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To UV or Not UV

by Simon Blanford
Trout Eyesight

Can this trout see wavelengths we can’t? Simon Blanford photo

I doubt there are many fly fishermen who pass up the opportunity to try out something new, something that might improve their chances of catching fish. That ‘something’ could simply be a new piece of gear, one that will send the fly farther or more accurately to its target. Or, perhaps it’s some new understanding about our quarry: where they may be at a particular time of day, what they might be doing under a particular set off conditions, how they might be locating and catching their prey.

One of our favorite debates revolves around how fish view their world, and more words have been written in the fly fishing literature on this subject than perhaps any other. Clearly, whatever other factors may influence our decisions, knowing at least the basics of a trout’s visual ability is helpful. And in fact we know quite a lot. For instance, we know they can see color—pretty well actually and across a color range similar to our own. That doesn’t stop us wondering how important color is to the trout and how that might affect our fly design.

From that fascination comes a question that has become the focus of much popular conjecture: Can trout see more? Can they see a “world invisible to man,” a world illuminated by ultraviolet wavelengths?

Our fly fishing community doesn’t entertain these thoughts alone. Hunters are told their white-tail deer quarry can “see ultraviolet” and those wanting to avoid a deer’s gaze would do well to shed any clothing reflecting this wavelength. Or so the advice goes. The trouble is, and despite its dissemination on the web, the study to which the hunting community often refers doesn’t actually say that white-tails see ultraviolet. In fact it doesn’t say anything of the sort. Indeed, recent work on reindeer (which can see ultraviolet, an adaptation to their arctic environment) points out that this latter study “… failed to find any UV response to such stimulation in the white-tailed deer….

It’s a salutary point because in the last few years ultraviolet vision has had a niche carved out for it in the fly fishing world. A book espousing its importance has been written (excerpted here on Midcurrent), the subject regularly appears on forum and blogs, and some retailers now offer ultraviolet-reflective materials for the fly tier. Despite this apparent acceptance, the importance of ultraviolet vision in a trout’s life has not gone unquestioned. So it’s probably worth having a careful look at our own homegrown attempt to imbue trout with this secret visual power—especially in the light of the less than sure-footed claims made by some hunters.

Let’s start at the beginning.

In vision, simply put, color perception is determined by two things: which wavelengths of light actually get through the initial barriers of the eye to fall on the retina, and then what range of color-interpreting cells the retina contains. To get to the retina, light passes through the cornea, the anterior cavity containing aqueous fluid, the lens and then another cavity containing a thicker fluid called the vitreous body. In some animals these barriers allow pretty much all the available light through. In others, like us, pigments in one or more of these barriers, or the structure of the barrier itself, can selectively limit the wavelengths that reach the eye.

Whatever light gets through to the retina then falls on two types of retinal cells, the cone and the rod cells. Cone cells do the daytime job of color vision and they are classified by the pigment they contain (a protein called opsin) and the wavelength of light to which these pigments maximally respond. For us, our Red-Green-Blue vision is created by a set of cone cells containing opsin, which is maximally responsive to long wavelengths in the red part of the spectrum (LWS cones); a set sensitive to medium wavelengths in the green part (MWS cones); and those sensitive to short wavelengths in the blue part of the spectrum (SWS cones). It is the combined input from cones containing different visual pigments, responsive to different wavelengths, which allows animals to perceive color.

Trout Vision  and Retinal Sensitivity

A simple schematic showing retinal sensitivity of four cone types responding to wavelengths in the red (LWS cones), green (MWS cones), blue (SWS cones) and ultraviolet (UVS cones) parts of the spectrum.

Our RGB color vision makes us “trichromatic,” meaning we have cones sensitive to three different colors. “Dichromatic” then obviously means sensitivity to two colors, and “tetrachromatic” sensitivity to four. Tetrachromacy is usually achieved by adding a fourth cone type that responds to even shorter wavelengths, those in the ultraviolet portion of the spectrum below about 400 nm (UVS cones).

The rod cells come into play during low light conditions. As dusk falls the cones and rods share the job of vision for a while (this is the dark adaptation period) and then once the sun is down the rods do all the work. They too have an opsin pigment, this one maximally responsive in the green part of the spectrum. It can be stimulated during dusk and dawn. But since color vision requires the interaction of two different opsin types at the same time, and as the cone cells shut down after dark leaving the rod cells on their own, trout, like us, are color blind at night.

There’s much more to all this of course (and if you really want to boggle your mind look up how mantis shrimps see the world), but for our purposes the basic description above is sufficient for the discussion below.

