UV colours for trout – fact or fallacy?

UV colours for trout – fact or fallacy?

Stillwater flies tied with materials that display fluorescent under black light illumination - flies and photo by Jan Korrubel
Stillwater flies tied with materials that display fluorescent under black light illumination – flies and photo by Jan Korrubel

UV colours for trout – fact or fallacy?

There has been a lot of talk about UV dubbing and the use of UV crystal flash in flies for trout (and many other fish species). I started using UV colours a number of years ago after Mark Krige insisted that flies containing materials reflecting UV light worked better for stillwater trout than flies without it. The ‘UV’ fly we used back then, called the Krige Taddy, was a smallish tadpole imitation. The body consisted of black UV dubbing and for some reason the trout went nuts for the ones that were tied with the UV dub, but not so much for the versions with a plain black dressing – or so we thought.

I have since used UV crystal flash as rib in many nymph patterns and found that the trout in South African rivers came to the flies like monkeys in the zoo go crazy for bananas. In all honesty, the flies notably out-fished ‘ordinary’ patterns without the UV flash. I then started fishing UV dubbed flies for yellowfish in the Orange River and to my surprise the yellows pounced on UV caddis larvae imitations like they hadn’t had a meal in weeks. However, I haven’t experienced the same for saltwater species. To the contrary, UV colours or any flashy material for that matter seemed to scare predatory saltwater fish off rather than attract them, or at least those fish species I tried to catch on flies with UV colours in them. I’ve been told that only milkfish hone in on colours that are enticed by UV light – not flash, but ‘fluorescence’.

Just prior to my recent saltwater fly fishing trip to the Socotra Islands I spoke to a number of fisherman, including competitive anglers, in the Western Cape that frequently use materials that either light up as ‘fluorescent’ under a black light (mostly LEDs that emit longwave UV light) or materials that reflect UV light. Most of the people were under the impression that UV light penetrates deep (the deepest of the white light spectrum) into water and that it would be the most sensible material to use when targeting trout of all sizes (since they were also under the impression that trout lose the ability to observe UV light as they age and that, therefore, although only juveniles should observe UV or ‘fluorescent’ colours, it shouldn’t make any difference to ‘big-ol’ lunkers) and up to the deepest areas that can be reached with a fly line.

Edwardsiella, on the other hand, predictably advised me not to add any UV flash in my flies for our saltwater expedition to Socotra, but that I should rather use hot orange and chartreuse (most hot orange and chartreuse materials are fluorescent under a black light). Ed Truter is a big believer in colour, but not so much flash for saltwater fish.

Man was I confused between ‘fluorescence’ and UV reflection, flies with it or without it, juvenile and mature trout eyesight, trout and saltwater fish eyesight, and UV light penetrating deep or shallow in water etc. etc. etc. I decided to take these questions to task and find sensible and rational explanations for most, at least – if laborious scientific explanations bore you, go to the concluding points at the end of this article (you’ll miss out if you don’t read the rest though…).

UV light penetrates best in crystal clear, nutrient poor water; it is also the juvenile trout <60 grams that will be attracted to your UV flies in these waters
UV light penetrates best in crystal clear, nutrient poor water; it is also the juvenile trout of less 60 grams that will see the UV colours in your flies in these waters

Question 1: How deep does UV light penetrate water?

Firstly, let’s look at the UV spectrum; UV is the abbreviation for ‘ultra violet’ – the colours ‘beyond violet’ in the white light ‘colour’ spectrum (I place colour in inverted commas here because although part of the white light spectrum, UV is not visible to the human eye). It is correct that violet light penetrates the deepest in water of all the white light spectrum colours visible to the human eye, but ultra violet light penetrates very poorly in water. As with all white light spectrum colours, UV light has a wavelength and due to the high energy contained in the electromagnetic radiation of this ‘light-form’, it reacts with many things, such as ozone and organic particles (including algae and bacteria in water) and also does not penetrate water well.

