Matching the hatch and why it can't be done.
Historically, some fly-tyers have gone to great lengths to match the colour of hatching flies with the greatest possible precision. Whilst there is certainly much value in experimenting with colours, there is a reason why we could never know whether the trout sees our colour mixture as an accurate match to the natural, as we do. The reason is technically known as 'metamerism'.
The word simply means 'changing colour'. It sometimes happens that two surfaces which appear the same colour in one condition of lighting become obviously different when the light changes; as, for example, from daylight to artificial light. It happens as a result of the two surfaces being coloured with different pigment mixtures. It is particularly a problem for commercial dyers who are commonly required to accurately match a provided sample. The dyer does not know what mixture of dyes were used for the original but must find a mixture which gives a visually similar result. This he can easily do, usually in daylight. But it often occurs that viewing the new match in artificial light shows them to be quite different.
The reason is that our colour vision is not such that we can recognise different mixtures of light wavelengths, unlike our ears which do indeed recognise different mixtures, or 'chords', in terms of music. And for any one colour mixture there are many others which can look the same - under one condition of lighting but not others. Imagine an object painted with green paint where the pigment in the paint is a true green, that is to say it reflects those wavelengths of the visible spectrum which give rise to the colour green. Then imagine that we mix blue and yellow paints to match the green colour, while viewing it in daylight. Then we view the result under ordinary tungsten light bulbs. This light is strong in yellow wavelengths, moderate in green but quite deficient in blue. So the original green paint reflects the greeen element of the light but although our matching reflects the yellow portion, there is no light which the blue paint component can reflect. So the balance of the yellow and blue is greatly altered and no longer appears the same as the green paint.
Since the way that our eyes filter the components of a colour can differ between individuals in the comparative strength of the three filters, red , green and blue, not everyone would have agreed that we had a good match in the first place. 'Colour blindness' is an extreme case, and any person's eyes are likely to change with age to the extent that an older person often disagrees with a younger as to what is or is not a good colour match. To this extent the result of looking with 'different eyes' is similar to the effect of changing the light source.
A trout's colour vision works in very much the same way as ours, with a small but significant difference. The red colour filter of the trout' eyes covers a slightly different and wider range than ours. So we can be pretty sure that when viewing a contrived colour match between two entirely different media, say the body of an insect compared with a mixture of coloured fibres, the trout will disagree with us as to the accuracy of the match. Most likely green is still green, red is still red, but probably not with the same differences of shade.
And after all, if the trout were so very picky about the precise shade of the Iron Blue, for example, they would be hard put to it to get a decent meal.
COLOUR VISION IN TROUT
The importance of colour in fishing flies and lures is a subject which
gives rise to a great diversity of opinion. In spite of living in a
'high tech' age there is still no widely accepted analysis of the
situation and understanding is scattered due to being based on partial
or faulty information; and on individual experience which may or may
not have been objectively assessed. I would like to try to clarify the
situation by examining scientific evidence relating to colour vision in
trout and thereby equip ourselves with a more realistic basis for
understanding the probable importance of colour in the trout's world.
IT'S ALL IN THE MIND:
To begin with we must understand the nature of colour vision in humans
because this is the yardstick by which we measure the rest of the
world. Colour is not a property of light. It is a physiological effect
- a sensation occurring in the brain in response to neural signals
arriving from the retina of the eye. Light falling on the retina causes
a reaction in two types of nerve receptor, "rods" and "cones". The
former register the presence of light but not in colour, like a
monochrome photograph. It is the cones which are responsible for our
seeing colour. But it is not correct to imagine that they respond to
coloured light since there is no such thing. There is no colour outside
our heads. (Note that in the following passages where reference is
made, for example, to "red light" or "red wavelength", this is loose
wording in order to avoid long phraseology)
There are three sub-types of cone receptor in the retina of the human
eye. Each responds to a different narrow band of light wavelengths and
each sends its own characteristic signal to the brain. The brain
replies by causing us to experience the sensations of red, green and
blue. Combinations of two signals give the remaining colours of our
interpretation of the spectrum. The fact that we see the spectrum or
rainbow as a graduated series of colour is because the wavelength
ranges of the three cone types overlap so that a single wavelength
causes response in at least two of the types, the ratio of the signal
strengths determining the shade of the colour. However, no single light
wavelength can cause a strong signal in the "red" and "blue" receptors
simultaneously because they do not overlap to any extent, so that if we
see a combination of short and long wavelengths, giving a strong
response in both those receptors, we see colours which do not exist in
the rainbow, that is the shades of purple and magenta. It is this
phenomenon which allows us to bridge the gap between red and blue and
create the colour circle, which is an analogy of our colour experience
and is at variance with the linear form of the light spectrum.