Evidence for animals seeing ultraviolet was first discovered back in 1882, when ants were shown to respond to these short wavelengths. One hundred years later the first fish species was quietly added to the ultraviolet list. A short paper briefly suggested that roach—a European member of the carp family and a very popular angling target—were receptive. In the same year a bigger splash was made by a study on the Japanese dace (rather endearingly known as the “ugui”) published in the prestigious journal Nature, showing it too could see ultraviolet. And a few years later still we get to the object of our interest when in 1987 James Bowmaker and Yvette Kunz demonstrated that brown trout were ultraviolet sensitive.

One wonders immediately what this additional color channel might be good for. Is it used to assess the health or social status of others? Does it help them navigate their world? Is it employed to locate, identify and catch food?

Sensitivity to ultraviolet light may be useful for all of these purposes, but of course  how it helps trout locate their food is what anglers are really interested in. Fortunately a number of researchers have also been interested in these questions. They’ve established that many zooplankton, like Daphnia, absorb ultraviolet and so might stand out more to an ultraviolet-sensitive predator. Initial studies on a number of species, including trout, were equivocal, some showing that being able to see ultraviolet aided in prey capture, and some showing it didn’t. It wasn’t until a couple of years ago that Professor Novales Flamarique carefully designed and conducted an experiment on trout (young rainbows in this case) to get to the bottom of it. He showed quite conclusively that the trout with ultraviolet cones could locate their zooplankton prey at greater distances, and across a greater field of vision, than the trout that didn’t have the specific UVS cones.

So where is the debate? Well, there is a reason Professor Flamarique chose rainbow trout parr, a reason that is revealed in the title of the study that first described ultraviolet vision in trout. The full title of Bowmaker and Kunz’s paper read: “Ultraviolet receptors, tetrachromatic color vision and retinal mosaics in brown trout (Salmo trutta): age-dependent changes.”

It turns out that trout and salmon do not maintain their population of ultraviolet-sensitive cones throughout their lifetime. Though the extent of UVS cone loss was the subject of a scientific spat a few years ago, the consensus now is that UVS cones are lost and replaced by blue-sensitive cones when the fish smolts—the process that prepares the fish for downstream migration and a period of life at sea. Resident trout go through a similar process, losing their ultraviolet cones, but don’t head for the ocean. A small number of the UVS cones may remain in the upper retina but are unlikely to be of any use for prey location. These cones would primarily respond to light coming from below—not the direction trout tend to focus on to find prey.

No dedicated population of ultraviolet specific cones then. Yet lately it’s been suggested that trout don’t actually need UVS cones. They can see ultraviolet using a secondary peak in sensitivity of cones maximally tuned to longer wavelengths. This secondary peak is called the beta-band or cis-peak. Have a look at this figure.

Adult Trout Vision

Retinal sensitivity of an adult rainbow trout. The beta-band shows a secondary level of sensitivity in the ultraviolet portion of the spectrum as shown. But note, these trout have had their cornea, lens and other ocular media removed. The light is shone directly on the retinal cells free of any blocking pigments and so represents the theoretical sensitivity rather than the actual sensitivity.


According to this conjecture, the beta-band shown in the figure above allows trout to see ultraviolet and see it especially well in the evening and early morning (when short wavelengths are more prevalent) as well as at night when rod cells take over the visual job.

Some of this is demonstrably true. There is more ultraviolet around at dawn and dusk, and there are a few animals known to use the beta-band. There is a bat for example, that uses it in the evening to locate flowering plants. And mackerel might view each other with the help of the beta-band to help maintain shoal cohesion—their vivid stripes apparently reflect ultraviolet.

But these appear to be exceptions to a general rule. More commonly the beta-band is something of a problem for vision. For trout this secondary peak only provides between a third and a quarter of the sensitivity they had when their young eyes were full of specific UVS cones. The beta-band is also problematic because short wavelengths get in the way of acute vision. This is especially true in the ultraviolet range as there is an exponential decline in acuity as wavelength decreases. It is one of the reasons we restrict the number of blue cones (which, though longer than ultraviolet, are still at the shorter end of the spectrum) from the area of our eye, the fovea, we use for our most acute vision. Additionally, ultraviolet is easily scattered across the retina, making it difficult to focus at anything but the shortest range. And the wavelength causes chromatic aberration (when the lens can’t focus color to the same convergence point) as well as reducing color constancy—an attribute that ensures an object remains the same color under different levels of illumination. All of these effects become more pronounced as the trout ages and the eye grows larger.

