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Sea Fishing – How To Read Your Fishfinder Correctly


“There’s loads of fish under the boat…RIGHT NOW!!!”. I wish I had a pound for every time I’ve heard that, even when tied up in the marina! Of course the chances of there being a huge shoal of fish under your boat whilst it is tied up is minimal at best but that still doesn’t stop people believing the magic under water all seeing eye instead of what their own brain is telling them. You buy the fishfinder and you believe everything it says. Don’t. So for beginners and experienced anglers alike here is the correct way of reading what your fishfinder is telling you.

Lets get some basics out of the way first. The sea around the UK is NOT like the North American lakes on which just about every Fishfinder instructional is based. It’s not even close. Configuring your fishfinder for UK salt water fishing is a totally different kettle of fish. The sea is literally full of stuff that will show as a return on your fishfinder, and really we want stuff to show! Around May/June there are massive plankton blooms, at times the sea can be so full of jellyfish that it is hard to get a line down without bringing up a gooey mess. Large shoals of tiny baitfish are common. Then you have the various bits and pieces that float around the bottom (especially over a rocky sea bed) and the movement of the constantly shifting surface (except for those very rare millpond days). In order to give you any chance of telling one of these from another, and more importantly from your target fish you absolutely 100% have to turn off the Fish ID on your fishfinder. Whilst those pretty little fish symbols look nice the vast majority will not be fish. If it’s in the top 20 feet of water there’s a 99% chance it’s not fish. The odds get a bit better a little deeper but if you’re using your Fishfinder in auto Fish ID mode you are wasting your time.

The Correct Fishfinder Setup

This is going to be a little general because there are so many different makes and models out there but it should serve as a good starting point for you to tweak from.

Sensitivity – Use manual. Start with a minimum setting and slowly increase it until you get a good solid bottom return, now turn it up a little more. At your berth or the harbour wall you should be seeing plenty of “blobby” and “speckly” returns in the water, these aren’t fish. As an approximate guide I have my Humminbird 798CXI turned up to about 60%-70% for fishing in the Irish sea between 50-120 feet of water.

Depth Range – Use manual and make sure you set it deep enough to show plenty of bottom. This means that when you change from a hard to soft bottom and vice versa over depth changes you can see the change in your gray line/ colour contours. It’s very hard to tell how bottom hardness is changing if your range is cutting off what you’ve just passed over. I also forget about the first 15-20 feet near the surface, there is so much going on up there that it’s almost impossible to pick out any hard fish returns.

Surface Clutter – Either medium at most or use my recommendation above to remove the top 15-20 feet from your display and forget about it. Your cone is so narrow at that point, combined with the usual slopping about of a boat at sea that it’s almost impossible to ping fish at that depth anyway.

Zoom Or Not To Zoom? – Up to 80 feet and I don’t bother, much deeper than that and I do. Of course this depends on if you are feathering Mackerel or not, if you are it’s best to show the whole column of water.

That’s just about it really, I know every modern fish finder has dozens of other settings but really that’s all you need to get an “honest” picture of what is going on under your boat. The next trick is to learn to tell Fish from Not Fish.

What Does A Fish Look Like On My Fish Finder?

Because how fish are repsresented can change so much depending on how well your transducer is setup, the depth of water and the resulotion of your fishfinder display I’m going to give tips to cover all bases. I’ve had many years playing around with fishfinders but I’ve also enlisted the help of some commercial fishermen for this section. It’s very hard to definitely determine fish above a certain speed so when looking I try to stick below 7-8 knots as a maximum, 5 is ideal.

The Basic Fish Arch – With everything setup perfectly and the fish sitting nice for you :), you’ll getting the perfect half crescent. In anything up to 100 foot of water this should show as a strong return getting much thicker in the centre of the arch. Please see 3rd image below.

Solid Straight Horizonal Line – This is usually an indication that your transducer isn’t quite right or your scroll speed is set too fast. It is still a definite solid signal and easily stands out amongst any general blobs and speckles. It’s not unusal if your scroll speed is set to maximum for this line to cross the screen from one side to the other.

Angled Lines (Wavy or not) – What happens when you combine your target fish feeding amongst baitfish? You get something that looks like a helter skelter of angled lines. Please see 4th image below.

