It's essential to understand the basic physics of how light behaves in water, which in turn determines what it is possible for the salmon to see.The colour of the fly line doesn't make much difference to anglers? but a fish will see a line coming into their vision at an angle way less than 45 degrees,and depending on where the fish is in a deep or shallower pool the colour will be visible ?
At angles below 41 degrees elevation what the salmon will see is a reflection of the underwater scene and little or nothing above or on the surface. Here is an example below, taken with a lens that approximates to a salmon's eye, at a depth of 3' 6" at 2pm in the first week of October.
The sky is at the bottom. The trees above the far bank are at the limit of direct vision and beyond them at 41 degrees is the dark shade of the reflection of the bottom and peat-stained water. A fly line on or near the surface in that zone is almost invisible.
Here is another example, taken on a brighter day in August near midday, again at 3' 6" depth, looking upwards at 45 degrees, of a salmon's left eye view of a shallow fished Blue Charm
Notice the clarity of the sky; the brightness of the margin between clarity and reflection; and the darkness of the reflected zone beyond 41 degrees. Again, a fly line in the reflected zone would be physically very difficult to detect.
It's the physics that limits what can be seen, not the physiology of the salmon's eye. However, it is essential to have some knowledge of the way in which the salmon's eyes work, because they are very different from ours.
- The focal length is much shorter (about 7mm) and the focus is fixed. They have no mechanism like ours for shifting between near and far objects. As a result evolution has optimised their focal distance around 2.5 metres because that's all you can usefully employ most of the time underwater, which makes them rather short sighted in relation to distant objects.
- In the ocean their primary means of detecting, locating and acquiring prey is through vibration detection via the sensor array in the lateral line and on the slope of the forehead. The eyes only come into play in the closing stage of acquisition.
- Unlike us they have no iris to control the amount of light entering the eye, or any eyelids. So when the conditions are very bright they have to rely on a naturally produced pigment to desensitise their extremely good low-light capability. Thus when confronted with scenes like that shown above the effect is to make the dark areas darker and objects within them less distinct.
Of course in much shallower, trout-depth water, you encounter much higher light levels. The example below is from the Dee in brilliant mid-April sunshine.
The water here is 18" deep in the foreground and about 2' at the limit of visibility at 8'. The edge of the sky circle is top centre. There is then about 5-6' of reflective area, and beyond that anything on or near the surface is invisible. In the foreground a salmon would be able to detect the colour of a fly line below the surface, but there again you don't find too many in water only 18" deep.
The deeper a salmon is lying the larger the sky circle (known as Window 3). For a fish 6' down it's about 6' across. As it nears the surface Window 3 shrinks the reflectivity of the Window 2 reflection zone increases. On many salmon rivers the reflection zone will be affected by surface waves and disturbances that create a kaleidoscopic effect, which breaks up the image of anything on or near the surface, making detection and identification correspondingly more difficult.
Then we start running into the problems of knowing what the salmon "sees". We can work out what it can detect and the limits of its vision from basic physics and the physiology of its eyes, but how its brain processes and forms images and resolves colours remain completely unknown. That said, we do know that the salmon employs a smaller proportion of its brain volume on image processing than we do (smell processing occupies the biggest single slice at close to 30%).
In any event it is essential not to make assumptions about what salmon can see in water - a less than ideal visual medium - on the basis of our own optical performance in air, which by any reckoning is quite outstanding.