Trout deal with this by increasing the amount of pigment in their lens restricting the amount of ultraviolet reaching their retinas. Gerald Jacobs, long-term researcher on animal vision, points out that for brown trout, “the 50% transmission curve progresses from about 310 to 390 nm over the first three years of life.” The cutoff for ultraviolet vision is at about 400 nm so there really isn’t much wiggle room for a trout’s visual ability in this wavelength. And this includes being able to see it at night too. Aside from all the above factors restricting short wavelength penetration to the retina, there also happens to be very little ultraviolet light at night anyway. One research paper even goes so far as to say “there is no ultraviolet after dark”.

Taking all this into account researchers have modeled what light gets through to the retina of an adult rainbow trout. As you can see from the figure below, their estimates suggest that a rainbow doesn’t see much in the ultraviolet region.

Color Perception in Trout

Estimated color perception of an adult rainbow trout (the same one as in the figure above) after retinal sensitivities have been corrected for the UV transmissibility of the lens and solar irradiance. Not much gets through in the ultraviolet region of the spectrum.

Now I suppose it might be possible to squint at this summary and still suggest ways adult trout use ultraviolet. But even then there is another aspect to consider. What about the trout’s prey?  Shouldn’t a grown-up trout’s food interact with ultraviolet in a way that makes them easier to detect, just like the zooplankton prey of young trout? After all, even the best ultraviolet receiver needs to see a signal.

John Acorn regularly uses ultraviolet photography in his work as an entomologist. One of the areas he finds the technique useful is in helping to distinguish between species of Colias butterflies. They look very similar under visible light but are easily told apart under ultraviolet. Acorn is also a fly fisherman, and since fly fishing entomologists are knowledgeable folk his local fly fishing club asked him to give a talk on ultraviolet vision. Before the talk, he turned his skill at ultraviolet photography to taking pictures of trout food—mayflies, caddis flies and the like. Then, having given his talk he wrote an article on the topic for the journal, American Entomologist. He found that ultraviolet photographs of trout food looked “pretty much exactly the same as any other black and white photographs of the same subject.” He also went on to say that this was not surprising: “… trout food insects just simply don’t possess strong UV-reflectance patterns, and that trout are not likely to identify their food by looking for UV reflectance”.

It’s now known that many fish (goldfish, carp, roach, rudd, bluegills, bass and more) possess ultraviolet sensitive cones when young but lose them when they get older and larger. In general, young (or small) fish feeding on zooplankton benefit from having ultraviolet vision, whereas older, larger fish, foraging on opaque macroinvertebrates and smaller fish don’t. The food items of these older fish don’t usually convey an ultraviolet signal.

Ultraviolet light is an important component of vision for some animals and an interesting topic, not least because we can’t see these wavelengths and often think there is something special about animals that can. But for fly fishers, there seems to be no reason to elevate this trait to any significant status. For trout and a number of other species we target, ultraviolet-sensitive vision does not seem to be a component of the adult fish’s behavior, and it is especially unlikely to be an important part of the way they locate and identify their food.

MidCurrent Fly Fishing
Simon Blanford has been a biologist for twenty years, fished for more than forty and has long been interested in how research on fish behavior helps us understand the fish we try to catch. He lives in central Pennsylvania where the trout, carp and bass routinely snub his carefully presented flies. He can be contacted at
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  • Nymphermaniac

    Great article. That is about as scientifically specific as you can get on the subject. I was happy that they verified collectively that blue is the color most recognized by trout. I have found that to be my favorite color in size 18-22. “Blue Assassin”, is a real killer.—Rather, catch & releaser.
    Thanks mid-current! You’re the best!

  • 3weight


    A+ … this was excellent.


  • Nate


    Thank you! My take away from this article is that the bulk of my fishing targets (Smallmouth, largemouth, and trout) see what I see when I am looking at materials and flies. Granted, our visual acuities are not identical, nor am I considering what happens to colors as they move down the water column or giving considerations to water clarity. It is way too much fun to overthink these things! 🙂

    Tight lines!

  • Larry

    Good article but I’m going to add a twist. I’ve been struggling with the UV dilemma for some time. It does seem that UV materials improve success sometimes but I’m wondering how UV treated colors are viewed in the visible or non-UV spectrum by trout. In other words, does the UV treatment enhance how trout see regular (not UV) colors? Does it make them pop or glow or be more visible from distance than the same non-UV treated colors? It certainly appears so when we look at the materials ourselves under regular and UV lighting. So would a UV treatment of bright blue or white be seen easier than bright blue or white untreated by UV?

