Part I: Depth sounder basics
  |  First Published: November 2014

These days, very few boat fishers venture out in search of angling action without a depth sounder, sonar unit or fish finder fitted to their vessels. Even kayak and canoe enthusiasts are increasingly relying on sonar technology to help find fish. But if you’re one of those people still struggling to interpret what you’re seeing on your sounder’s screen, then read on.

Modern recreational depth sounders evolved from the SONAR (Sound Navigation and Ranging) equipment developed during the First and Second World Wars to help surface ships detect and locate enemy submarines. These technologies have come a long way in a century, and the calibre of entry level sounder available to recreational anglers nowadays would’ve completely amazed those wartime destroyer captains!

Today, very few boat fishers head offshore, or even onto our estuaries, lakes and rivers, without having some sort of sounder unit on board. These tools are now viewed as an almost essential piece of kit, with base units priced keenly enough to fit almost any budget. Yet I constantly encounter anglers who struggle to make sense of what their sounders are telling them. Hopefully, this series of entry-level columns will help those people.

Without going into all the technical details, sonar works by transmitting sound pulses into the water, then listening for their return as they bounce off objects such as the sea bed, rocks or fish. By measuring the elapsed time between the transmission of the sound pulse and the return of any reflected echoes, a depth sounder can very precisely measure how far away those objects are. Most modern sounders then use this information to illuminate pixels on a screen, providing a graphic interpretation of what lies below.

Modern sounders are incredibly sophisticated and well-developed pieces of equipment. Rather like state-of-the-art cameras and computers, most casual users will only ever explore (or need) about a quarter of the potential power, functions and capabilities of their sounders! In fact, just as with modern point-and-shoot cameras, in the vast majority of cases, these casual users are best served by simply turning the sounder unit on, setting it to auto and letting it do its own thing!

So long as it has been installed correctly in the first place and you’ve remembered to switch off the showroom ‘simulator’ mode (a trap for young players!), the sounder should do a perfectly adequate job of telling you how deep the water is under your hull, showing you the basic contour of the bottom, and alerting you to any significant objects in the water column under the boat, all without you ever needing to push another button (except the ‘off’ switch when you get back to the boat ramp).

At the most basic level, which is what most of us want our sounders to do most of the time: tell us the depth, show us the bottom and give some kind of indication of the presence of any ‘targets’ (which may or may not be fish) between our boat and the seabed. That’s really about it! So, don’t get carried away or confused by all of the other stuff you read about and see on TV or in DVDs. Start with those basics, study the images (carefully read the captions) and you’ll already be well on the road to better understanding your sonar or depth sounder! Next time we’ll move on to some slightly more advanced stuff.


Try to mount your sounder where it’s reasonably easy to see, both while your travelling and also while you’re actually fishing. If you spend a lot of time standing up in the front of the boat while fishing, consider placing the unit there instead of on the console or at the helm — or fit two units if you can afford to.


Here’s a very basic, un-tweaked sonar image. It shows the bottom, a little less than 6m below the boat, and a blob of ‘something’ in mid-water, plus some smaller possible scattered targets. The messy blue stuff near the top is just clutter and interference. All we can know for sure from this image is that the water is just under 6 metres deep, the bottom is pretty flat and that the boat has passed over something reasonably solid in mid-water… but that is actually a lot of information!


In this screen image we can see that the bottom is slowly rising as the boat travels along (the freshest part of the scrolling image is at the right). The water is currently around 12m deep under the boat. There are also half a dozen or more ‘things’ in mid-water and they may well be fish. As a bonus, the sounder is telling us that the water is 15.7 degrees Celsius.


Now things are getting interesting… The water here is 5.6m deep (we can see that from the large, numeric readout at top left of screen, and also the hard line separating the yellow section at the bottom of the screen, which is the lake bed, from the rest of the image). But there’s a stack of ‘something’ between the boat and the bottom too, some of it extending up to 2m into the water column. This might be weed, timber, small organisms such as bait fish, or larger fish. Next month we’ll look at working out which option is more likely!

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