Both deep water and soft bottoms tend to return a weak signal. In deep water, the signal travels a long way on its round-trip from surface to lake bottom and back to the surface again. As it travels, it weakens and becomes harder to detect, translate, and display. On soft bottoms like muck and silt, much of the signal is absorbed by the substrate. To improve readings over such substrates, turn up the unit’s sensitivity in manual mode to a level higher than the default one the unit’s auto mode selected. In water shallower than 5 feet, the return signal can be too strong because of the short distance it travels. In this case, auto mode may not reduce the sensitivity enough to produce a clear picture. Instead, the whole screen may “gray out.” To adjust, turn down the sensitivity in shallow water or over a hard bottom, thereby providing a narrower cone angle. In deeper water or over softer bottom, increase sensitivity to provide a broader cone angle.

Vegetation provides another set of challenges for adjusting sensitivity. It’s common for fish to hold along edges of weedlines or above the tops of weedbeds. Locators set in auto mode have difficulty handling such situations. If you increase sensitivity to penetrate to the bottom in a weedbed, the screen will be saturated and lack detail. Remember that increasing sensitivity raises the unit’s listening ability, not the locator’s power. When you power up in auto mode in dense vegetation, you get a strong return signal because vegetation is a good reflector, sending back a strong signal that lacks detail because of the multiple surfaces of the plants’ leaves. Your unit is on overload. To reduce its sensitivity around vegetation, use the sonar in manual mode, experimenting with sensitivity settings to learn which works best.

The sensitivity control is the locator’s most important function. Learn how it affects what you see on screen, so the unit works for and not against you. Launch on a favorite body of water and check familiar locations. Position your boat in shallow water, deep water, over hard bottoms, soft bottoms, rockpiles, and around vegetation. See how the unit interprets each of these areas. Adjust its sensitivity settings to see how doing so affects the information displayed. Decide which work best in situations you’re familiar with. Doing so will give you more confidence in your sonar’s capabilities.

Fine-Tuning

Today’s sonars are highly sophisticated—some even include a computer-style hard drive. Faced with multiple buttons and arrows, which lead to layers of menu options, most anglers opt for the auto button. And given that this setting provides serviceable images on most good units, that’s as far as many go. But to achieve the best possible depiction of bottom, bait, and fish, given the computer-like capabilities of modern marine technology, adjustments are needed.