It follows, of course, that neither is colour a property of surfaces
that we see as coloured. Feathers, flowers, pigments, dyes all contain
chemical compounds which have the property of absorbing most
wavelengths of visible light and reflecting a narrow band. This
selective reflection is perceived by us as colour via the mechanism
In asking whether a trout or any other creature can see colour we must
not confuse ourselves. Since we have already established that colour
does not exist in the material world, the real question is whether a
trout has a neural mechanism similar to ours which causes selected
bands of light wavelength to be perceived as having distinct character
rather than simply assessing the total light energy.
It has been deduced that vision must have developed in early life forms
as the ability to detect movement as a result of change from light to
dark and vice versa. The rod receptors of our own eyes without the aid
of a focussing device would do this. Research has indicated that the
first evolutionary receptor was a cone with sensitivity to the longer
wavelengths of the Sun's radiation, i.e. red. This does not mean that
those creatures saw the world in red since a single cone response can
only show differences between light and dark without reference to
colour. Distribution of a number of such light sensitive spots would
reveal direction of movement. Evolutionary modification of cells to
form a focussing lens made it possible for later life forms to
pin-point the precise position of all the light/dark differences around
them - that is, to see objects. Two such optical devices makes it
possible to accurately judge distance.
The usefulness of this mechanism is, of course, to detect both prey and
danger; but where the vision is monochrome, camouflage is easy. Two
materials of entirely different constitution - say the shell of a crab
and the pebbles surrounding it - might to our eye be different colours
but if they both reflect the same total intensity of light as assessed
by the eye which has only one receptor, then the two will be
Evolution overcame this limitation by introducing another cone, one
which was sensitive to the shorter wavelengths of daylight, i.e.
blue-violet. With two characteristically different sets of neural data
arriving at the brain, the brain had to be able to superimpose the two
images and assess the quality of the difference between them which is
additional to the light/dark differential. This quality is that which
we refer to as "hue", and its combination with light and dark results
in colour. The combined siganl of two cone types does not give rise to
the experience of the colour circle. It gives a linear colour
graduation with no means of bringing its "ends" together to form a
circle. About 2% of the human population is deficient in one cone type
and has this kind of colour vision which also occurs normally in some
mammals - dogs and cats, for example. The 3-cone system, referred to as
trichromatic vision, is common to primates. So the relevant question is
- do salmonids have cones in their retinas and if so how many types?
HUMANS 3, TROUT 4:
The three types of cone which occur in the human eye are defined by the
chemical compound they carry, called a "diopsin", which is a
photo-sensitive pigment. Each of these has the capacity to absorb a
range of light wavelengths and is characterised by the wavelength at
which maximum absorption occurs. The three are, each for the colour
sensation it creates:
erythrolabe; peak absorption at 565nm; red
chlorolabe; peak absorption at 535nm; green
cyanolabe; peak absorption at 440nm; blue
Light wavelength is measured in nanometres and the visible spectrum
ranges from 700nm (red) to 400nm (blue-violet).
Analysis of the visual receptors of Salmo trutta has shown that there
are four peaks of cone response -
600nm, 535nm, 440nm, 355nm.
The second and third conform to the green and blue cones in humans. The
first is analogous to the human red, but its sensitivity extends to
longer wavelengths than its human counterpart. The fourth is outside
the band of wavelengths visible to primates and is active in the region
we refer to as "ultra-violet". However, it is found that this last
class of cones disappears by the time a trout is two years old.