The sunlight spectrum
The sunlight spectrum

Several studies confirmed this by either measuring the amount of UV irradiation in pure water or natural water systems, such as tropical seas (Bolton et al., 2011; and Dunne and Brown, 1996). It was demonstrated scientifically that 100% of what’s left of UVB in the 300 Nanometre (nm) range after penetrating the atmosphere of Earth, does not penetrate pure water deeper than 0.021 metres (Quickenden and Irvin, 1980). For UVA in the 350 nm range, this 100% maximum penetration depth in pure water was approx. 0.013 m (Fry, 2000). Bolton et al. (2011) reported that the most trustworthy data for UVB light penetration in pure water is that of Quickenden and Irvin (1980), and Fry’s methods were based on that of Quickenden and Irvin (Fry, 2000).

Furthermore, it was demonstrated that in natural, tropical sea water (which was very clear) containing very little organic matter in the form of carbon, bacteria and algae, most of the UV light penetrating the water surface was absorbed and 1% of UV light was visible at a maximum water depth of approx. 11 metres in the Maldives and only 3 m in Phuket (Dunne and Brown, 1996).

If we take secchi depth into account (which is likely the most reliable conservative water clarity measurement technique making use of visible light reflection), one of the clearest lakes (Crater Lake) has a secchi depth of 44 metres, while my personal studies have shown that Western Cape mesotrophic waters (mesotrophic is a term used for water with a moderate level of organic matter in it and represents the trophic status of most of the ‘high-land’ stillwaters, those lakes situated in mountainous terrain above agriculture, in the Western Cape) seldom have a secchi depth >2 metres. That is a ~22 fold reduction in secchi water clarity (secchi measurements take into account the absorption and scattering of light by particles and dissolved substances in the water) if compared to ‘very clear lakes’.

Let’s say the water in the Maldives, for instance, was not as clear as Crater Lake, but clear enough to give a conservative secchi depth of 22 metres (which is pretty good for saltwater). The most stillwaters in the Western Cape would have an approximate 11 fold decrease in water clarity if compared to the secchi depth of the Maldives. It is unlikely to be highly accurate, but a rough indication when this 11 fold decrease in water clarity is applied to UV light penetration leaves a 1% UV penetration depth at approx. 1 m in most of our ‘high-land’ trout stillwaters. UVA (320 – 400 nm) also penetrates deeper than UVB (280 – 320 nm) and would also be the UV rays mostly responsible for illuminating materials that reflect UV light in deeper water (at 1 m). Therefore, the UV theory holds true to some extent and assuming that trout can see pretty well, it is not impossible that 1% of UV light is picked up at a 1 metre water depth in fairly clear natural freshwater by these fish.

Something else to bring into account now is the angle of the sun. The 1% maximum UV light penetration measurements were performed in midday hours, when the sun was at its highest point in the sky (day- and season-wise) referred to as the lowest sun zenith angle and abbreviated as Z01. Therefore, UV light penetration through water and also visibility under water greatly depends on the time of day and time of year. Our best fishing period is that of high sun zenith angles, winter, when UV light penetration would be at its least in stillwaters. This leaves UV light visibility around a 1 metre depth, and deeper, questionable and flies containing UV reflective or fluorescent materials would be ‘ineffective’ if the fisherman relied on their UV properties to catch trout.

Hot orange beads are fluorescent under a black light - flies and photo by Jan Korrubel
Hot orange beads are fluorescent under a black light – flies and photo by Jan Korrubel

Question 2: What’s the difference between Fluorescence and UV reflection?   

There is only one true form of fluorescence; it can be defined as luminescence caused by electromagnetic radiation (such as UV light) – this is the fluorescence we observe when we shine a black light on hot orange and chartreuse fly tying materials. Other forms of luminescence are bioluminescence (caused by luciferase in insects, such as glow-worms, and other animals, including squid) and chemical phosphorescence [chromophores with triplet-producing aromatic aldehydes and triplet-promoting bromine, in which crystal-state halogen bonding can produce phosphorescence (Bolton et al., 2011)] – these two forms are responsible for the ‘glow-in-the-dark’ effect. I am mainly interested in the first form, i.e., when UV light is reflected in the form of colour photons by an object in such a manner that it appears to be a specific colour, such as red, orange, or yellow. The other two forms are important to especially saltwater fly fisherman – many squid and fish, as well as plankton in the sea produce bioluminescence due to the protein luciferase – see Fred’s experimental fly: https://feathersandflouro.wordpress.com/2013/01/17/the-atomic-poodle-a-night-fly/

Back to UV fluorescence…The different UV wavelengths can be summarised as follows: UVA = longwave ultraviolet (the longer wavelength is the reason why UVA penetrates slightly deeper in water); UVB = midwave ultraviolet; and UVC = shortwave ultraviolet, which is 100% blocked out by ozone in the atmosphere.

