A Note to the Reader: There has been quite a bit of interest in ultraviolet (UV) tying materials over the past decade. What follows is a scientific look at the UV world in fly fishing. If high school science class was a great opportunity to nap after lunch, this article ‘aint gonna keep you awake. Skip to the bold text at the bottom. It’ll feel like the nerdy guy did your homework again. On the other hand, if you’re motivated to be a better angler, and want to begin learning about trout vision, read on. We’ll start with the basics, and get a bit more complicated as we go.
Remember “ROY G BIV”? That’s the human visual spectrum of light. Red, orange, and yellow (ROY) is on the longer wavelength, low frequency side of the spectrum. Blue, indigo, and violet (BIV) is on the shorter wavelength, higher frequency end. UV has an even shorter wavelength and higher frequency than violet light. Hence, the name ultra-violet. UV light lies just outside the human visual spectrum of light. When you shine a “UV light” on a material to see if it glows, you’re not actually seeing UV light. What you are actually seeing is fluorescent light. Fluorescent light is totally different from UV light. Remember that part.
The common belief seems to be that trout see the world differently than us humans. They see in an extended spectrum that includes not only the same colors we see, but also colors in the ultraviolet range. By taking a look at a trout’s underwater world, the bugs she eats, and the materials we use to mimic those bugs, we can learn to make more appealing flies. Heck, with enough insight, we could create the ultimate fly. The fly-to-end-all-flies. The ultra-fly. Right?
The argument for UV was popularized in Reed Curry’s book, “The New Scientific Angling – Trout and Ultraviolet Vision”. While UV fly tying materials have been available for many years, it wasn’t until after the publication of Curry’s book in 2009 that the shelves of our local fly shop exploded with them. I will admit the Tenkara Guides LLC crew has spent our fair share of coin experimenting UV materials, and not without some apparent success. One of our favorite flies, the Grave Digger, relies on a base and collar of UV chartreuse thread. You might find a few Utah Killer Bugs tied with UV pink yarn in your guide’s box, too.
I never thought to take a closer look at the evidence behind UV trout vision. That is, until a post by our friend Anthony Naples lead me to the work of Eccles. On his blog, “Turning Over Small Stones”, Eccles presents a well-cited argument AGAINST the utility of UV materials in fly tying. What caught my attention was the well-cited part. In contrast to Curry, Eccles presented a rather formal reference list of peer-reviewed journal articles. Hmm.
Who is right, Curry or Eccles? To answer the question of UV trout vision for myself, I took advantage of the fact that I paid a ton of money for a terminal degree, and performed a systematic review of existing literature to identify peer reviewed scientific journal articles related to UV trout vision using three available source engines and a host of search term combinations. Here’s a summary of what I found:
- It all started in the 1980’s when a series of studies confirmed the presence of UV sensitive cone cells in the retina of very, very small brown and rainbow trout (1,2)
- These little guys may even use their UV vision to hunt for food (3). Then again, a later study refuted this claim (4). Note at this age these tiny fish are eating plankton. I can tie a size 26 midge, but I don’t know anybody that can tie a plankton fly.
- While UV cone cells are present in these super tiny trout, the same cones were absent in two year old brown trout (1).
- This loss of UV sensitive organics in trout and salmon has been confirmed in multiple studies. It happens during the alevin-parr-smolt transformation, somewhere between a paltry 10-60 grams weight (2,5,6).
- There is limited evidence that these ultraviolet cones could regenerate. The loss of UV cones during smoltification has been linked to the hormone thyroxine. Give a tiny trout (19 grams) thyroxine, and they loose their UV cones just like they would naturally. Give a slightly larger trout (>90 grams but still really, really small) thyroxine, and some of those cones come back (7,8).
- In fact, there are a few studies that appear to confirm the presence of UV cones in anadromous salmon (but not trout) when transitioning from sea to freshwater to spawn and die (9,10).
