How to Interpret Your Fishing Boat Sonar | Sport Fishing Magazine

Interpreting Your Fishing Boat's Sonar

Learning to properly use and understand sonar takes time on the water.

school of fish swimming under fishing boat

Learn how to see what's under your boat.

Walt Stearns

I’d like to start this article with a quiz! No pressure: I just have one question, and I won’t be scoring you.

Your fish finder shows a thick red line at 200 feet and a large red blob just above jagged structure that rises from the bottom to 190 feet. Which of the following statements accurately describes this picture:

a. The blob is a fish positioned at about 185 feet directly below the transom of your boat.
b. That blob is the only fish in the area.
c. There’s no dramatic drop-off in the immediate vicinity.
d. None of the above.

While all of those statements could be true, they also could be false. So the answer really has to be “d” — none of them accurately describes the picture. Confused? If so, you’re not alone. Many anglers look at a fish finder as a pinpoint locater device, but what we really see on our display screen is a two-dimensional view of a ­three-dimensional world.

"We always joke that fish swim from right to left in the world of sonar," says Greg DeVries, Garmin's director of marine sales and marketing. But, in all seriousness, he adds that "learning to interpret traditional sonar does require some getting used to."

Understanding Traditional Sonar

For instance, a fish that marks at 185 feet is simply 185 feet from the transducer — whether straight down or at an angle — within the width of the sonar beam. The fish could be ahead of the boat, or to the left, right or behind.

If your transducer shoots a particularly wide beam, the fish's distance from the boat could be 50 or more feet away. It might also be swimming in any direction and at any speed. For a variety of reasons, nearby fish might not mark.

"The cone, or beam, is shooting down in three ­dimensions. The fish can be anywhere in the cone, but the fish finder sandwiches it into two dimensions," says Geonav's product manager, John Luther. "It could show two fish, one on top of the other, when they really could be separated in the cone by many feet."

How much that affects fishing success can't really be determined. But it's important to understand both the advantages and the limitations of sounders as you motor out for a day on the water.

The Cone of Resonance

Sounder makers say several functions common to most units can help anglers better comprehend the picture. In addition, the advent of new recreational technologies such as CHIRP (Compressed High Intensity Radar Pulse) — also known as Spread Spectrum — from Simrad and Garmin, as well as Side-/Down-Imaging from Humminbird or StructureScan/DownScan from Lowrance adds perspective to the underwater view.

Anglers with a traditional sounder and dual transducer (commonly 50/200 kHz or 83/200 kHz) can greatly enhance their understanding by displaying screens showing both frequencies side by side. Reason being: A lower-frequency signal (50 kHz, primarily used in deeper water) generally features a broader/wider cone — 15 to 45 degrees, according to transducer-maker Airmar — and a higher-frequency signal (200 kHz, primarily for shallow water) shoots a narrower cone — 5 to 12 degrees.

Wide- and Narrow-Beam Coverage

wide-beam and narrow-beam transducer coverage sonar signal illustration

(From left to right): 1. A transducer’s wide beam (50 kHz) covers a greater area but provides less bottom detail. In addition, the sonar signal returns as soon as it strikes the highest point on the bottom, creating a dead zone below. 2. A narrow beam (200 kHz) covers less area but o ers better bottom de nition. 3. Using both beams provides the coverage of a wide beam and the precision of a narrow beam. Source: Humminbird.

David Shepherd / Sport Fishing Magazine

For an easy analogy, picture the beams from two different flashlights. One flashlight, with a broad head, throws a wide swath of light across many objects. The second light, with a narrow head, shoots a very concentrated beam, showing great detail in a small area.

"The shape of a [fish mark] will change depending on whether it passes through the center or the edge of either beam," says Jim McGowan, U.S. marketing manager for Raymarine. "It will also change based on the aspect of the fish — for instance, if you're headed north and the fish is going south. Comparing simultaneous images from both frequencies can help you gauge whether the target is closer to the center of the narrower cone (200 kHz) or if it shows brighter out on the lower frequency, broader side."

Using an A-scope or "live" view of the bottom also helps anglers pinpoint fish, McGowan says. A-scope shows instant feedback in the form of horizontal lines when anything appears in the transducer cone.

On Raymarine units, A-scope features multiple modes that can favor the port, starboard or center parts of the sonar beam. If you split the screen to show both low and high frequencies, you can add A-scope views to the display for each frequency as well. (Ray's A-scope also uses the known beam angle and depth to display the width in feet of the bottom area you're viewing on your monitor.)

Remember too that the first object a beam hits graphs the bottom, says Humminbird's marketing director, Mark Gibson. If a wide beam hits the top of a rock pile, it marks that as bottom, "and then anything it hits farther away is shown in the bottom return."

That phenomenon, called target masking, causes a "dead zone" where fish and structure near the tallest object on the bottom will not display. That dead zone could easily mask a productive drop-off, a bait school or the true mother lode of game fish.

