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Colour in the Fishes Eye - The Light (part 2)
by Max Garth

Following his Colour in the Fishes Eye article, Max follows up with a second, expanded edition.

It is important to understand that you are encased in the light from the sun, passing though the atmosphere and falling on the land and the surface of the sea. When you look up at the sky, on a clear day the sky, or rather the atmosphere is blue, and that indicates that our atmosphere is, like oceanic water, a blue filter. The light that falls on the ocean is filtered by the Ozone layer and the atmosphere over many kilometres, filtering and attenuating the various light wavelengths until the predominant colour is blue.

The atmosphere also collects and holds water vapour in the form of clouds, which are really just water droplets which refract the coloured photons, like little prisms, and produce white light. So the clouds are white. Often the water droplets are large, and absorb the light photons and the greater the absorption the darker the colour, ie storm and rain clouds. The darker the clouds the greater the light attenuation and the lesser the amount of light which falls onto the ocean surface. But looking down on the clouds, regardless of the type, the colour is white.

The Atmosphere

Those things that affect you, the night/day cycle, the seasonal changes, the variations in the weather conditions, winds, storms, rain whatever, all affect the passage of light into the oceans, rivers and lakes. The fish, their vision and their day to day activities are affected by the same things that affect us.

In the rivers there are variations relating to bank vegetation, which is varied by sun angle, seasonal growth, wind movement etc. If you understand that, and can modify your fishing to accommodate these changes you are more than halfway there.

Light is modified as it passes through the atmosphere depending on latitude, time of day or sun angle, and cloud cover all of which produce large variations in the amount of light that falls on the surface of the sea. Rain causes surface distortion and wind causes surface waves which move in relation to the direction and force of the wind.

These things also affect our visual distance but, in reality, the only thing that drastically reduces visual distance for humans is thick fog or smoke, which puts a heavy particle content into the atmosphere.

Light penetration

High level surface activity, strong swells and breaking waves introduce great amounts of bubbles near the surface, which further decrease the amount of light transmission into the water, as well as creating very strong grating light. All of these things effect the transmission of light from the atmosphere into the water column.

There will be surface reflection which will vary from 2% for vertical incident light, noon, to 100% for light striking the surface at the critical angle for a smooth surface, i.e. sunset and dawn, and will also be effected by surface conditions and irregularities.

As well as these effects the light varies with the seasons, as the sun angle moves from the north to the south over time. This varies the transmission angle and the spectral illumination. The effects of illumination change due to seasonal effects is also affected by the location, ie the actual latitude and longitude of the specific locations.

Snell's Window

Directly above the fish, as it looks up and swims around in that area close to the surface, is a bright spot of light called Snell's window. This spot covers an area, a cone, of about 97 degrees, relative to the fishes eye.

It is a result of the refraction of light passing through the water surface. If varies in size, brightness and shape depending on the depth of the fish in the water column, light intensity and the position of the sun and is visible not only to fishes but also to divers looking up at the water surface.

The edges of Snell's window are clearly defined and outside the area of the window the surface appears to be a mirror. Photons arriving from deeper water at angles greater than 48.5 degrees are totally reflected back into the water column creating a mirror effect.

Any object, regardless of its colour, a lure, a small fish whatever, which is on the surface in the bright Snell's Window light, will appear to a viewing fish looking up, as dark, or high contrast. Basically a body illuminated from above and viewed from underneath will appear dark, or black.

The underside of lures, or fishes, is not illuminated by light from above, it is in shadow and is viewed against that very bright circle of light. That makes it appear black.

This is what causes baitfish to hide under boats. The boat represents a very large black blob, and the bait fish a smaller black blob. A small blob under a large blob means the small blob can actually disappear. It is also the reason for baitfish balling up, they present a large dark blob to pelagic predators, such as Tuna, mahimahi, marlins and mackerels, which are blue/green dichromats, and red colour limited. There are no individuals just a large black blob, which may itself, disappear inside a bigger black blob presented by a boat hull.

Banded light

Light passing through the surface appears to be transmitted in beams, some lighter and some darker and this can cause objects to appear banded. This is modified by sea state because wave action causes a flicker effect underwater. Surface waves focus sunlight at different depths and causes illuminated objects to flicker. The rate depends on wave height and frequency. These grating effects are very visible underwater and become obvious when one observes underwater film footage and the effect is well known to divers and underwater photographers.

Added to this is the fact that the refractive index of air is one, and the refractive index or water is 1.6, and the sea water is 800 times denser than air. The variation in refractive index means that while fishes see things in the correct perspective, since their eye has a refractive index of 1.6, humans, with an eye refractive index of 1.0, in the water, see objects as 30% larger.

