Marine Electronics


Do you know what your fishfinder is trying to tell you about bottom composition? 

If you’ve been fishing long enough to end up here on the pages of BD Outdoors and you’ve spent some time perusing our Marine Electronics section, we’re guessing you already have a decent level of competence when it comes to reading a fishfinder. Yet there are a few aspects of using fishfinders which are misunderstood by many anglers. One of them is reading bottom composition. 

Getting over the right bottom type can make the difference between hooking ‘em two at a time, or going without a bite. 

Hard Bottom, or Soft? 

One of the key features of bottom composition that has a huge impact on which species frequent which areas, is hardness. In some places oyster beds or rocks pull in the fish, and in others finding pockets of sand or mud is what does the trick. Whatever may be true in your neck of the woods, the big question is, will you know it when you see it? 

The visual display of a hard area can be misleading, since softer bottoms can absorb the sonar waves and appear thicker than hard bottom spots. Imagery by Raymarine. 

Part of the reason so many people misinterpret what they see on-screen is that it can be counter-intuitive. Since sonar waves can at least partially penetrate some substrates, like soft mud, it can create a very thick line at the bottom of the fishfinder screen — which the human brain interprets as a “big” return…so the bottom must be hard, right? But the exact opposite is true. If those sonar waves were pinging against bedrock, the line would be narrower. And critically, displayed in a hotter color. 

The best way to judge bottom composition with a modern machine is by forcing yourself to ignore how “big” the return seems to be, and instead look for the hotter colors like reds and yellows. And when you see that thin red line, you know you’re over something that’s rock-solid.  

What about telling the difference between rock and shell? Or locating rocks surrounded by hard sand? Now it’s time to start looking at that bottom line itself. Areas of isolated rock will almost always extend out to either side under the bottom line, beyond where they’re exposed. You (and the fish!) are just looking at the tip of the iceberg. So, when you go over rock in most cases you can see the shadow of its edges to either side of the main return, under the bottom. When you’re looking at a blue bottom line capped with a band of yellow and see a yellow streak beneath bottom that’s coming closer and closer to the top, you’re looking at the edges of the rock underneath the sand, shells, or mud. Red just popped through the bottom line? There’s the tip of your underground iceberg. 

The Best Tech for Determining Bottom Type 

Furuno’s Bottom Discrimination color codes different bottom types to display mud, gravel, sand, and rocks. Imagery by Furuno. 

A few years ago Furuno introduced Bottom Discrimination, which remains the best tech around for judging bottom composition — because you don’t have to judge a darn thing. Bottom discrimination turns the returns into color-coded graphics that go beyond the usual: deep red is rock, but light red indicates gravel, tan is for sand, and gray is for mud. It can also break down the composition into percentages (if you’re going over 10-percent gravel and 90-percent sand, for example), and can color-code your boat’s track on the chartplotter so you can see just where you’ve gone over the rocks versus sand versus whatever. 

Scanner-style views are two-tone, and it can be tougher to determine bottom type with some pallets as opposed to others. 

With modern scanner-style fishfinders, determining bottom type is a whole different ballgame. Although they all offer different color palettes, the views they provide are in two tones just like black-and-white. However, the tones you choose can make a difference in how easy it is to see the differences in bottom composition. As a general rule, most people feel that on most machines using a blue or purple palette makes it easiest to differentiate. But, not everyone agrees. Different people find different palettes easier or harder to interpret so if you use a scanner, the best thing to do is go to an area where you know there’s a mixed bottom, start off with blue or purple, and cycle through the different palettes until you find which one you like best. 

Bottom hardness is also showing up in some digital chartography, and with the advent of self-charting, in the community layers in some charts. Both C-Map and Navionics have their own ways of displaying it, however, they don’t differentiate between bottom types beyond hard, soft, and in some cases vegetation. On top of that, most of the coverage for these is freshwater-based. At the moment it won’t do much for you salty guys, but as the tech and the coverage expand, we’d expect to see more utility for saltwater anglers moving forward. 

When it comes to interpreting bottom composition on a fishfinder, as with all things fishing, there’s no substitute for experience. Spend more time on the water and sooner or later you’ll feel the different “ping” of your spoon connecting with rock as opposed to the “poof” you feel when it lands in mud, and a look at the fishfinder as you feel each will help develop your ability to know which is which at a glance. 

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