The significance of the fourth cone is difficult to deduce. If it
interacted with the other three as they themselves interact then the
richness of the trout's experience of colour would be as impossible for
us to imagine as is the concept of shapes in a fourth dimension of
space. It has been suggested that the small fauna on which the immature
trout feeds reflect u.v. radiation and are therefore more visible in
that range. Sensitivity to u.v. exists also in other fauna: it is of
great benefit to some insects, for example, because many flowers
reflect strongly in the u.v. range. It is also suggested that u.v.
cones re-grow annually in mature trout as spawning time approaches and
are used to track polarised light as a means of navigation. This would
appear to be more significant for salmon and sea trout. In any case it
seems unlikely that a mature trout in which these cones do not persist
should use them for prey detection. It is reasonable to suppose in the
absence of positive evidence that the neurological response of the u.v.
cones is treated separately from that of the trichromatic system and
does not assist in prey detection. It should be mentioned at this point
that some people have made the statement that trout respond to
fluorescent colours because they see in the u.v. range. This is an
erroneous assumption and will be more fully explained later.
We should not overlook the shift of the long wave (red) cone response
of the trout as compared with ours. It has its peak response at a point
where the response of our "red" receptor is tailing off. This means
that where we see a red colour, which though a rich hue we regard as a
darkish colour compared with orange or yellow, the trout sees a much
brighter colour, visible in lower light conditions than we can see red.
Research has indicated that the trout's ability to distinguish between
small differences in shade is highest in blue, second but much lower in
red and lowest in green.
An important feature of the trout's visual mechanism is that the rods
and cones physically swap places at the start and end of daylight. In
the evening the cones which give rise to colour response but need high
light levels to operate are withdrawn into the surface of the retina
and the rods obtrude. At dawn the reverse action occurs.
So it is clear that trout possess the mechanism for full colour vision
as we know it and with a somewhat wider range. How the trout brain
assesses the combined cone response and the resulting subjective
experience of the trout is perhaps impossible to prove; but then, any
sceptic might claim that we can not prove common subjective experience
between any two human subjects. In fact the subjective experience of
colour is of no importance. It should be realised that when we talk
about possession of colour vision we are talking about possession of a
mechanism which enables qualitative assessment of received light in
addition to quantitative; and it is well established that trout do in
fact possess such a mechanism. If you opened a trout's head and found
it full of a coil spring and little meshing cog wheels, you would not
deny that it was driven by clockwork simply because you could not
"feel" the wheels running for yourself.
Rationalisation of the view that trout possess full colour vision does
not provide sufficient information to allow us to judge how it actually
affects their mode of life. They might well have colour vision similar
to our own but there are major differences in their environment most
particularly with regard to the available light. The usefulness of
their mode of vision must be limited by the quality of light which
enters the underwater world, i.e. the full potential of their 4-cone
system can be effective only if the full spectrum of sunlight from
infra-red to ultraviolet (so named because they are invisible to us -
but not to the trout) is available to them. On the other hand it must
be borne in mind that natural selection does not favour useless
Colour vision underwater is more complicated than in air. A given
colour can be perceived only if light of the appropriate wavelength is
present, or to put it another way a surface which reflects a given
narrow waveband must appear black if wavelengths in that band are not
present, and the situation is that all visible light is absorbed and
turned to heat as it passes through water, the distance required for
full absorption of any waveband depending on the condition of the water
at the time.
In perfectly clear water short wavelengths - blue to ultraviolet - are
scattered, causing the whole background to appear pale blue. This is
indeed what happens in the atmosphere giving rise to a blue sky, but it
is much enhanced in water. This occurs because the short wavelengths
are small enough to find that water molecules present an obstacle to
their passage and so any blue light which is reflected by, say, the
shiny scales of a fish, arrive at the eye of an observer by a
circuitous route from all directions. At short distance the image of
the fish is blurred and at a greater distance invisible. In water which
is not pure the scattering is exaggerated by suspended particles and
animal and plant organisms.
