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Fish Vision - and the Salmonids
by Dave Wallbridge


This article appeared originally as a post on the Board. As a follow up to Max Garth's "Colour in the Fishes Eye", I've pulled it out with Dave's permission because I think it's of good general site use - Paul

A couple of months ago I posted some comments on fish vision in the "nymph design" topic on the Board. As the information given then was of a generalised nature, I decided to do some searching to see if I could find any up to date information which related specifically to Trout, Seatrout and Salmon. Here is a very brief summary of what I have been able to find so far, most of which has been kindly supplied either directly by the researchers themselves or from publications which they pointed me to.

General vision

It seems that compared to ourselves, trout and salmon have pretty poor vision - in fact compared to the fish, our vision is about 14 times better at resolving images.  This is more than a little reassuring to me- it's good to know that even my ageing vision must still be at least 12 times better than theirs!

From what I have gathered it appears that a trout's eye can detect relative size, overall shape and general colour pattern, but even at its sharpest focus, which is about 2-3 inches from the fish's mouth, viewed objects will appear as a blur.

As you might expect of a surface feeding fish, the photoreceptors are packed more densely in the lower part of the retina giving maximum resolution to upward vision. Interestingly, because of the unusual shape of the lens, they are able to see both distant and near objects simultaneously - like looking through both parts of a pair of bi-focals at the same time.

There are two areas where the trout's eye excels. The first is contrast - anything which stands out against the underwater background space light is easily seen, as are stripes, bars, and particularly, circles and spots (although the detail would be `fuzzy`) and the second is movement - even remarkably small and rapid movements provoke an instantaneous response in the trout's brain.

To me the above observations seem to validate the impressionistic approach to tying imitations. If the fish cannot see fine detail then there does not seem to be much point in worrying about exact imitation. Until now I have always tried to tie close copies, but from here on I will be concentrating more on overall shape and size and incorporating more mobile materials in my tying.

Colour

The trout's eye is capable of detecting colour over a slightly broader bandwidth than our own, extending into the far red (to about 800nm) and is very similar to ours having both rods, which detect only monochrome differences and are very sensitive, and cones which detect colour.

During the day the sensitive rods are withdrawn below the surface of the retina and shielded from bright light by dark pigments whilst the cones move to the upper layer to give optimum colour vision. At night, when the light level falls to below about1-foot candle, the reverse is true with the cones withdrawn and the rods exposed. Even in the brightest moonlight the fish will not see colour, only shades of grey.

Studies on Brown and Rainbow trout shows that there are however two periods of light adaptation within this cycle, these coinciding with dawn and dusk - the main feeding times.

The changeover from cones to rods starts well in advance of darkness and takes about 5 hours but the reverse change from rods to cones can take even longer.  This might explain Faulkus` “second half” observation that a large fly fished deeply seems to work better. At this time the fish's eye would be well into the change to “daytime” vision with the sensitive rods partially withdrawn, but no usable light for the emerging cones to detect. Could it be that with its vision at such a relatively low level, a large fly drifting across its nose would be just about all it can easily see?

Trout and salmon can see all of the colours that we can, but whilst our eyes are most sensitive in the green area of the spectrum, the trout's eye can discriminate best in the blue region.

Experiments show that not only will the fish show a preference for blue under most background and light intensity conditions, but that they are able to differentiate between small, subtle differences of shade. Second in sensitivity to blue comes red (about 10 times lower sensitivity than blue) then black, orange, brown, yellow and green in that order, but what they actually see depends on a number of factors including the frequency (wavelength) of the available light and the turbidity (cloudiness) of the water in which they live.

As white light passes through a column of water it is progressively absorbed – the deeper we go the less light can penetrate. The longwave light (red) is absorbed first and is virtually non-existent at around 12-15 ft so that any red materials in a fly will appear to be black at this depth. Orange survives to about 25- 30 ft. and so on until at about 60-70 ft. only blue light penetrates. These are approximate figures typical of very clear water illuminated by bright sunshine- the light penetration will obviously be less in dirtier water or in low light conditions. As an example, in a rising spate river with a high level of suspended solids, red light would disappear only a few inches below the surface.

The light as it passes through water is reflected off any small particles (or even large molecules) and is scattered in random directions. The more animal and plant life, i.e. the murkier the water, the greater this effect. Because of this scattering effect, objects appear indistinct and fuzzy in anything other than shallow water – just as they do in mist or fog on land. Shortwave light (blue) is the affected the most, longwave (red) the least – this is why freshwater fish generally have a colour response more red shifted than fish of the clearer open seas.

UV or not UV

Most research indicates that salmonids have cones to detect UV light when small, but as the fish grow these cones gradually disappear. Their diet in their early period of life consists of zooplankton and other small creatures that reflect UV light, but as the fish get larger they can no longer filter such food with their gillrakers. This is given as the main reason why no UV receptors are found in fish above 2 years old.  

Other studies, however, have shown that new temporary UV receptors are created annually to coincide with the spawning migration and that these are used to detect polarised light as a navigational aid.  

This would mean that returning seatrout and salmon do have some ability to see UV but I have yet to see any evidence that would lead me to believe that this is used for prey detection. It would seem unlikely, both because this is the time when their appetite is suppressed and, if it was a useful tool for finding food, it is logical that they would retain it throughout their life.

Colour shift

The reason that I began the search in the first place was to answer a question that came to mind when I made the original postings:

I had known that fish of the open sea have a colour vision response blue shifted relative to freshwater fish (as a broad generalisation) - but what sort of colour response do seatrout and salmon have - living their early life in freshwater then migrating to and from the sea?

It appears that the answer to this question is that when these fish migrate to sea, they shift the spectral response of their colour receptors towards the blue end of their spectrum. The process is then reversed when they return to freshwater.  

How significant this is I do not know but it seems to tie in neatly with the anecdotal evidence.  On my local river, many of the old hands swear by predominately blue flies when fishing the lower reaches for fresh run fish. If the new arrivals colour vision was still blue shifted or in the transition phase, this might be a contributing factor to its success.



Dave Wallbridge is a seatrout angler from Wales and a Board member who refers to himself mysteriously as "The Silver Stoat".

Related reading: Max Garth's "Colour in the Fishes Eye"

 

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