Finding fish

Al McGlashan | VOLUME 27, ISSUE 4

Being a good fisherman these days is all about being in control of your electronics. Getting the most out of technology will give you the edge.

To the uninitiated, the sounder screen looked like a TV with bad reception, but to all of us on Strikezone it was pure bliss. Through the array of green and red dots we could make out a huge red patch of bait, while above it three very distinct dark red marks stood out – marlin.

I pressed the Man Overboard (MOB) button on the plotter as I pulled the Honda into reverse, allowing the live baits to sink down. The air was electric with anticipation as we all watched and waited. Then, seconds later, the line on the left rigger snapped free. A split second later, the other rigger followed suit. I raced for the short bait as the crew attended to the other two and ripped it in just before the third marlin could nail it (I needed to film, not catch them!). The circle hooks found their mark and we were into our second double hook-up of the day – woohoo!

It doesn’t seem to matter what I fish for these days, I am finding I rely more than ever on my electronics to help me find my quarry. My fishfinder has become my eyes underwater, while the GPS gives me precise positioning. Together they are paramount to my success. I would have never known those marlin were there if I didn’t mark the bait school on the GPS the day before and then come straight back to the same mark and spot them on the sounder. It doesn’t matter whether you are offshore drifting for yellowfin tuna or hunting snapper inshore, without a fishfinder and GPS you’re simply not in the running.


With more than two thirds of the earth’s surface covered by water, finding fish can be akin to finding needles in a very big haystack. There is a lot of water to cover out there, but to make matters worse the fish aren’t spread out evenly. Inconveniently, they are found all over the place, but they generally congregate in productive pockets of life. Finding these pockets is a lot easier if you have the right electronics and, most importantly, know how to use them correctly.

These days I seem to be spending more and more time transfixed to the sounder and GPS as opposed to searching the horizon for signs of life. In fact, I have to admit that a little while ago I had an electrical fault and my sounder wasn’t working so I just turned around and went home. For me, given that I make my living fishing, it was like being naked in public!

The first sounder I saw was very basic. In fact, it printed out the results of its scans on a scroll of paper. Not only was the unit very expensive, but I had to pay for the paper as well. It is certainly a far cry from what we are using today, with high-resolution colour units that can distinguish a marlin from a minnow.

But having a fishfinder is one thing; knowing how to use it correctly is a completely different story. The modern sounder, like my Furuno, is so advanced that it can even distinguish between different seabed terrains as well as identifying bait schools and individual fish – if you know how to read it. A skilled skipper can even distinguish between leatherjackets and schools of slimies.


The best way to learn how to get the most out of your sounder is firstly to read the instruction manual. Now, while this goes against the psyche of most males, I have to say it is still actually very good advice. Think about it – if you invest a few thousand dollars in a decent unit then you owe it to yourself to know how to use it properly.

Once you are familiar with your unit, the next step is to get out and use it on the water. Admittedly, to begin with I had the advantage of being able to ‘cheat’ when it came to reading the images on the screen. Running underwater Towcams in conjunction with the fishfinder really helped to give me a jump-start on how different fish show up. What I saw on the sounder I could verify on the video screen so that I soon came to understand exactly what the images on the fishfinder actually were.

While I was able to use other technology to help me understand the images on my sounder, the reality is that way too many anglers know little more than how to turn their units on and mostly use Auto mode. The problem is that they are really only using a fraction of the capabilities of most modern sounders and I find that many anglers need to develop their skills to take full advantage of the technology now available. Anglers really do have to get seriously ‘geeky’ these days to understand their fishfinders. It doesn’t matter whether you are jigging for kingfish or trolling for mackerel, you need to know how to work your fishfinder properly if you want success.


Once you have developed a fundamental understanding of your unit, you can go to the Manual mode of operation. In Auto mode your fishfinder’s computer is set to specific parameters, which don’t necessarily suit your particular situation at the time. If you are serious about finding fish, you need to fine-tune your unit to maximise its capabilities. There is little point spending $5000 on a unit, but only utilising $1000-worth of its capabilities.

The Gain control is critical (called Sensitivity in cheaper units) and varies the amount of information that is displayed on the screen. Better quality units, like my Furuno, have a Gain knob that can be twigged to get the perfect picture. A balance where there is enough gain to get the right amount of information without too much interference or clutter on the screen is what you need to strive for.

Once you get really good, you will even be able to pick up thermoclines – boundary layers in the water where colder and warmer waters meet – which show up as a thin fuzzy line, as well as mark bait and fish clearly. To do this you need to constantly vary the Gain control for the best possible picture.

Fishfinder fundamentals

Transducers are the key to fishfinder performance

The way a fishfinder workers is pretty basic, but it really is essential that anglers understand the fundamentals. The transducer sends a pulse into the water, which hits anything in its way and then bounces back to the transducer. The bigger the transducer surface area, the better the signal return.

The strength of the signal return is based on what it hits and the more dense the subject, the stronger the return. In other words, structures like rock or shipwrecks will be defined more clearly than soft targets, such as sand and seagrass. But conversely, while metal and rock produce solid return signals, so does air. This is why it is so imperative to make sure the transducer is not affected by aeration under the hull. The positioning of the transducer is critical to the performance of a fishfinder. If you are having problems, best to check with your local boat shop.

Most fish have air sacks to help them move through the water column. Happily for fishos, air sacks are also useful as they produce a really good return signal, which makes it really easy to mark them up. Basically, the bigger the air sack, the better the signal. Species like marlin, jewfish, snapper and kingfish all show up well because they have big air bladders. Other species, like sharks, don’t have an air sack so they don’t show up as well, but their body density is still greater than the surrounding water so they will still produce a mark on the screen.