    • Simon Blanford

      Thanks for comment and the interesting observation. If I were to speculate, and it is nothing more than that, what you’ve observed may have something to do with the manufacturing process. For example the manufacturer may have used an optical brightener like stilbene – the same stuff found in some laundry detergents – which absorbs UV and emits blue wavelengths enhancing colors in the process. Trout may simply see this brightening of the material as we do. If that’s true (or in fact whatever the process) it might imply that your UV material simply acts as a good attractor, an attention getter in a similar way to the effect adding a gold bead or flashback has to a fly. Perhaps it would be a good way of rebranding UV material…….

  • Fred Rickson

    I know it is heresy, and those I fish with out of West Yellowstone certainly let me know they agree, but for the last 30 years I have tied my flys in black only. Maybe a contrasting rib; both wet and dry. I guess my epiphany came when tying pale morning duns and reading where I was supposed to match a nuanced yellow body shade to the particular stream in question, or I had no chance of success. Also, being a SCUBA diver most of my life, I realized that, looking up, a floating silhouette of about anything was some shade of dark. Hence black flies, to me, offered in the correct size, provided the perfect image for a fish. Not a great suggestion for those offering a selection of tying colors, but the fish seem to appreciate my myopic point of view.

  • Pierre Dunant

    So could we then assume that any perceived increase of success through the use of UV materials is (was) due to other factors, i.e. user confidence? If so, that could be an interesting study; take one fisherman’s “go to” fly and have a number of other fishermen use it in similar situations etc.
    Great article!

  • Captain Mark Hlis

    Articles such as this and specifically the video tutorials are why I refer all my fly customers to MidCurrent. Hope you are well Marshall.

  • UVRC

    For a refutation of most of the above article, I direct the curious to . I will deal with a few particulars below.

    Mr. Blanford stated: “Trout deal with this by increasing the amount of pigment in
    their lens restricting the amount of ultraviolet reaching their retinas. Gerald
    Jacobs, long-term researcher on animal vision, points out that for brown trout,
    “the 50% transmission curve progresses from about 310 to 390 nm over the
    first three years of life.” The cutoff for ultraviolet vision is at about
    400 nm so there really isn’t much wiggle room for a trout’s visual ability in
    this wavelength.”

    However, Jacobs was mis-quoting R.H. Douglas’s work “The spectral
    transmission of the lens and cornea of the Brown Trout (Salmo trutta) and Goldfish
    (Carassius auratus) – Effect of age and implications for ultraviolet vision”.
    The goldfish had a greatly reduced transmission curve in the UV, which makes
    sense since the goldfish is a domesticated descendant of the Prussian carp,
    which is a bottom feeder in murky water. However, the brown trout did not
    significantly alter its UV range as it aged. Douglas hypothesized pigments in
    the trout lens might have contributed to the shift toward less depth in the UV,
    but did not find such pigments. Douglas presented the following graph (see the graph at the end of this comment).
    Douglas records the diameter of a trout lens at 2 years as 4mm in diameter. The 6.23 diameter lens is from a much older, larger fish. Note that the 50% transmission is in the 360nm range, not the 390nm range. 360-380nm is an optimal range for many UV sensitive species.

    Blanford says “Taking all this into account researchers have modeled what light gets through to the retina of an adult rainbow trout.” Which researchers, in what peer-reviewed journal? And since the premise is fallacious, the result is equally so.

    Blanford says “Shouldn’t a grown-up trout’s food interact with ultraviolet in a way that makes them easier to detect, just like the zooplankton prey of young trout? After all, even the best ultraviolet receiver needs to see a signal.”

    Mr. Blanford avoided presenting the 2013 study “Ultraviolet reflection enhances the risk of predation in a vertebrate” by R. Modarressie, et al, which states “UV reflections seem to be costly by enhancing the risk of predation due to an increased conspicuousness of prey. This is the first study in a vertebrate, to our knowledge, demonstrating direct predation risk due to UV wavelengths.” The prey was three-spined sticklebacks, the predators were yearling Brown trout (yearling brown trout range in size from 7-9″). The trout aggressively chose the sticklebacks which had visible UV markings.

    Then Mr. Blanford tells of the UV-reflectance photography of John Acorn. Alas, Mr. Acorn was not taking UV photos. Acorn stated in his article the filter, camera, etc. that he used. His filter, a Tiffen 18A, leaks a lot of near-IR. The lens of his camera – he didn’t mention the type – would effectively reduce the UV reaching the sensor to a very small amount; however the IR would get through just fine. Effectively he was taking IR photos. [Note: I design and sell UV-bandpass filters around the world. They are used by forensic scientists, botanists, the US Navy Advanced Weapons, astronomers, etc. So I do know something of UV photography. See] If you look at the photos in my book – “The New Scientific Angling – Trout and Ultraviolet Vision” – you will see that the UV images differ greatly from the visible light images. For example, one yellow-dyed marabou might be dark in the UV, another similar in color, might be light in the UV.