You might therefore expect directional sunlight passing through water
to tend toward red (as it does in atmosphere at sunrise and sunset) and
becoming more red with increased distance. However this is opposed by
another phenomenon, which is the absorption by water of long light
wavelengths. That is to say, the energy of the longer wavelengths - red
- is absorbed and converted to heat. In clear water the red end of the
spectrum is completely absorbed in about 12 feet. At about 60 feet only
the blue and ultraviolet wavelengths remain, even though they are
scattered in all directions. Note that these distances apply not only
to depth but also to horizontal viewing. So this is why, when sunlight
is reflected back from white sand, clear sea water looks green in the
shallows and bluer with increased depth.
So much for clear water. However, the relative penetration of different
wavelengths can be completely altered by impurities which might consist
, for example, of mineral particles, algae and/or peat staining. In the
first instance vision can be completely obscured in a few inches. A
given strain of alga is likely to filter out selected wavelengths. Peat
staining, when the water looks like weak tea, is interesting in that it
offers no obvious particle suspension but acts as a colour filter,
removing the ultraviolet and blue in a short distance and allowing long
wavelengths to penetrate the farthest - but no farther than the clear
water limit of (about) 12 feet.
It is therefore apparent that the colour vision mechanism can be fully
implemented only in relatively clear, shallow water and at relatively
short distances, though this does not take into account the
significance of luminescent organisms at greater depths.
SO - WHAT?
The ultimate aim here is to ascertain the extent to which our
perception of colour is potentially significant to the trout in its
environment. How the trout's behaviourism is governed by such potential
is outside the current scope and is more a question to be answered by
anglers' experience than by science. Nevertheless it is worth
considering that the reason for trout preferentially taking a
particular colour one day and a different one on another occasion might
be due to water conditions making that colour more visible rather than
any fickleness on the part of the trout.
In rivers and most of the time in still water, we are fishing near the
surface in clear water. At close quarters there can be no doubt that a
trout can see full detail of a lure, including colour. At longer range
absorption of the long wavelengths and blurring of the image due to
scattering become significant and it at this point it would be useful
to examine more closely the limitations of viewing horizontally, close
to the surface.
Consider a red object in the water. At a depth of more than 12 feet no
long wavelengths remain so it can reflect no light. Therefore even at
close quarters it appears black. Near the surface the full sun spectrum
reaches it and it reflects the red portion. This is visible at close
range and as already deduced is probably a brighter colour to a trout
than it is to us but as distance increases the reflected light is
absorbed and at some distance approaching 12 feet the object is no
longer visible - only the blue scattered light remains. If the object
were white, i.e. had the property of reflecting all incident
wavelengths, then it would remain visible at longer distances.
It is important to realise that the attenuation of light over a
distance is on a logarithmic scale, i.e. is not directly proportional
to the distance. Therefore if we observe that an image has finally
disappeared at 12 feet, then at 6 feet there might be as little as 10%
of the light energy left - not 50%.
It appears then that the full colour spectrum will be significant at
close range near the surface. So what is likely to grab the fish's
attention at greater distance? Of course, notwithstanding the
hypothetical distances given for light absorption, the stronger the
light is the longer it will persist. The flash of specular reflection
from a shiny surface such as metallic tinsel or the scales of a fish
will carry a much greater distance than body class="normal" colour.
It is in this respect also that fluorescent colours are important,
because they increase the intensity of reflected light - usually of a
particular colour. The increased visibility that we observe in our own
environment is also valid underwater given that the same modes of
attenuation apply as for "natural" light. A point needs to be explained
here. It has been stated by other authors that fluorescence is
important because "trout see in the ultraviolet". This is a
misunderstanding. Fluorescence occurs where a surface has the property
of absorbing ultraviolet radiation and converting its energy to be
reflected as a lower wavelength which is within our visible range. This
converted reflection is added to the reflection of normally visible
light wavelengths and thus appears more intense than "should" be
possible. The point about the trout's possession of a receptor which is
sensitive to ultraviolet is that it can see unconverted ultraviolet
reflection, which has nothing to do with fluorescent surfaces.
