Using Depth Sounders

In this and the next part of our ongoing series about getting more from your depth sounder, we present a few more random sounder screen images captured from our Lowrance HDS units and offer our interpretations of what they might possibly reveal.

The image above and at left (or possibly below, depending on the device you’re using), shows what a submerged tree looks like on Lowrance’s StructureScan. This is a fallen tree in a southern estuary. You can see that it’s loaded with small to middling fish. The ones higher up in the water column are most likely mullet (it was taken in early May, when the mullet were schooled up prior to running to sea, and we could also see a lot of mullet milling near the surface). Some of the targets down deeper, near the bottom, are almost certainly bream, estuary perch and/or blackfish. However, we failed to pull a single fish off this snag, or even register a hit. Just goes to show that seeing/finding them and CATCHING them are two entirely different things!

The Lowrance StructureScan image shown at left (or possibly below, if you’re on a smaller device) shows how well DownScan captures the “texture” of big boulders, giving an almost 3-D look to the image. The “grains of rice” or confetti scattered through the image are small fish. You can make an educated guess about their size by examining the vertical scale. Have a look at the length of a metre on that vertical scale… Then think about how long each of those little fish signals is in relation to that vertical metre measurement.

My guess is that the fish are very small: just a few centimetres long. This is confirmed by what I know about the real-life situation. This image was captured over submerged boulders at Wyangala Dam, in central western NSW. Looking down into the fairly clear water, I could actually see schools of little baitfish: smelt, gudgeons, etc. I’ve included a shot (at left or below) of one that ended up in our shrimp trap on that trip. It was about 3cm long. Fish this small don’t always show up on a sounder, but in calm, relatively clear and still freshwater scenarios, they often do.

There are a few messages here, but the most important, in my opinion, is not to base your “reading” of the situation solely on what is displayed on the sounder screen. Take other items of intel’ into consideration as well. Be situation aware…

The next two images at left (or below) demonstrate the difference in signal returns between sonar (the left half of each image) and StructureScan (the right side of each image).

It would be easy to interpret the “targets” on the sonar signal as some sort of structure lying on the bottom, or a dense weed bed. However, the narrower, more intense and clearer StructureScan image clearly shows that they are fish.

Have a look at the vertical scale and see if you can make an educated guess about the likely size range of the fish. This can be tricky if the fish are moving or active (or the boat is moving), but in these instances both are fairly static, giving a good indication of the size of the targets.