At the same time, fishes, refractive index 1.6, looking out through that bright Snell's Window hole into the atmosphere, with a refractive index of 1.0 see the reverse effect. If they see anything at all, because what they do see is moderated by the surface conditions, and the outside hemisphere, of 176 degrees is condensed into the small circle on the surface. The only time the fishes vision could be as clear as a bell, is if the sea surface was as flat as a large pane of glass. This is not an every day situation.

Common sense tells us that if the water surface, fresh or salt water, is ruffled or disturbed by wind, rain or waves the picture would not be clear or coherent.

During the time of full daylight through twilight and sunset the brightness of light changes by a factor of one million, and through the period from sunset to dark, light is further reduced by a factor of one hundred. This effect is also apparent in the ocean.

Water purity

Passage of light through water is effected by the water itself, chlorophyll, breakdown products of plants known as Gelbstoff, other organic matter such as plankton, and the sea state, since wave action, height and frequency can effect the available light.

Photons striking particles and water molecules are reflected and scattered randomly, and light becomes diffused underwater. The effect of these things provides a wide variation on light transmission and spectral irradiance at different depths.

When light photons are scattered they travel further through the water to reach a given point. As a result of this random scattering and filtering every direction, or line of sight, is a source of photons so the water appears to be coloured in all directions.

Space Light

This is referred to as background "space light" and it varies with the line of sight direction. It also varies with the particle content, which determines water colour and filter frequency which effects the background space light colour.

Fish must detect objects, food, against a background space light that has a relatively constant colour along any particular line of sight. Therefore, considering the fishes optical system the appearance of objects can change in contrast depending on the line of sight, whether it is vertical, horizontal or at some other angle.

Under the water surface things are entirely different from our gaseous atmosphere. The fishes live in salt and fresh water which can be clear, filled with organic particles or almost mud. These conditions cause varying degrees of light transmission or illumination attenuation, and varying degrees of frequency attenuation relating to depth.

Violet and Indigo are heavily attenuated as is orange and red. That in itself is important for, while anglers understand that red light is heavily attenuated they are generally unaware that indigo and violet are attenuated equally as heavily.

Colour and illumination

The water appears to be blue with a maximum frequency of 470nm and illumination exceeds 400 metres. In these waters, at 60 metres, spectral irradiance is only 5% of the available surface light.

In water that has green organic matter, or other particles, such as coastal oceanic and fresh water dams the frequency attenuation is different and the maximum transmission shifts from blue to green/yellow with a frequency maximum of 520 nm.

The water colour appears to be green and the maximum illumination shifts from 400 metres to 150 metres depth. Spectral irradiance is 5% of surface illumination at 20 metres.

In dark or brown water light is greatly attenuated and the colour range shifts to the red and infrared with frequency maximum of 680nm.

Most colours, violet/indigo/blue/green/yellow are heavily attenuated. Illumination is limited to a depth of 5 metres. The spectral irradiance 5% level of available light occurs at 1.5 metres.

In general, in clear oceanic water, of the total light that arrives from a clear sky at noon only 45% remains at 1 metre depth, 15% remains at 10 metres of depth and 1% remains at 100 metres. Ultra violet and infrared light is heavily attenuated at 1 metres and red and orange light are absent at 10 metres.

The colour of the water surface, as we see it, is the reflection of light photons of that colour and is a pointer to the underwater conditions i.e. the colour attenuation and light transmission in that particular body of water.

Because colour vision depends on high illumination and the spectral irradiance varies depending on water colour, or particle content, the cut off point for the illumination level, or threshold, for acceptable colour vision must vary with the particle content. In some areas because of the water colour and minimum levels of illumination the fishes colour vision may not be a viable option.


The light underwater is generally polarised and is the reason that anglers can see underwater objects with greater efficiency when using Polaroid glasses. Sensitivity to underwater polarisation could provide oceanic pelagic fishes with migratory information relating to sun angle and attitude. This could enhance their ability to navigate in an environment without significant landmarks.

For a fish with polarisation sensitivity the relationship between time of day and sun angle could provide high visual acuity with increased contrast which could determine feeding times. This could relate to the solunar tables used by anglers to indicate times of peak activity.

Fishes are different from humans in that they never stop growing, and as they grow the eye becomes bigger and the retinal packing density increases, which means that the vision becomes more acute. It also means that the fishes can repair injuries to the eye.

Some fishes, because of their specialised feeding habits have different visual acuity in different areas. Fishes which feed off organisms which attach themselves to rocks, like barnacles, oysters etc, would need to have very acute close up binocular vision, more so that a very long range acute horizontal vision.

