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saddo that I am, I was thinking about this today. no expert, but there are a couple of scientific principles that may come into play here.

firstly, we see line in the water looking down - ie with the light source (sun/sky) behind us. hence what we see of the line is reflected light bouncing back to us from the line's surface/core. any colour we see in the line is caused by that colour being preferentially reflected by the line's chemistry, and its interaction with the optics of the water between it and our eyes.

whereas, fish will by and large see the line from below, in other words backlit by the light source. in this case, any colour they see in the line will be the opposite of the first case - it will be the light frequencies (which our eyes interpret as colour) which are NOT absorbed or reflected by the line. If all light is reflected by a particular line, we will see it as white, but fish will see it as black, in other words a straight, black line through their line of vision.

So, as an extension of this, any light absorbed by the line (ie any apparent colour to us) will make that line show up as a darker feature against the relative clarity and neutral 'colour' of the water.

Therefore, it would seem that having any colour in the line or even reflectivity(?) of the line, will make that line visible to the fish.

One further element: it would seem obvious that the thicker the line, the more visible it is to us, and the fish. To an extent that is true. But there is a complication.

Since line has a circular cross-section, it acts as an effective lens in two dimensions. So that, even if all light passes through the line, ie it is totally transparent, the light path can be distorted.

It has been well understood for many (hundreds?) of years that, the tighter the curve on the lens (ie the smaller the radius of the circle), the more magnfication or distortion it creates - try looking at a raindrop on a window. Early microscopes used tiny bead lenses, since the radius was as small as possible. Can you see where I'm going with this?

In my opinion, it would appear that thinner line can actually distort light more than thicker! Hence to a fish that is looking upwards, that increased distortion could make a thin line just as visible as a thicker one (anyone seen the Schwarzenegger film 'Predator'?).

Since the water surface is uneven, there is always distortion of the image of objects above the surface, since that image must pass through the surface layer (seen from the fish's point of view) but anything below the surface, ie anything actually in the water, will be part of a pretty uniform liquid, and hence any distortions will be quite unusual, and probably quite visible, especially at close range.

So, to conclude my argument, given that, especially in the late day, red light is the most prominent in the sky, a thin red fishing line will be seen to fish below as a distinct, black line, with further light-distorting qualities the nearer they get to it!

Matt Hayes is an intelligent, successful, experienced angler. He is also sponsored.

The scientific deduction above is my own, and proves quite clearly that the best fishing line is in fact a thin, flat sheet of cling film.

You work it out!

dan

even in 'cloudy' water, fish visibility is possibly better than we think.

the 'cloudiness' is probably more apparent to us, since the opacity and dark bottom coupled with refraction at the surface boundary, cause a mirror effect, reducing our ability to see down into the water. polarising lenses cut this down some. looking up through the water towards the light is much easier.

consider the perceived difference in water visibility between a swimming pool and garden pond, even though the water can be 100% clear.

If the fish could not see well (at least, better than surface predators) even in cloudy water, they would be at a distinct disadvantage, even given their other sensory capabilities.

when was the last time you saw a fish 'frozen' in cloudy water when hooked? or run headlong into the bank?

walk towards the light, little fish. sob.

regards

dan

ps: as most will be aware, the surface refraction, or straw-in-a-glass-of-water effect, means that fish can effectively see round corners in the vertical plane, hence the importance of staying low, even away from the water's edge.

pps: fish eyes are optimised for visibility within the optical environment of water. wonder if things look different when we put them on the bank? do they have to readjust when we return them to the water?