Interpreting your display means more fish

In this pair of images, part of the bottom drops and another area gets shallower (shown in red).

Color Units: Color sonars add another dimension that significantly improves an angler's ability to differentiate echoes coming from the sides, from echoes below the boat. By brightening and changing the color of strong signals, these units offer better clues. Faint echoes in the weakest color range are likely out in front, off the side, or to the rear. Faint returns also are most likely to be shown well below their actual depth. For all but the most casual anglers, color units are worth the extra cost.

Experienced anglers can use the apparent depth change of these more distant echoes to home in on schools of shad, gamefish, or cover objects not directly under their boats. The echo from a large and highly reflective object 30 feet to the right of the sonar can come back with the same strength as one from a weakly reflecting fish 30 feet directly below the boat. Both appear to be at 30 feet. Approaching a weak echo will move it upward in depth, however, and perhaps brighten it, while moving away adds apparent depth and fades the signal.

Each manufacturer has its own definition of "tear-drops" of ­coverage, filtering criteria, and factory settings, so each vertical sonar unit has unique ­characteristics. To better identify these differences, sonar users should make tests of their coverage with varying gains and intensity settings, perhaps also varying chart and ping speeds as well as the split-screen and enlarging features.

Sonar Tests

Starting in 2007, Dugald McMillan, a veteran deep-water angler from Georgia, conducted tests with anchored balloons suspended at various depths. He photographed and studied images as a sonar unit approached, passed over or near objects simulating fish, then moved away. His images show the formation of arches.

They also show that small balloons created echo signatures similar to 12- to 18-inch bass, and how much distortion in depth measurement occurs when the outer or forward and trailing edges of the zone of sonar ­coverage first contact any reflective object. He also found that in a lake full of shad, crappie, and bass, balloons functioned as fish shelters, collecting bass within just a few hours.

Several of his sonar images accompany this article. Others are available on his posts on the Bass Fishing Home Page, wmi.org/bassfish/bassboard/fishing_tactics as "LTBama." His images from Lowrance and Humminbird vertical color sonars illustrate differences in performance, and the effects of varying intensity, chart, and ping speed on their respective displays. These posts and images can provide new sonar insights and show what you could learn about your own unit by building a test rig.

Ralph Manns, Rockwall, Texas, is a veteran contributor to In-Fisherman and Bass Guide. A certified fishery scientist, he consistently provides breakthrough information on a variety of fishing topics.