Worms & Boomerangs – Learn To Use A Fish Finder

Over the years of installing, using and conducting seminars about sounders, I’ve heard the same operational questions over and over. So I thought I’d put together a group of the most commonly asked questions to give you a basic overview on how to learn to use a fish finder to find more fish.

I reviewed my seminar notes and recalled a few Q and A sessions from on the water and in the store to come up with the following list of the most asked questions. Compiling the list was easy enough. Explaining the material in a way that makes sense is another matter, but here we go…

1) Basic Operation: A depth sounder or fish finder is nothing more than a sophisticated timing device. Here’s how it works: The display takes a sip of energy from the battery and sends it hurtling down the wire to a crystal in a housing that is called a transducer. The crystal gets all nervous when the energy shows up and it gives off a little click of acoustic energy (about as loud as if you were to flick your fingernails on your thumb and forefinger together).

That short burst of noise is sent to the bottom to reflect back all the life it encounters during the trip down such as fish, kelp, bait schools and the plankton layer on the thermocline. Once it hits the bottom itself, the whole pulse of energy is turned around and sent back up towards the surface. Once reflected, the returning energy is called an echo and the transducer hears these echoes. The acoustic energy is converted back to electrical energy and sent to the display where it is modified, analyzed, amplified and instantly converted into the dazzling display of confusing colors that are so instrumental to the enjoyment of our time on the water.

2) Transducers: These units are the heart and soul of your operation. There are several concepts to become familiar with, and they define the ability of your unit to do what you require. Years ago, we didn’t have to pick and choose as the options available were limited. Today, a basic working knowledge of beam angles and frequencies will let you fine-tune your sounder to your style of fishing.

If you stand straight over the location of your transducer and look straight down, that’s the perspective the transducer uses to see the underwater world. But the energy doesn’t just go straight down, it broadens out into a cone shape, much like an ice cream cone. The shape the energy takes is called the “beam angle” and helps to determine what is displayed on your screen.

The acoustic energy is sent off into the cone-shaped beam at some specific frequency. It used to be just 50 or 200 kHz, but today it can be some combination of 28, 38, 50, 88, 192, 200 or 233 kHz depending on your needs. You’ll see major differences between the different frequencies and this is where things start to get somewhat technical.

The beam angles in the higher frequencies (200 kHz) tend to be very narrow, in the 8- to 12-degree range. This means that if you are standing over your transducer with your meter on, the energy is sent down in a cone shape, about 5 degrees off vertical in any direction. If you are in a 100 feet of water the diameter of the cone on the bottom will be about 10 feet across. All the energy is concentrated and the pulse lengths are short, so you can get extraordinary definition and detail to show what is sitting under the boat. When you see fish being drawn on the screen, they are pretty much directly under the boat.

In contrast, the lower frequencies may have very wide beam angles, as much as 46 degrees in some of the older 50 kHz transducers. Standing on top of one of those units in 100 feet of water, the coverage at the bottom will be about a 100 feet in diameter, almost 50 feet to either side, fore and aft and all points in between. When you see fish on the meter, they can be anywhere in the beam, which may mean they are well away from being directly under the boat. Because there is a huge amount of coverage and since the pulse length is longer in the bigger cone, there is typically less definition with lower frequency transducers.

“The other major difference between the high and low frequencies is depth penetration. In the high frequencies you get excellent detail but only down 300 to 400 feet. In the lower frequencies you can follow a fathom curve in 500 fathoms of water but you will lose some definition. Because of these differences, manufacturers started offering dual-frequency machines to combine the best of both worlds. These now represent the industry standard.”

Beam angles also have a weaker but much broader component to their shape called side lobes. These side lobes are responsible for the large “boomerang” fish marks as well as the long tails under the main bottom trace when the bottom is harder. Why this happens will have to be the subject of another article.


