/GPS system. I know that’s a tough assignment, but as you gain experience, you’ll be delighted with the way your brain begins to integrate the images.