Upping the Marine Electronics Ante

Technology does exist now that can improve an angler's fish-finding ability, but much of it costs tens of thousands of dollars ($12,000 to more than $65,000) — a cost barrier only serious tournament anglers and commercial fishermen cross. For instance, Furuno builds searchlight and omni sonar systems that use a deployable transducer that extends and retracts from the hull of the boat.

"It drops out from the bottom of the boat and spins. You can see 360 degrees or as little as 6 degrees," says Steve Bradburn, assistant product manager for Furuno USA. "It's like an underwater radar."

Furuno also builds "slewable" sounders that anglers can tune, dialing in the frequency they want to use. Bradburn says the company expects to release a CHIRP-type sounder soon.

Raymarine also has a CHIRP machine on its drawing board. Currently, its high-end DSM 400 black-box unit ($2,795) supports two transducers across four frequencies — 28/38 kHz and 50/200 kHz — all capable of operating simultaneously. "It marks depths in excess of 10,000 feet; it's pretty close to CHIRP," McGowan says.

Today's Sonar Symphony

The current dazzle, however, comes from Simrad's BSM-2 black-box sounder ($2,495) and Garmin's GSD26 module ($1,999.99), expected in August. Each features CHIRP technology, though Garmin uses the phrase "Spread Spectrum." CHIRP units send pulses containing a whole band of frequencies instead of just one frequency. They literally "chirp" instead of "ping," and consequently a lot of information comes back.

As a complement to those units, Airmar is introducing a full line of broadband transducers with CHIRP abilities; one includes a medium-frequency ceramic — 85 kHz to 135 kHz — commonly used by commercial fishermen, who claim that band captures some species other frequencies don't. The two transducers available at press time included the R209LH/R209LM ($2,995) and B265LH/B265LM ($1,795) through-hull units. Airmar will launch other transducers in different mounting styles this spring and summer.

"The big thing I think this [technology] will do is greatly improve the clarity between the bottom and structure, and the separation of that structure and fish around it," says Garmin's DeVries. "Spread Spectrum allows the transducer to spread frequencies over a single chirp. Spread Spectrum will go a long way for sport fishermen toward getting deeper water penetration with less power, better target separation and superb bottom detail."

The GSD26 sends a pulse over any frequency range available from the transducer. Both the GSD26 and the BSM-2 feature two separate transceivers; one can work with a fixed frequency while the other uses CHIRP/Spread Spectrum.

Simrad's BSM-2 uses low (25 kHz to 45 kHz), medium (40 kHz to 60 kHz) and high (130 kHz to 200 kHz) bands of frequencies, based on the transducer setup. "Traditional sonar sends out a ping that's fairly long," says Lucas Steward, product manager for Navico (the parent company of Simrad). "If there are multiple fish together, all you see is one return. Multiple frequencies separate that out."

This frequency sweep should also show species that might not appear or might appear faintly in a 50 kHz or 200 kHz beam. And while the technology won't resolve the issue of where in the beam the fish resides, it does add detail and provide bottom clarity that anglers can greatly use.

Side-Imaging Sonar Technology

One tool that might help better pinpoint fish is side-imaging sonar, a technology now used by Johnson Outdoors (for Humminbird and Geonav) and Navico (for Lowrance and Simrad). In shallow water, in particular, even a wide sonar beam has a fairly small bottom-coverage footprint. Comparing what's out to each side of the boat — as much as 240 feet left and 240 feet right — with what traditional 2-D sonar shows helps anglers locate structure and fish.

One caveat about using side-imaging sonar for finding fish: Its fore- and aft-beam width is very narrow, about 1.5 degrees. Picture a set of fans emanating from the transducer, aimed toward either side of the boat. One side of each fan touches the bottom and the other might be just below the waves.

If your boat is under way, a fish that swims through the beam fairly quickly might not even register a blip. However, structure such as rock piles or wrecks show up in splendid, photolike detail. Slow down and watch for streaks on the screen to better identify fish.

On-the-Water Sonar Practice

Regardless of what sounder technology you embrace, learning to properly use it and understand what you're seeing takes time on the water. One particular tip that can help: While on a wreck-fishing trip, motor repeatedly over a piece of structure such as a wreck; if it's a wreck you might have dived or snorkeled, so much the better. Approach it from a variety of angles and watch how the sonar returns paint the picture.

Pay attention to the fish marks you see compared with the fish you catch. Patterns should emerge.

Know the beam width of your transducer's pulses so you can determine how much of the water column and the bottom you might be seeing.

Diameter of Sonar Beam at Depth

diameter of sonar beam at depth

Diameter of sonar beam at depth

Chris McGlinchy / Sport Fishing Magazine

Having all the puzzle pieces together can make every day on the water more productive and rewarding. Plus, imagine how you'll amaze your friends.


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