3) Real Time: Your sounder generates the picture on the right side of the screen and sends it left, one line of data at a time. The picture crawls across the screen at whatever speed the advance is set for, and ultimately draws out what was, or is, under the boat. If you are on anchor or tied to the dock, the whole picture shows what’s under the boat. If you are under way and moving, most of the picture will be history, as far as what was under the boat. Remember, only the lines drawn at the far right show the real time of what is directly under the boat

4) The A-Scope: One of the most common questions I get in seminars has to do with what the A-Scope is and how to use it. I can certainly relate to the frustration, as in one manual for a very expensive sounder the factory definition says, “The A-Scope displays echoes at each transmission with amplitude and tone proportional to their intensities.” Some definition! It’s no wonder many users never take the time to figure it out. It’s actually relatively simple to understand A-Scope and very helpful to use.

When you turn on the A-Scope function, you split the screen into the regular display with the A-Scope on the right side. In the A-Scope section, what you see is an expanded version of each line of transmission. The colors parallel the ones your meter is using, that part is the same. In the definition, the word amplitude refers to the width of the display, with the darker colors (stronger echoes) being much wider than the weaker echoes.

With the A-Scope on, two things happen: Since each line is expanded to almost an inch wide, you get instantaneous information about what is under the boat without having to wait as the unit draws out the picture line by line. You get a fascinating kaleidoscope of moving color that is fun to watch no matter what’s on the screen. I love the A-Scope function and use its instant access to real-time data as a constant reference.

5) Colors: Whether your machine displays only 8 or 64 colors, the basic principle is the same, the weaker targets are displayed in the softer blue, green and yellow scheme while the harder targets get into the oranges, reds and dark reds to nearly black. This makes it easy to distinguish between the pale pastel blue of a squid mark and the heavy dark-red display you’ll see from the dense body and large air bladder of a big sea bass.

6) White Marker: Here’s where the color setup gets fine-tuned. The white marker function let’s you manually select one color to change to bright white. I usually select the darkest color since that is the one the machine uses to draw the actual bottom (and the bigger sea bass).

“By turning the bottom into stark, bright white, anything on or slightly above the bottom becomes very easy to distinguish from the actual bottom. This makes the white marker a very helpful feature!”

7) Advance Speed: Most of the newer meters use one of two displays to indicate the movement of the picture across the screen. Some use numbers on a scale to show speed, usually from one to six. The higher the number the faster the speed will be. Others use a fractional display like 1/4 or 1/2 which means, respectively, one line of data drawn for four actual transmissions from the transducer or one line for two transmissions.

Get the display moving too fast (2/1) and the picture becomes too spread out. Run it too slow (1/8) and the picture becomes too compressed to see good bottom and fish detail. I usually run my meter in the 1/2 position for a good representation our underwater environment.

8) Worms: These are one of my favorite sights when sea bass fishing. They might actually bring a rare smile to my face, as its usually just a short time thereafter and the rods start bending! The actual mechanism that spots the worm is simple enough. With the boat anchored and stationary, a slow moving fish that takes up residence in the beam for a long period of time will display continuously on the screen. As the boat rises and falls in the swells, the mark appears to undulate, and it looks like a worm on the screen.

Sand bass on the local grounds often will mark like this, as will barracuda at times. Sea bass make classic, very distinctive worms as they will hang under a boat for long periods of time. Additionally, when they are on the move they often get in a line of several fish, following closely — nose to tail — in a chain.

Every sounder is a bit different and the new machines out there offer even more functionality, but the eight topics mentioned in this article represent the most commonly asked questions I hear. If you want to attend a seminar to get more details and see some great sounder shots, check the schedule on our website, www.PacificEdgeTackle.com.

Well-known California captain, writer, speaker and tackle shop owner, Capt. Mark Wisch has been helping anglers catch fish along the West Coast for decades. As an author, Mark has written three books, including “Between Two and Twenty Fathoms,” “In The Gray” and “Lessons from a Lifetime on...