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Fish Vision - What do Salmonids See?

by Dave Wallbridge

Some time ago ago I posted some comments on fish vision in the `Nymph design` topic on an internet message board. As the information given then was of a generalised nature and knowing that fish vision is extremely species specific - i.e. different species have widely different eyesight - I decided to do some searching to see if I could find any up to date information which related specifically to Trout, Sea Trout and Salmon. Here is a very brief summary of what I found, most of which was 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 our vision is about 14 times better at resolving images than theirs is. 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 slightly blurred.

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 focus 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 - such as stripes, bars, and particularly, circles and spots. The second is movement - even remarkably small and rapid movements (like the gills of a nymph) 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 but not into the Infra Red (we see to about 700nm) and is very similar to ours - the retina having both 'rods', which detect only monochrome differences and are very sensitive, and 'cones' which detect colour, however, unlike us, the rods and cones have their own distinct operating conditions dependent upon light levels.

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 about 1-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.

Work done with 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 4-5 hours but the reverse change from rods to cones can take even longer. This might explain Falkus`s `second half ` observation that a large fly fished deeply seems to work better. At this time on a short summer night 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/green region. A number of behavioural experiments have shown 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 it actually sees depends on a number of factors not the least being the turbidity (cloudiness) of the water in which it lives..........

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 12-15ft 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 the effect. Because of this scattering effect objects appear indistinct and fuzzy in anything other than shallow water - just as they do to us in mist or fog. Short-wave light (blue) is affected the most by this, long-wave (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, along with the change in habitat as the fish develops, is given as the 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 but that these are used to detect polarised light as a navigational aid.  This would mean that returning Sea Trout 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 as we know that at this time their appetite is suppressed and also, if it was a useful tool for finding food it is logical that they would retain it throughout their life.

Finally, 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 kind of colour response do Sea Trout and Salmon have - living their early life in freshwater then migrating to and from the sea? The answer 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. The change appears to run more or less parallel with the changes in the fish's physiology which enable it to tolerate the change in water salinity and this can take many weeks (and may be different for Salmon and Sea Trout).

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 a predominately blue fly 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 the success of this colour choice.

Most of the above information seems to simply confirm the methods developed over the years by anglers, but although some might argue that it is mainly of academic interest, it has certainly made me think a little more deeply about fly tying and presentation.

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