Trevally on the other hand need to have acute horizontal vision as well as binocular vision, because of the feeding habits of that species. These variations are categorised by the different areas of increased retinal density of the eyes of different species.

Lure visibility

The light photon reflection theory effects lure colours underwater, particularly deep diving lures or jigging lures. The deeper the lure below the surface the lower the photon density and the lower the reflective effect.

The lighter and brighter the lure the greater the number of reflected photons and the standard white reflects all light photons and black absorbs all light photon theory applies but directly in relation to the spectral illumination. In other words lures are as bright as the underwater illumination allows.

The visibility of lures is totally dependant on water colour and presentation depth, which is related to particle content, which effects the numbers of photons striking the lure which effects photon reflection which directly effects the visual distance.

Everything affects the fishes vision. The atmospheric conditions at the time, the amount of particle content, the greener the water and these effects change from minute to minute as the atmospheric conditions change.

All of which means that the effective range of a fishes vision can vary quite drastically as it swims around the ocean, as it gets deeper in the water column, as the particle content increases, the water gets dirtier, or as the surface conditions vary and surface bubble content increases.

Nothing is static in the ocean; these things change rapidly and often. But the fish is not insensitive to these changes in illumination and it can, and does, regulate its depth to maintain eye comfort, that is to keep the illumination level in the comfort zone.

Vision specific

This can be a barramundi in tropical creek water, where the surface illumination is low because of particle content and the fish is operating in grey scale mode. Under these circumstances the fishes will be at a comfort zone depth, where they are most comfortable. Under these circumstances colour is irrelevant, since the fish is not going to see colour.

The facts are that you live in atmosphere, the air around us, the fishes live in water, a totally different medium, with totally different characteristics. The underwater illumination is directly effected by the atmospheric conditions, and also by the underwater characteristics. Our eyes are geared to our conditions, the fishes eyes to their situation.

There is but one human species, and 25,000 species of fishes. We have a single visual system; the fishes have many visual systems, depending on their lifestyle, their habitat in the water body they live in, the depth, their food source etc. We are not fishes, and we need to understand the vast differences between our habitat and the habitat of the fishes. Without that understanding we are just guessing, and sadly, most of our guesses are wrong.

These articles on "Colour in the Fishes Eye" are a very brief part of the story, a mere smattering of bits of information out of a subject that needs, to get a complete picture, a lot of research.

Science and the Mumbo Jumbo

Fishermen generally do not know enough about this subject; they seem to regard science as mumbo jumbo. Some do not even know that this research has been going on continually for many years. But it is a solid fact that the so called "Mumbo Jumbo" got man fishing with some very sophisticated tackle and lines. It was not done by observation, although observation may help in its usage.

Observation can be helpful, providing you understand what you are observing, particularly in regards to the water colour confronting you, and your fishing will improve, because you will know what the fish is going to see of your offering. How far it might see the fly or lure, and whether it can see things in colour or not.

Sitting in front of someone lecturing on the subject, is not a sure guarantee that what you are hearing is worth listening to. What is a sure guarantee is to do your own research, find out "de trute" rather than "de furfy".

Over the past few years Dr Julia Shand PhD of the WA University Zoology Dept has been researching the visual systems of West Australian black bream, with particular reference to Swan River black bream, from the time of spawning to adulthood. Bet there is some surprises in that paper, especially for adherents to the "mumbo jumbo" theory.

You could try the following. From your local University Library.

Scientific American Vol 246 1982 "Colour Vision in Fishes" by Joseph S Levine and Edward F. MacNichol, Jnr.
"Light and Life in the Sea" 1990. Ed's P.J.Herring, A.K.Campbell, M. Whitfield, C.L.Maddock. Cambridge University Press with particular reference to Chapter 10 of that work, "The Colour Sensitivity and Vision of Fishes" by J.C Partridge.
"The ecology of the visual pigments of snappers (Lutjanidea) on the Great Barrier Reef", J. N.Lythgoe, W.R.A. Muntz, J.C. Partridge, J. Shand, D. McB. Williams.
"Adaptative Mechanics in the Ecology of Vision" Kluwer Achademic Publications Netherlands.
"Specialisations of the telost visual system: adaptive diversity from shallow-water to deep sea". Shaun P Collin 1997. From "The Visual System of Fish" Ed Douglas & Djamgoz 1990 Chapman & Hall London.

Max Garth has been saltwater flyfishing Australian long before it was possible. Inventor of the term "hucking" (to cast a flyline, much like a brick, whilst standing on a rock), I get the impression that there's not much Max hasn't tried. He lives and fishes West Australia, where I hope to meet him on my travels.


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