Black lights emit longwave UV light = UVA (which is the safest form of ultra violet light). Very few ‘fluorescent’ materials respond to longwave UV light; in other words, most of the products we test with our black lights will not show up fluorescent – only 15% of natural minerals respond well to longwave UV light. This means that although UVB may only penetrate natural mesotrophic freshwater at a maximum of say 1 m, we wouldn’t even know if our fly-tying materials responded to it…There goes the black light theory and it may be an explanation for why some ‘colours’ or rather ‘materials’ work better than others, even though the types show up as fluorescent under black light = UVA = longwave UV light. Some of these materials may only be fluorescent under UVB or fluorescent at a much higher intensity under UVB and may therefore show up ‘brighter’ in shallow water, but we’ll never know that…

UV reflective materials are those materials that do not absorb UV light at all and do not display fluorescence either. They simply reflect UV as a whole and appears sharp to the eye under a black light (if UVA is reflected of course). This also leaves the question: which materials actually reflect UVB? The layman that loves to fly fish will never know; it is only the minority of fly anglers that are scientists with access to midwave UV lights that will be able to tell the difference between materials reflecting UVB and appearing fluorescent under UVB light – in case you do have access to a UVB light, please wear protective goggles.

Question 3: Can trout see UV ‘colours’ and does age affect vision?

Some enthusiastic bloggers posted very interesting arguments against the rumours that trout observe ‘colours’ in the UV spectrum. The arguments were staffed with several scientific references, which in a nutshell pointed out that only very small trout of less than 60 g can observe ‘colours’ in the UV spectrum. I am not interested in what size trout have UV cone cells in their eyes, my interest lies with fluorescence – and this was correctly pointed out by the Tenkara Guides at the end of their blog post: http://tenkaraguides.com/category/trout-literature/

If humans can observe longwave UV fluorescence under black light LEDs, then it is highly likely that trout of all sizes can also see it – I have no doubt that they can, the question I have is to what extent can trout see these fluorescent colours (is the UV electromagnetic radiation from the sun alone enough)? Food for thought…

Concluding points:

  1. UV light penetrates water poorly;
  2. 1% of UV light will be visible at an approximate depth of 1 m in our stillwaters in midday sunlight in mid-summer;
  3. UV reflection or UV fluorescence will not show up in flies fished beyond a metre deep;
  4. UV flies will unlikely give the angler an advantage in the early morning and late afternoon hours;
  5. UV flies will unlikely give the angler an advantage in most Western Cape winter conditions;
  6. mature trout cannot observe UV light, but UV fluorescence;
  7. UV fluorescence is key and can be picked up to some extent with a UV black light;
  8. UV dry flies or UV ‘surface’ flies should have a greater advantage than UV sinking flies, especially when the sinking flies are aimed at fishing 1 m or deeper, in murky water, in early or late afternoon light, in winter etc.
  9. Go tie flies now and experiment with this radical ‘new’ info ;P
Which fly - the UVB one - which is that!?
Which fly? The one that’s fluorescent under UVB light! Which is that!?


Bolton, O., K. Lee, H.-J. Kim, K. Y. Lin, and J. Kim. 2011. Activating efficient phosphorescence from purely organic materials by crystal design. Nature 3:205–210.

Dunne, R. P., and B. E. Brown. 1996. Penetration of solar UVB radiation in shallow tropical waters and its potential biological effects on coral reefs; results from the central Indian Ocean and Andaman Sea. Marine Ecology Progress Series 144:109-118.

Fry, E. S. 2000. Visible and near ultraviolet absorption spectrum of liquid water. Applied Optics 39:2743–2744.

Quickenden, T. and J. Irvin. 1980. The ultraviolet absorption spectrum of liquid water. Journal of Chemistry and Physics 72: 4416–4428.

PS – the author’s calculations on UV penetration in Western Cape stillwaters is a rough estimate and not a scientifically calculated or proven value; however, the author believes that the value obtained in this article is not irrational and may well be close to what is fact. Note that a semi-submersible scanning spectroradiometer would be required to measure actual values in different types of mesotrophic stillwaters.

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