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."
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.
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 (see illustration).
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.