Marine Electronics

How Deep Is That Bass? by Wayne Heinz

Nobody knows. A depthsounder tells us how far away a fish swims, not how deep. If a bass swims off to the side of your boat, at the edge of the depthsounder’s cone, 50’ from your transducer, your depthsounder will paint that fish as a blip, 50’, on your screen’s vertical range scale.

But that bass may be swimming only 40’ deep in the water column. The deeper you fish, the wider the cone, the greater the potential error. A narrow cone transducer reduces the error. Side-scan sonar, and forward-scan sonar try to estimate both distance and depth, via trigonometry. If boat and fish stay perfectly level, the estimate might be accurate. More likely, your boat’s bobbing will ruin the triangle’s math.

Back to down-scan sonar: You stare at a down-scan screen, turn to your partner, and say, “There’s a bass right next to a boulder, 35’ under the boat. Drop your jig straight down.” The boulder’s there, sure. But is the bass? What if that bass is swimming way off to the left, 35’ from the transducer, but only 20’ deep? Where will your depthsounder paint that fish? Right beside that 35’ deep boulder!

This is bad news, I agree. Like a seven year old boy in December who’s hearing dark rumors from his classmates, we want to believe in our depthsounder. After all, that bass might well be by the boulder. Then again….

Let’s look at cone coverage at modest depths – less than 100’. If we assume that only a fourth of the main area of the cone is directly under the boat, then three fourths of the time the fish will be off to the side, and shallower than it shows on the screen.

When you scroll the range cursor to the fish, always think 3-D. Picture an upside-down ice cream cone in your mind. Fish can be anywhere in that cone. Narrow cone – fish likely swims under your boat. Wide cone – fish likely swims off the side of your boat.

Get into the habit of reading the numbers on the vertical scale as, “distance away from the transducer.” Say out loud, “Marking fish, 35’ away.” Think, “These fish are somewhere above 35’.” Then you won’t make the mistake of fishing exactly 35’ deep for 20 minutes without a strike.

Learn more about your electronics. Read, Depthfinders, A Guide To Finding & Catching More Fish, 2015, by Wayne Heinz, available at Griggs, Pasco, and


Similar Posts