Side Scan Analysis – Shark Island, Sydney Harbour


Side Scan Analysis – Shark Island, Sydney Harbour

This is the first blog in what I hope will be a series detailing how I’m currently unlocking the power of my side scan sounder.

While my little Humminbird Helix 5 doesn’t have a reputation for being the most powerful unit out there I’ve been getting some pretty good images off it lately by fine-tuning the settings. I’ll go through some of the images I’ve recorded and discuss my interpretation of what is appearing on the sounder.

Why Shark Island? Well, I’ve mentioned it in a previous blog, thousands of Sydneysiders travel past it daily and it has always interested me. So let’s get into it.


  1. The seam line

    That’s my track where the two sides of the side-scan image stitch together. You can see I almost did two complete laps of the island in a clockwise direction.

    Notice that I had to stop my scan slightly early (for a group of ladies in kayaks that were spread out across the route) near location 2.  I also missed a little section around location 3 while I was trying to minimise the overlap of my two tracks.

  2. Drop-off #1


    Side scan sonar works a bit like shining a torch out the side of the boat. When the beam hits something it reflects back brightly. When the beam has nothing to bounce back off then the reading is much darker.

    At location 2 the bottom slopes away from the boat making returns on the side scan much dimmer. This drop-off displays as a dark patch on the map.

    Using the other sounder screens, we can see the bottom drops away from 3m down to 12m. Being right on the point of the island this area experiences quite a bit of water movement as the tide rises and falls.  This is likely to be an important location for fish.

    The first school of fish (circled in red) can clearly be seen on the conventional sounder and right side of the side scan image. The second school of fish (circled in black) can only be seen on the conventional sounder. (Click image to zoom)

  3. Drop-off #2


    Moving further around to location 3 there is a second drop-off. While not as interesting as the first there does appear to be some scattered structure on the left side of the scan which might be worthy of further investigation.

    I’ll probably come back here later and do another wider pass to try and work out if anything significant is along the remainder of this drop-off.  (Click image to zoom)

  4. Soft Returns

    At location 4 on the map we can see the soundings remaining bright but becoming slightly fuzzy. There is quite a bit of weed throughout this section and I’ll probably be heading back here at some stage to see if any squid live here.

  5. Pile of large rocks

    Shark-Island-RocksLocation 5 on the map shows a pile of large rocks in deeper water immediately before a significant drop-off. I have the sounder set to scan 30 meters either side of the boat. This structure must extend around 15 meters out from the left side of the boat.

    As we head into deeper water we can see a school of bait (circled in red). To put the size of the reading into perspective, water depth at the bait ball is approximately 7 meters.

    Just beneath the bait we can see 2-3 much larger fish lurking in deeper water (circled in black). On the zoom these show as quite clear fish arches.

    This area of the harbour carries a huge amount of recreational, commercial and public transport boat traffic. The prop wash (ie tiny bubbles in the water) is starting to really interfere with the readings on the sounder through this section.

Equipment Used:

Sounder: Humminbird Helix 5 SI GPS
Range: 30m
Sensitivity: 12
Contrast: 8
Speed: 3
Boat Speed: 3kts

If you found this useful head over to our Facebook page and let me know which area you want scanned next.

 Got fishing reels?

Did you like our blog?

We also make the best fishing reel cases available anywhere!

Take a look!

Fishing Reel Case for Spinning Reels


  • Ben / 30 December 2020 1:05

    Great article! Did you use Reefmaster to stitch the images together? or some other GIS program?

  • Ocean Addiction / 30 December 2020 10:08

    Thank you. Images are created using a combination of Reefmaster and Google Earth overlays. Reefmaster is a great bit of software. It takes a bit of time to create your first map but then it becomes quite easy. I highly recommend the investment.

Leave a Comment

eighteen + nineteen =

The charm of fishing is that it is the pursuit of what is elusive but attainable, a perpetual series of occasions for hope.

John Buchan