- More recently, however, we find a series of studies that refuted the claim that UV receptors can regenerate naturally. In fact, not only do they refute them, they pretty much take the prior papers to school for methodologic error (11,12).
- At this point, I will point out that we have uncovered ZERO convincing evidence for the presence of functional UV vision in the size of trout we target in fly fishing.
- Even if all of this is complete bull, it’s unclear whether UV light even reaches the mature brown trout retina. Turns out there may be an age-dependent decrease in lenticular short-wave transmission in trout. In other words, similar to humans, the lens of a mature brown trout may act as a UV filter (13).
To sum it all up, if you are tanago micro-fishing for 60 gram or smaller trout, or happen to be casting a fly to dying salmon, then perhaps UV materials are for you. But there is no evidence in the current body of scientific literature to support the use of UV materials in fly tying when targeting game sized trout like the German brown or rainbow. They simply don’t see the world in ultraviolet.
Reed Curry, your book has some very useful information in it, and is a worthy addition to an angler’s library. But there is misunderstood info, too, and should be read with healthy skepticism. I simply cannot agree with your basic premise.
Well done, Eccles. Well done.
One last note. It may be that the apparent success some of us report with certain UV materials is actually the result of fluorescence or iridescence. If so, then we need to change the way we evaluate our fly tying materials. More on that to come . . .
- Bowmaker J, Kunz Y. Ultraviolet receptors, tetrachromatic colour vision and retinal mosaics in the brown trout (salmo trutta): age dependent changes. Vision Res 1987;27:2101-2108
- Hawryshyn C, Arnold M, Chaisson D, Martin P. The ontogeny of ultraviolet photosensitivity in rainbow trout (Salmo gardneri). Vis Neurosci 1989; 2:247-254
- Browman H, Novales-Flamarique I, Hawryshyn C. Ultraviolet photoreception contributes to prey search behavior in two species of zooplanktivorous fishes. J Exp Biol 1994; 186:187-198
- Rocco et al. (2002).how much does ultraviolet radiation contribute to the feeding performance of rainbow trout, Oncorhynchus mykiss, juveniles under natural illumination? Environmental Biology of Fishes, 63, 223-228.
- Kunz YW. Tracts of putative ultraviolet receptors in the retinal of the two-year-old brown trout (Salmo trutta) and the Atlantic salmon (Salmo salar) Vision Res 1987; 43:1202-1204
- Novales –Flamarique I, Hawryshyn C. Retinal development and visual sensitivity of young Pacific sockeye salmon (Oncorhynchus nerka) J Exp Biol 1996; 199:869-882
- Hawryshyn C, Martens G, Allison T, Anholt R. Regeneration of ultraviolet-sensitive cones in the retinal cone mosaic of thyroxin-challenged post-juvenile rainbow trout (Oncorhynchus mykiss). J Exp Biol 2003; 206:2665-2673
- Allison W, Dann S, Veldhoen K, Hawryshyn C. Degeneration and regeneration of ultraviolet cone photoreceptors during development in rainbow trout. J Comp Neurol 2006; 499:702-715
- Beaudet L, Novales-Flamarique I, Hwryshyn C. Cone photoreceptor topography in the retina of sexually mature Pacific salmonid fishes. J Comp Neurol 1997; 383:49-59
- Novales-Flamarique 2000. The ontogeny of ultraviolet sensitivity, cone disappearance and regeneration in the sockeye salmon Oncorhynchus nerka. J Exp Biol 2000; 203:1161-1172
- Cheng C, Flamarique I. Chromatic organization of cone photoreceptors in the retina of rainbow trout: single cones irreversibly switch from UV (SWS1) to blue (SWS2) light sensitive opsin during natural development. J Exp Biol 2007; 210:4123-4135
- Cheng C. Thyroid hormone induces a time-dependent opsin switch in the retina of Salmonid fishes. Investigative Ophthalmology and Vision Science 2009; 50: 3024-3032
- Douglas RH. 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. Vision Res 1989; 29:861-869