Using sonar to improve your fishing results

[Originally published in the January 2022 issue of Ski-Boat magazine]
By Stuart Simpson

THERE are many ways of targeting marlin and many of them — whether trolling livebait dead and strip baits or, the most prevalent, trolling artificial lures — have not only proved very successful, but have also been refined over the last 30-odd years to a point where, here in South Africa, the sport of marlin fishing has become extremely popular.
I grew up with the passion of not only targeting these fish, but also exploring the numerous techniques available to catch them. I set out to learn as much as possible from marlin charter skippers worldwide to improve my strike rate, and to discover the best techniques for turning strikes to hook ups.
I had read and heard of skippers picking up sonar verification of possible marlin showings, but to an extent I only used my sonar to pick up structure and drop off lines in deep water. I used that as an indication of where to troll, presuming that marlin would follow the upwelling and bait availability to hunt in a specific area.
Did I go with the idea of hunting a specific mark that I thought could possibly be a marlin? Probably not. At least not until I started fishing Cape Verde four seasons ago. There experience taught me that one could pick up a marlin on one’s sonar, and establish a technique to work this area to get the marlin you’d found to come to the surface and attack your lures, or be excited by the dredges so that you could to pitch a bait to it.
This pitch bait way of targeting marlin has been described at length in previous articles I have written for SKI-BOAT magazine. The 2021 season proved once again that this is not only the most successful way of hooking blue marlin, but without doubt also by far the most exciting form of marlin fishing I have ever encountered as an angler and now as a charter captain.
Over the last few years I have learnt to fine tune the way I use my sonar to pick up specific fish I want to target, so let’s take a closer look at that…

A 700 lb marlin on the dropoff. She was caught one minute after being marked.

The best chance of finding subsurface fish comes back to the way we were all taught from day one fishing offshore — look for birds, and if the ocean is relatively calm look for water that runs like a river (current) or floating objects. Once you find this, keep your eyes on your trusted sonar. However, it’s not always as easy as finding birds diving and beautiful current lines and floating objects full of fish. That only happens on those perfect days, right?
Technology has come a long way, even in my relatively short career; it changes every year and it’s getting better and better, but fishing is fishing —some days are good and some days are hard. However, if you have the technology and you know how to use it, it can turn those not so good days into spectacular days.
Different moon phases, the current running the wrong way, and wind blowing in the wrong direction can all chase fish down. Eye balling for birds and current lines might not help you at times like this, but using what you have in-front of you can expose everything that you can’t see from above.
As you can see on the screenshots alongside that I have taken off my trusted Raymarine Axiom Pro, I’ve set the sonar at 90 metres. All new machines these days have this option.
Now look at the far right of the screen which shows an extra depth column. On my unit I set that to 300m and keep the main screen scanning at 90- to 100m. My reason for this is that fish might be down too deep to mark anything at 90m.
I can’t say I’ve actually marked marlin at 150- or 200m, but what I’m looking for down there is bait! Where there is bait there should be fish big enough to eat your lures. Even though the bait might be sitting down at 200m, and you’re not marking anything from 0- to 60m, you’re onto something already.
Remember that when bait are down deep they are safe, but there comes a time of day that they have to come up into the light; this might be at first light or late evening, and when that happens the billfish will be near the bait.
Billfish might also be sitting right at the surface, in the top 10m of the water column where you cannot mark them unless you have side scanning which is unaffordable to most. In this case you’re still in the right area, even if you haven’t had a bite. Just change tack to suit your lures in the sea conditions on that given day, and work around that area to see if there are any higher showings which you will often find, then fish that area where you’re marking deep bait. I’ve caught countless fish off deep bait when I have not even marked one marlin on my screen.
When trolling around from point A to B or B to A, depending on what your favourite tack is, just keep your fish alarm on. Fish can be anywhere and you cannot have your eyes on a screen all day. You could be in the middle of nowhere, trolling to your next waypoint unaware that something has changed underwater which is not detectable to the naked eye, and just like that your alarm will sound either with bait or that beautiful dark red, quarter moon shape indicating a marlin mark.
You could be preoccupied chatting to your mates when a “Beep, beep, beep” suddenly warns you that you have driven over something of interest. If you’re on auto range showing the bottom, then don’t use the fish alarm option because it will beep at you all day and you will never mark any billfish unless you’re in 100m of water or shallower. I suggest you switch off auto range and set your machine; read the manual to fnd out how to do this.
Obviously the fish will not always come into your spread after you’ve marked it, but the area where you have pinned it on your GPS gives you a much better chance of success than the empty spot you came from, so stay and fish that area where you marked the fish. Once you’ve marked a fish turn down your radio and ignore other boats’ hook ups; chasing other people’s fish doesn’t often have a good outcome. You have found a fish already, so rather try to catch that one even though it may not have bitten yet — it may well bite after a few more passes. Not only that, there will be more fish in that area than what you think!
Work the area where you marked the fish in a way that allows the sea to favour your lures and enables them to swim the way they were designed to. Don’t worry about running with the current as you might have wind against current against you. Do some tacks to figure out where all your lures are swimming at their best, and keep working that area for at least 30 minutes before thinking of leaving for another spot.
If you do move on, always go back later to the spot where you marked bait or some fish on your machine. As the day progresses and the tide changes, an area that was quiet earlier could be mayhem one hour later. Sound around that area as the fish would not have moved too far. If some lures are jumping, don’t be lazy — change them until they are all swimming correctly. The fish might just not be on the snap yet, so by working around within 1nm maximum, trying different tacks, you will most probably start working something out — drawing a map out on your plotter and in your head — that shows smaller, tighter balls of bait and grouped marlin marks. After a while you will see a pattern.
I’ve fished on a few boats at Sodwana and I see those skippers often leave their sonar screen as is all day long. Remember that water changes with current, so as this changes you need to set your gain differently. If the screen gets too cluttered on your machine, turn down your gain as you are obviously in an area with greater salinity. Conversely, as your screen becomes much cleaner, turn up your gain so you get clearer markings and it’s easier to identify what your sonar is trying to show you. This is as easy as sending a text on your mobile — a couple clicks on your machine and you will have it running at its optimal level at all times. If the water is really clear from the shoreline all the way out, then you can leave your gain on “Auto”.
The transducer I’m using is a B275LHW which comes as an in-hull setup. I set my machine on high frequency as this is best for shallow water. Don’t use low frequency just because you’re in deep water; you only want the machine to draw out the first 100- to 150m clearly at its optimal power for you to see what’s going on.
I’m not one of those that really reads manuals unless I’m stuck; instead I prefer going to YouTube for advice. The guys out there doing videos for these units are great salespeople. They show you things that you could not even imagine your unit can do, enabling you to get your money’s worth by using it correctly.
Here’s my rule of thumb plan for marking bait and fish:
1. Set your unit to 100- to 150m. If you have the option of the column on the right, set that to 300m.
2. Use your gain to your advantage; in slightly dirty or milky water turn your gain down, in clear water turn your gain up
3. Use high frequency for marking mid- to surface level fish.
4. Don’t share power with any other fancy electrical units; your GPS & sonar unit should have a direct current with its own power source.
Using these suggestions when targeting blue- and striped marlin can be construed as being a lot more effort intensive, but I find it is not only a lot more exciting but also keeps the skipper, gillies and anglers wide awake and a lot more involved in the “hunt” and any action resulting from it. It’s certainly lot more exciting and more visually pleasing than just sleeping on the deck while pulling konas endlessly all day!


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