Magnetic Island Shoals – Using your Sounder and GPS to find shoal marks

With massive advances in GPS and sounder technology over the past decade, more and more people are discovering the huge potential of fishing an area off Townsville that is collectively know as the 'Magnetic Island Shoals'. But a lot of people new to fishing, or who have recently upgraded boats and put the shoals into reach, don’t really understand what a shoal is and therefore what to look for to find one. It’s a big ocean out there, and to the untrained eye it’s a very basic bottom all the way from the back of the island to the reef. The rewards form fishing this area can be huge, and at half the distance so are the fuel savings compared to going to the reef!

So what is a shoal? Generally speaking a shoal is a section of the seafloor that has a harder rubble appearance rather than a basic mud bottom. This is usually created by a raised area in the seafloor. When the seafloor is raised above the height of the surrounding area, current and tide remove the fine sediment leaving a harder substrate exposed. This harder substrate is critical for sedentary life forms such as sponges and corals to attach to and grow. The bottom may look something like the picture to the right. Once these structures grow and form, smaller baitfish will be attracted to the protection provided. And with the bait come the bigger predators. The problem is that these subtle changes in seafloor can be very difficult to detect, especially with cheaper sounders or a sounder left on automatic. The aim of this article is to show people what to look for on a sounder, and how to best adjust your sounder to pick up the minute changes in seafloor structure.

Firstly, a cheap sounder with a poor resolution simply won’t cut it out here. Finely tuned you might pick up some of the larger pieces of structure or bigger bait schools, but you can’t beat a good high resolution colour sounder for those faint details. One of the best ways to tune your sounder to pick up on smaller details and individual fish in deep water is to zoom in on the bottom. And I don't mean by using the zoom button built into most sounders. I mean adjusting you upper and lower limits of the sounder to only show the bottom few meters of water. When you use the zoom feature of the sounder all you are doing is taking whats already displayed and making it larger, no new information is rendered. By changing the upper and lower limits you force the sounder to re-allocate all the pixels available to the new depth range. This increases resolution and detail, and more information will be rendered. Think of it like this, if you have a sounder with only a 100px count in the vertical direction, and you are in 100m of water, then every one of those 100 pixels is representing 1m of water depth! If you change the range of the sounder to show from 80-100m only, then every pixel now represents about 20cm of the water column. So you stand much greater chance of picking up smaller details. In reality, most modern sounder will have a vertical pixel count of about 640px or more, and most shoals are in about the 30m water. So by telling the sounder to only look from about 15-35m you are giving the sounder a good chance of picking up individual fish and small bottom structures. You also stand a better chance of separating fish from the bottom. You also want to sent the sensitivity up quire high. Don't worry about some clutter showing, you will read fish and bait through this. But its important not to miss out on information that may be removed by setting the sensitivity low. 

The image below shows how my Humminbird 997c has been set to 17-35m in 29m of water. The fish showing, probably Nannygai, are sitting very close to the bottom. In automatic mode the fish would be barely visible. And without a good sounder they probably wouldn't have been picked up at all. If you are interested in talking sounders then go along to Townsville Marine and have a chat to Reece. He really knows his stuff, and Townsville Marine are stockists of Humminbird sounders. 

Generally speaking most shoal areas will be covered by live corals and other marine life. As a coral is softer than the bottom it will return a weaker signal to the transducer. Strong signals, such as from the bottom proper, will return as red, and weaker signals will return green and yellow. So a good shoal will have the red signal for the bottom and scatterings of green/yellow attached to this. The image below shows quite nicely the red bottom with a yellow and green line sitting above, fish are clearly making off the bottom. This is a good Nannygai show. 

If your sounder isn't quite up to picking up the smaller details, or your just not sure what to look for, probably easiest is to find the bait and fish themselves. If its a big shoal, large bait balls and predators will be present, sounding around should locate these easily. The image below is from a popular shoal that holds good numbers of Spanish Mackerel. There is clearly a large bait ball present with predators above. The Side Image of the Humminbird sounder has the added feature here of showing most of the school to the left of the boat. Notice the depth is still locked in on the bottom half of the water column.

This shot below shows a very large bait school holding close to the bottom with Spanish Mackerel narrowing in from above. Spanish Mackerel, by virtue of their body shape, return as longer streaks on the sounder than Nannygai. 

The discovery of new shoals is the 'holy grail' of shoal fishing. Shoals unknown to others and that haven't been fished much will hold the biggest and best of the Red fish. So when running around out there its a good idea to keep an eye on that sounder! Even at 30knots its possible for good sounders to pick up new territory. But at that speed you more likely to pick up bait balls than good bottom. The image below shows where I have picked up a couple of patches while running between marks. The speed indicated is only 11knots, but I had pulled the boat out of gear already before marking the position. The advantage here, of a good sounder, is that you can pause the screen and bring the cursor back to the patch of bait and mark its 'exact' location. Spin the boat around and have a better look. Holding bottom at 25+ knots requires excellent transducer positioning and calm conditions. Air passing under the hull from choppy weather can break up an image, but its still worth watching. If your sounder simply won't hold the bottom at running speed, then the same thing can be achieved by trolling around while enjoying a quiet ale or two.

Here is another shot that highlights the advantages of a Side Image capable sounder. This was taken on the popular Mackerel Grounds. I was sounding around for a patch of Spanish Mackerel that had moved away from the boat. I knew they were there, but couldn't get onto them. There was nothing showing on the 2d sounder at all, the fish simply were not under the boat. But a quick flick over the SI and it was reviled that the fish were sitting off to the left of the boat. Very handy feature!

This shot shows what a bit of shoaly bottom looks like on the Side Image. The features are small, and with the sounder set to look 65m either side of the boat, you don't get much definition. But it does work, even in 30m of water.

One last tip I will give for finding shoals has nothing to do with a sounder. Keep and eye pealed for SEA SNAKES. These reptilian creatures live around good structure and come to the surface to breath. They will NOT live over mud bottom. So if you see a snake on the top of the water stop the boat and/or mark the position on your GPS. Hunt around with your sounder in the area he was spotted, there will be a lump of some kind on the bottom somewhere nearby. Think to yourself, how many times have you been anchored on a mark and a snake has popped its head up?

I hope this helps some people in fishing an area that is very productive but also very frustrating? If you have some success as a result of reading the info here then please use the comment section and let us know how you went.