Once the transducer picks up the signal, it sends it up to the fishfinder for processing. How the computer in the unit processes the picture is critical to what you see on the screen and the more you can manipulate the variables to maximise your picture, the easier it will be to read. And, of course, how well you interpret the data is the key to successfully finding fish.

The basic difference between transducers is power. Like a car, the bigger the engine the more powerful it will be. A 300-watt transducer is like a Honda Civic; it will get you around town, but it won’t set the world on fire. With minimal power, this type of transducer is best suited to shallower bodies of water, like lakes and estuaries. A 600-watt transducer is more powerful, but is still only useful for inshore work or driving around town. If you’re into offshore fishing you will need a 1kW unit or more – the equivalent of thundering around in a Mack truck.

To complicate matters further, there are different frequencies, and variations in beams. The most common come with dual 50/200kHz frequencies. The 200kHz is fine for inshore work, while offshore anglers use 50kHz. For the really serious angler, there are a number of specialised transducers varying from 30-80kHz, which are ideal for picking up fish in deeper water.

And then, as I’ve mentioned, where the transducer sits on the transom also plays a big role in its performance. This is the single biggest problem most anglers have with their transducers so it is critical to bring in the experts when fitting a new system.


Another hint is to change the colour palette. By taking out specific colours, you can make it easier to find the fish you are targeting. When chasing marlin around bait concentrations, I like to run about 30 per cent reduction on my colours. The reason is so I can see the green edges, which indicate feeding bait, while the more dense red colours highlights bait that is packed in tight and under attack. Quite often you will find a bait school that is green on one side, but red on the other and instantly you will know what side the predators are on.

Alternatively, when I am chasing tuna I reduce the colour spectrum by as much as 70 per cent, taking all the greens, yellows and blues out of the picture. The reason here is that I am not looking for feeding bait, but rather focusing on the big fish, which mark up really well. When trolling, I drop the gain down a bit to minimise interference, but when cubing I push the gain right up to get the best return possible. It is a similar case when hunting bottom fish like snapper and jewfish, which show up really well when slowly sounding over the seabed.

Running the screen on ‘Auto range’ is another trap a lot of anglers fall into. The bigger the picture, the more detail you’re going to get, which, in turn, means the more information for you to decipher. If you run the unit in Auto mode, chances are the seabed will usually be one third of the way up the screen, completely wasting the remainder of the display.


Instead, you need to run the unit on Manual range so that you can physically change the range to match the depth. For example, when I am marlin fishing along the continental shelf I preset my ranges at 80 and 90 fathoms. Nearly all my fishing is done between 80 and 90 fathoms, but the moment I slip into deeper water I can change the depth at the flick of a switch, which gives me the best possible picture all of the time.

Alternatively, when I am fishing inshore for kingfish, I set the depth ranges at 10 and 20 fathoms, which covers the drop-offs I like to fish perfectly. Manual also works really well in deep water. The problem is in Auto mode a lot of sounders really struggle to hold the bottom, especially if the water is rough, so the Manual approach is often the only option. To be honest, the only time I even run my sounder in Auto range is when I am running between spots.

Another important function on the sounder is the Zoom facility. Most serious sounders these days have the ability to zoom in on a specific depth. Enlarging the search area is great when you’re targeting specific species in deep water, like kingies over a reef, or hapuka. If you still want to monitor the whole water column as well you can split the screen and zoom in on one side, but have the whole picture on the other (as above).

Location, location

Knowing where you are is almost as important as knowing where the fish are.

While sounders and their various functions can be quite complex, generally speaking GPS units are much easier to understand and use. GPS units allow anglers to see exactly where they are all the time and, better still, display details such as navigation markers and, increasingly, even depth contours. I really do believe that if you’re looking for fish, knowing your exact position is almost more important than seeing what’s going on down below.

In the old days we had to line up landmarks to be on the mark, which was a real skill, but now all we do is match up a series of numbers on a screen. As a result, anyone can get on to that magic spot so long as they have the right coordinates. In particular, this has made offshore fishing a lot safer and more productive, as well.

There are a couple of tricks to employ to get the most out of your unit. Firstly, make sure Tracking is on. Seeing where you have been is imperative, especially when you have marked some reef on the sounder or had a bite on the troll. One trick I can offer is to zoom right in when you turn around to find the spot again.

Alternatively, you can press the Man Overboard (MOB) button as soon as you mark something. In fact, on some more advanced units, if you spot a fish while on the move, a quick operator can move the cursor onto it, press Mark and instantly it will be saved onto your plotter as part of your track.

Finally, remember also that even the best GPS units still have a slight delay. This is particularly important when running at night. If you are wanting to get right on top of a particular mark or onto a certain heading, you might end up running around in circles. The best way around this is to double up and use a compass as well to confirm your heading.

Ultimately, when it comes to catching fish you simply have to use technology to keep ahead of the game. Understanding the full capabilities of your electronics will mean you’ll catch more fish. It’s that simple.


The Fish ID mode is a real pet hate of mine. Fortunately it has been phased out by most of the better quality brands these days because it is only really there to appease anglers, who like to see lots of fish figurines swimming across their screens. Interpreting what is and isn’t fish is something you, the angler, will always do a lot more accurately. Technology can only do so much.

Fish show up as a line, mark or arch depending on the species. Some fish display as arches because the sounder beam is cone shaped and, using basic trigonometry, the edges of the cone are greater in length than the centre. As a fish passes through the beam, the image appears to rise up and then drop down again – hence the arch shape. It is not because that is the shape of the fish’s swim bladder, as some anglers believe.