    Since Mr. Acorn did not actually take any true UV photos of insects or fly tying materials, any conclusions he might draw from his photos are erroneous.

    I hope that this short rebuttal, trust me I could go on 🙂 , will serve to awaken the curious to examine the possibility that the article by Mr. Blanford was erroneous in almost all particulars.

    • Simon Blanford

      Hello Reed,

      Thanks for dropping in and adding your comment.

      You and I have been round the houses on this topic before and I (and other biologically literate fishermen) have found the experience rather frustrating and intellectually unrewarding. You’ll understand if it’s ground I’d rather not go over yet again. Besides, I’m sure the discerning Midcurrent readers are able to make up their own minds.

      best wishes

      Dr Simon

      • UVRC


        In order for them to make up their own minds the discerning readers should have access to the facts, don’t you agree?

        How you could tell your readers that “One research paper even goes so far as to say “there is no ultraviolet after dark”.” after you spent years studying mosquitoes as a dipterist. You know that An. gambiae, a nocturnal mosquito, expresses the UV-sensitive op8 rhodopsin in the mosquito’s major visual field. Without UV light at night, this insect would be largely blind. You also know that nocturnal swarming mayflies also rely upon polarized UV light in order to find their river… and often end up squished on a wet road because the UV reflection is similar to a river. Knowing these things you say “there is no ultraviolet after dark.”

        I trust the Midcurrent readers to sift the wheat and discard the chaff.

        Best wishes,

        • UVRC


          I noted one more statement of yours that was contrary to recent scientific findings.

          You said “Indeed, recent work on reindeer (which can see ultraviolet, an adaptation to their arctic environment) points out that this latter study “… failed to find any UV response to such stimulation in the white-tailed deer….””

          You failed to mention the 2014 study – “Behavioral measure of the light‐adapted visual sensitivity of white‐tailed deer” by Bradley S. Cohen, et al., which states “Deer were able to detect and train to ultraviolet and near-ultraviolet light (360 nm and 380 nm) and were more sensitive to these wavelengths than to the long-wave length spectrum (590 nm and 650 nm).”

          So contrary to your statement, white-tailed deer have ultraviolet sensitive vision.

  • RogueAngler

    As one who enjoys learning, I have found fly fishing and fly tying to be prefect in keeping me from getting bored. The discussion on UV and how it does or does not affect fish is really causing the mind to work. I have had a couple of experiences using personally tied flies with UV materials where I have caught way more trout than one could reasonably expect to do so. The last time, our fly club was fishing a trout lake it the Cascades of Oregon. Out of 15 or so anglers, I caught the second highest total, but I have so little experience in stillwater angling that something had to be assisting. The woolly bugger had a UV tail.

    I suppose one of the best ways to respond to the question, “Does using UV material improve the fish catching ability of the angler?” is to set up a controlled experiment. How about this.
    Two anglers using the same pattern in size and color, fishing from the same boat, keep track of both strikes and hook ups. The anglers would use the same size lines and measure the same depth of presentation. One angler uses flies of normal or traditional material and the second uses the same pattern made from UV materials.
    I would expect that the same methods could be used in flowing waters recognizing fish hold in different locations relative to the food sources.

    Once the season long results are tallied and analyzed, it might prompt a conclusive study from the scientific community as to the relationship of UV to the fishery.

    I see a longer term study as follow up, this time in saltwater…………..

    • Great idea!

    • UVRC

      Actually, we have always been using UV materials in our flies, as everything reflects a certain amount of ultraviolet light. Some tying materials, such as peacock herl, absorb most of the UV, thus they are “dark”. Others, such as the white bands in natural grizzly hackle, reflect most of the UV and appear very bright.

      We have already tested natural “UV-reflective” materials for their efficacy. In my book, I look at the top dry flies of 1984 as chosen by Dave Whitlock. Most of these patterns had been catching trout for thirty to fifty years. Thus, if the UV markings of the patterns were in sync with the actual insects they represented, it would be fair to postulate that ultraviolet light played a factor in the trout’s acceptance of those fly patterns. In all cases, the patterns approximated the naturals in their reflection of ultraviolet wavelengths. See the book for photos –

      Below is the Hendrickson in visible light and reflected ultraviolet. The natural blue dun hackle is quite bright in UV light. (Love that natural blue dun 🙂

      OTOH, below that are images of five bucktails, one natural and four dyed. Notice how dark the yellow is in the UV.

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