Using Depth Sounders

Part 2: Beyond The Basics

In Part 2 of this series covering the basics of using a modern depth sounder, I take it a step further and moves into the realm of more advanced sonar signal interpretation.

This DownScan (StructureScan) image from a Lowrance unit shows a bunch of big black bream sheltering under an arching log in just under 3 m of water. I know they're bream because I caught several of them, and I know it's a log because I can just see it when the water is clear enough. It makes sense to combine other observations with what you are seeing on your sonar screen.
Here's another look at those bream under the log, as shown the main image above right. Note from the chart plotter where I've been standing off a short distance north of the log to cast to it..  I caught several and then they stopped biting so I moved in for a look. The way they're hunkering under the log probably indicates the fact that they've wised up to my presence.
A final look at those same bream under the log from a different angle and perspective. Note the shadow of the log on the lake bed and the fact that it is clearly attached to the bottom, while the fish signals exhibit a slight separation. I can count 12 or 13 bream in this image. Can you see them?
Here's a StructureScan image of a submerged fence in an inland dam. Note the weed growth on the wires. I've moved the cursor onto the fence in preparation for recording it as a GPS waypoint. How do I know it's a fence? Because it extended out of the water and up the bank at the other end.
I suspect that this is a submerged water tank or brick structure (it's in an outback impoundment). The large, single target to the right of it is most likely a yellowbelly, but could be a Murray cod or even a carp. Despite what lots of people would like you to believe, it's only rarely possible to be able to positively identify fish species from sonar returns, although you can certainly make educated guesses!
This is how weed beds or aquatic vegetation tend to show up, in both fresh and salt water. A sounder can be invaluable for identifying the edges of this sort of vegetation, which can be fishing hot spots. Those are possibly a couple of smallish fish near the bottom on the far right.
There's plenty going on here, with baitfish schools (mid left) and larger fish underneath them. The elongated signals in the middle of this image most likely indicate active fish moving fairly quickly. What species are involved? The best way to find out for sure is to catch a couple!

In the first part of this series, I briefly described what depth sounders or sonar units are, outlined the basics of how they work, and explained that most new chums and casual users are best off to simply turn their units on, hit auto and let them do their own thing (after first ensuring that the showroom “simulator” mode has been deactivated, of course). For some folks, this will be enough.

However, many keener sonar users will soon wish to begin exploring the wider capabilities of their sounders by tweaking and twiddling the various controls. How these work varies somewhat from one brand and model to another, and I can’t stress strongly enough the importance of carefully reading the manual that came with your sounder, as well as jumping on-line to watch any video clips or tutorials pertaining to it. There’s a remarkable amount of information out there, and a simple search engine query listing the make and model of your sounder will most likely bring up all sorts of useful (and not so useful) links.

I can’t stress strongly enough the importance of carefully reading the manual that came with your sounder!

You can refine this search process further by adding words or phrases such as “sensitivity”, “frequency”, “surface clutter”, “colour line”, “scroll speed” and so on to find out more about those various functions and tweaks, then apply them when on the water. In many ways, this is not unlike tuning a television in order to obtain the clearest and most pleasing picture for your eye.

The best settings to use will vary slightly, depending on the depth of water you’re operating in, as well as the speed of your boat and the amount of suspended matter in the water column. There’s no “one size fits all” answer to the best combination of settings, so you may need to fiddle a bit from day to day or even hour to hour, but don’t become fixated with all this button pushing and forget that you’re out there to catch fish! Also, learn how to return your unit to its factory default settings in case you screw things up!

Learn how to return your unit to its factory default settings in case you screw things up!

Finally, it’s worth noting that sounder technologies have improved at an exponential rate in recent years, and the quality of underwater imaging now available at a reasonable price is nothing short of staggering. Most makes now offer enhanced, high definition (HD) imaging technologies in their premium units under a range of titles such as CHIRP (compressed high intensity radar pulse), StructureScan, Side Imaging, DownScan, SpotlightScan and so on. Basically, these technologies all make use of more powerful and focussed “beams” of sound waves, combined with enhanced imaging software, to paint a clearer, more defined and detailed picture of what’s under, alongside and even ahead of your vessel. In many ways, these modern systems are like narrow-beamed spotlights compared to the virtual floodlights of older-style sonar. It’s really worth bearing that analogy in mind, because there are obviously times and places where the broader (albeit less bright) illumination of a floodlight can actually be more useful than the narrow, confined but intense beam of a spotlight, and vice versa… It’s a matter of horses for courses and both formats still have their place.

I’ve included some StructureScan images with this piece… Be sure to click on them to enlarge the images and read the detailed descriptions. Hopefully you’ll find these screen shots and their accompanying captions describing them “illuminating”, if you’ll pardon the pun!

Any questions about this tutorial?


  1. kmc

    I’m hooked, keep the info coming Steve (sonar for dummies please). Always had trouble interpreting sonar, especially having experienced the early types years ago, seemed like drunken bird tracks in the sand to me. Worth the effort of putting in more study and devoting more practical time on the water with this topic. May even consider upgrading to some of the more new beaut stuff available.

  2. james reeve

    is the next issue due soon just brought a new sounder lowrance hook2 5x gps /splitshot so looking forward to the update and my learning curve

    • Steve Starling

      Hi James. Yes, more sounder stuff is in the pipeline! I reckon you’ll love the Hook2.

Submit a Comment