Water visibility

ronje · Post by **ronje** » Fri Nov 01, 2024 10:52 am

Visibility in water
We see that there several barramundi under our boat 2m down. So we run a 2m lure through them several times. Nothing happens. Did we do something wrong??

Probably we did but because we obviously didn’t know much about visibility in water.

As we know already, visibility in water is entirely dependent on water clarity and the clearer the water the better the visibility.

So how can we mere fishermen measure visibility? That’s easy.

Get our rod, put a bright colourful lure on the tip and push it down into the water beside the boat until the lure disappears. Mark the water level on the rod at that depth.

Measure the distance from the mark to the tip. Say 1 m. Is that the visibility? No, its not. That’s half the visibility because the light travels down 1m and is reflected back up 1m to our eye.

Visibility is the total path length which is 1 + 1m = 2 metres.

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The exercise above is a very simple one but the important thing to note is that visibility is actually total path length.

The distance from the surface to the lure is only half the story. When the sunlight hits the lure, the colour is reflected. One direction of the reflected path is back to the eye (surface). Its also reflected horizontally and down deeper.

However, with a water visibility/total path length of 2 m, if a fish's position exceeds 2m measured from the surface to the lure then to the fish, the fish will not see it.

Some examples?

There are 4 barramundi stacked directly under our boat at very convenient depths ‘cos we can see them on our sounder. Remember that our water visibility (total path length) = 2 m.

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We have water visibility (total path length) of 2 m ‘cos we measured it on our rod with brightly coloured lure wound to the tip.

So what swimming depth lure should we use so that the most number of fish will be able to see it? Improve our chances.

Let’s use a 2m diver. ‘cos they’ll all see it? Won’t they? Let’s have a quick think about that. Maybe try out a very shallow diver (.5m) first?

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Green lines are good news . Red lines are bad news. A .5m (very shallow) is all good news. All 4 barramundi can see our shallow diving lure.

What about a 1m diver?

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Uh-oh, it looks like we’ve had our first failure. Fish 1 can’t see lure 2 because the total path length of 2.1m exceeds the true visibility of 2m. Yet fish 1 is only 50cm under the surface of the water.

Let’s forget about fish 1. There are more down deeper anyway so let’s use a 1.5m diver.

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More red lines. Our second failure already. Neither fish 1 nor fish 2 can see lure 3.

This wasn’t supposed to happen. But it has and its simple physics at work. Bit different to what was expected?

Where’s that 2m diver that we were thinking of using.

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An unmitigated disaster.

Four more deeper barramundi have turned up (fish 5,6,7 and eight) while we’ve been playing around and still only one fish out of the 8 can see lure 4.

How come? Why? We are going down deeper and deeper to where more fish are so why can’t they see the lure?

Because we ran our2m lure (lure 4) at the true visibility depth and that consumed all of the total path length of 2m.

There’s none left to reflect to any nearby barramundi and THAT’S the limitation of simple physics when running lures at the true visibility depth.

Go deeper and none of the 8 fish will see the lure 4 at all (not even fish 4).

How then do anglers catch fish on coloured lures which they run at depths greater than true visibility? A 4m lure in water with a true visibility of 2 m?

Many do (including me) but if we think that it’s got anything to do with lure colours or fish vision, then we’re kidding ourselves.
At that point we are lateral line fishing (lure vibration) because the laws of physics have determined that colour doesn’t get down below the measured (we measured it with our rod and bright lure so its correct alright) true visibility depth of 2 metres.

Not even a white lure will work beyond the true visibility/ total path length limit.
So there we have an environment set by visibility following the law of total path length and a tool (a coloured or white lure) ill-suited to the job because it runs too deep.

Wow! Didn’t expect that.

So what depth should we run our lures at so fish can look admiringly at our work of art as if it was done by Rembrandt? What lure can be seen by the most fish?

That’s easy. 1 metre deep will give the best compromise. Pretty shallow eh? Will it work?

The results obtained over the years by shallow diving lures speak for themselves as probably the shallower lures that have caught more barra than any other lure.

That is but one of the secrets.

Go any deeper and you won’t be relying on colour to attract a fish’s attention.

Instead, you’ll be relying on the lure bib to produce action (vibrations) and its effect on fish lateral line detection.

It's a straight out exercise in geometry. No science involved at all let alone rocket science.

There is one reliable way to make the lure colour brighter and thus be seen from greater distances (more green lines on the diagrams and fewer red ones).

That reliable way is called fluorescence (physics again) and we’ll look at that subject a little later.