Short Horizontal Lines In A “Cloud” – Usually Mackerel. Mackerel do give a solid return on most fish finders that have their sensitivity set correct. They are not the large cloud that many people claim (the cloud associated with North American baitfish in the lakes is usually Sandeels around UK waters). Even in a shoal of thousands of Mackerel it is possible to pick out individual fish in up to 70 feet of water. Beyond that the extra width of the sonar cone and screen resolution can cause a more general cloud but in shallower coastal waters it becomes very easy to spot the “more solid” difference between Mackerel and Sandeels.

All those results above are for general fishing in depths of water up to around a maximum of 120-140 feet. Please keep in mind though that the deeper you fish the more sqaushed the signals become. Fish archers become more pointed, horizontal lines thin out, angled lines get more vertical and you’ll lose the more solid horizontal lines in your cloud. Hoever in all instances fish DO show a much more solid return than the general crap, baitfish around them.If you have to ask yourself “is that a fish or not”, then the chances are it isn’t.

If you’re fishing deeper water then there is only 1 sonar return that the commercial fishermen consider to be a 100% sign of fish and we’ll cover that last.

The Short Vertical Line – Nothing to get excited about in shallower water but over deep water wrecks and in several hundred feed of water this is what fishermen are looking for on their fishfinders. Large clouds of short vertical lines. If you get this whilst drifting over a wreck then you know you’re on to a good day.

Finally I’m going to finish of with just a couple of examples of good and bad fish returns.

Fishfinder Images Showing Fish?

Not A Fish

Fishfinder No Fish Showing

If you look at the right hand side of this image you’ll see a return just off the bottom, this isn’t fish. If you look at the bottom contour you’ll see we’ve gone from a softer (sandy) bottom on to rocks. This image actually shows the problem of not having your sensitivity turned up enough. The return we see is actually the start of the weed which if I had the setup done correctly you’d have seen across all the rocks we were about to cover. There are just the slightest indications of this to the left of the main sonar return. I can confirm that the 2 hours we spent drifting various lures and baits over this usually productive mark produced nothing.

Still Not A Fish!

Fishfinder Not Showing Fish Again

This is one of my favourites, if there’s anything to get my dad up out of his seat and working that rod harder than ever it’s something like this appearing on the fishfinder. But it’s not fish, well not anything we want to target. The sea that day was particularly heavy with plankton, you can see I removed the top 40 feet of water because the plankton was thick down to that level (just visible). Line coatingly thick. So what did we pass over? A very small shoal of little sandeels. You can also clearly see that there is nothing feeding on them or around them. A small blob like this in open water is not a sign to start fishing, well unless you like foul hooking sandeels! 😉

We Have Fish!

Fishfinder Showing Fish

Things are starting to look up here! This is actually an image of a small reef passed over at around 7 knots. You’ll notice above the pinacles there is a concentration of weak sonar returns forming a cloud. This is nice but still isn’t a definite sign of fish. If you look lower down between the peaks you can however see much stronger distinctive green and red fish arches. It’s also not hard to see where the majority of the fish on this mark are waiting to ambush their prey, a drift over the right hand side of that last pinacle should produce fish? And it did. In a little under 40 minutes 2 of us managed to catch and release around 20 Pollack and Coalfish to 5lb. A great bit of fun until the tide turned and the fish moved off to another holding position.

We Have Lots Of Fish!

Lots of fish on Fishfinder

Here we have a much slower pass over a different location on the same mark, on a different day and at slackwater (HW). With the tide not running the fish had decided to move off the pinacle and persue baitfish in more open water. You can see we are still over a rocky bottom by the hard bottom return and clear weed signals (the yellow,green and blue line above the hard bottom). This shows us we have are sensitivity set much better than the very first image in this set. Just off the bottom we have a slaughter going on as good sized Pollack and Coalfish are gorging themselves on Sandeels. This was a case of lots of short drifts because we didn’t have long on them but using extremely light spinning rods we pulled up some very sizeable Pollack that were literally packed with Sandeels.

I hope some of this helps and gives you the confidence to shut off your fishfinders auto mode and start interpreting sonar returns for yourself. I have no doubt whatsoever that even the most extremely expensive and sophisticated fishfinders would have shown fish returns on all those signals had Fish ID mode been left on. It’s really just a matter of practice, eventually you get to the point where you can start calling out when a fish will take and that adds a whole new level of satisfaction to fishing from your own boat!


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