Obviously if it were necessary to have ultraviolet sensitivity in order
to appreciate fluorescence, then we would not be able to appreciate it.
It is interesting to note the experience of divers with regard to
colour visibility in various water conditions:
In turbid water: fluorescent red, orange, yellow are best.
In clear water: any fluorescent paint
At long distance fluorescent green and yellow-green are more important
(though this probably applies to deep water)
An important principle is involved here - since uv penetrates deeply
(deeper than the visible blue wavelengths) all fluorescent colours are
visible to the uv limit, way beyond the depth at which their "natural"
couterparts have become invisible.
But the situation is very different in peat-stained water. The
red-brown colour of the dissolved tannins is a result of their
efficiency in filtering out the short wavelengths which causes a
complete reversal of the clear-water filtering sequence. Ultra-violet
and blue are filtered out first, possibly in the first few inches,
whereas the distance that red penetrates is not greatly altered. Beyond
its limit, however, no light remains. Fluorescence is therefore useless
in peaty water at any depth but it should be borne in mind that where a
lure is in use near the surface such that it does receive uv rays then
red/orange fluorescence will be visible at greater distance than
shorter wavelength colours.
It is a matter of common observation that a trout closely examines a
relatively static or slow moving lure, for example an emerger, before
taking or rejecting it. Add to this the (elsewhere) suggested
probability that trout are sensitive to contrast and it can be seen
that a strong feature of the fly dressing such as ribbing might be
highly significant. At depth a fluorescent or shiny rib might have a
Insects sometimes carry a bubble and it is to be expected that this
would have high visual impact. Of course, its visibility is due not to
colour but to a difference in optical density between water and gas.
Such a difference can be provided by transparent pearly mylar ribbon.
A dry fly is seen by the trout mainly as a silhouette broken by its
"footprint" in the surface tension. It seems reasonable to suppose that
colour has less importance than in the case of submerged lures but
common experience suggests that it does have some importance. It is
worth considering, then, that translucent colour is much more likely to
be visible from below than an opaque mass of colour. For example,
tightly wound red floss forming the body class="normal" of a Royal Coachman might be
less effective than a winding of dyed red goose feather herl or dubbed
wool or seal's fur which has been fluffed out. Particularly in this
context, it is difficult to reconcile the trout's reported lack of
acuity where shades of green (olive) are concerned with the lengths
that people sometimes go to achieve accurate colour matching. It seems
probable that a pattern of contrasts (body class="normal"/thorax?) is more important.
Furthermore, accurate colour matching between different materials
(insect/dubbing) is likely to be utterly impossible for the following
All natural colouring matter has a complex spectrographic character. A
particular blue pigment, for example, reflects not only that narrow
band of "blue wavelengths" which mark its apparent position in the
spectrum, but actually reflects a mixture of separate wave bands which
our eye (and the trout's) can resolve only as if it were a single wave
band. (Very much unlike our ear which with training can identify every
note - or wavelength - in a chord.) Different blue pigments are likely
to have very different wave band mixtures. . The result of this is a
phenomenon known as "metamerism" whereby two surfaces which are
coloured one with, say, a reddish blue pigment and the other with a
greenish blue toned with a touch of red might look alike in one
condition of lighting but will appear different when the light changes
as it does when filtered through water. They will also appear similar
only to people with exactly the same balance of 3-cone response. Since
the trout has response which is quite different to ours at least in the
red zone, it can be more or less guaranteed that any attempt at
accurate colour matching will fail.
Finally, with reference to the aforementioned rod/cone changeover at
dawn and dusk:
the change occurs over several hours so there is no sudden change of
response. Nevertheless, at night a trout has no colour response
whatever and in the evening it is fading with the light. Contrast
therefore becomes much more important so that black and white is likely
to be the most effective combination. Silver tinsel could have value if
conditions are such that moonlight is significant.
The author hopes that this description of the objective aspects of
colour vision, together with the last section which examines some of
the ramifications, will assist in realistic deduction as to what to
offer according to the conditions of the day