We’ve written extensively in In-Fisherman about the UV connection to fish based on the current science which tells us that many of the fish we pursue—bass, walleyes, sauger, pike, muskies, catfish, stripers, crappies, and a variety of sunfish species—don’t see UV light because they don’t have UV receptors in their eyes. Species such as trout, salmon, and goldfish do have UV receptors.
Humans also don’t have UV receptors and thus we can’t see UV light. But we’ve all seen baits marketed as “UV” brighten under blacklights. This isn’t UV light we’re seeing emitting from these lures, however. In this case, UV light emitted by the blacklight is causing the lure to fluoresce.
Fluorescence is a form of luminescence. Recall that the sun emits a solar radiation spectrum made up of various wavelengths. The visible portion of the spectrum ranges from violet (shorter wavelengths, higher energy photons) to red (longer wavelengths, lower energy photons). Ultraviolet light, or UV, has a shorter wavelength (higher energy) than violet and falls just outside the visible spectrum.
Fluorescence occurs when certain substances absorb light of a shorter wavelength (higher energy) and emit light of a longer wavelength (lower energy). Molecules in substances called fluorophores are responsible for this action. Various fluorophore compounds absorb light of a specific wavelength and re-emit light at longer wavelengths. In some applications, the fluorophore used fluoresces under visible light, such as a fluorescent orange construction cone or safety vest.
Certain fluorophores may only fluoresce when excited by “invisible” UV light. In the case of a lure that’s coated with a UV brightener, it may appear pale and ordinary looking under room lighting, but fluoresce under a blacklight, which is emitting UV as part of its emission spectrum. The UV light from the blacklight results in emission of longer wavelength light (say blue or green) from the lure causing it to fluoresce. UV light from solar radiation also causes it to fluoresce.
This brightening is what plays into the presentation process and not that fish can see actual UV light (unless we’re talking trout and salmon). Fluorescence adds to the contrast of the lure. More contrast makes a lure easier to detect in the environment, and fish may be more attentive to a brighter lure from a greater distance and show more interest.
It follows then that if and how much a lure with UV brighteners fluoresces depends on the amount of UV light in the environment. How much UV light from solar radiation penetrates into a water body has been shown to be affected by chlorophyll content and dissolved organic matter (DOM)—organic material dissolved in water from decomposition of plants and animals.
In very clear lakes with low amounts of DOM such as portions of the Great Lakes or high mountain lakes, UV can penetrate relatively deep. In Crater Lake, Oregon, one of the clearest lakes known to exist, UV can penetrate over 300 feet. In lakes with high DOM, it may penetrate less than a foot. Only a small amount of organic matter is needed to absorb UV rays.
In a previous article on UV and walleyes, written when the UV lure phenomenon was gaining steam, we reported that UV penetrated deeper than visible light. While this is the case in pure water, we understand now that UV penetration in natural waters is affected by dissolved organics and other factors, and that in many cases UV doesn’t penetrate as deep as visible light, and may even be attenuated at depths of just a few inches to a few feet. Again, it depends on the optical properties of the water in question.
We also wrote about some anglers who’ve reported greater success with UV lures in murky water, or in low-light conditions, situations we now know run counter to what the science says. In these cases, perhaps it was the silhouette, working depth, vibration, contrast, or other aspects of the lure’s overall presentation that made it more effective in these conditions. Or perhaps these lures contained glow characteristics along with UV properties. More on glow below.
This leads to questions about the effectiveness of UV lures—from a brightening standpoint—in various lake types or at certain depths. We’re talking here about the fluorescence of the lures, as UV lures also offer other presentational attributes such as vibration, profile, action, scent, or flash.
Do UV lures have “brightening” benefits under ice? That question starts with whether UV light can penetrate ice to reach the water below where it’s available to “excite” the fluorophore in a lure’s coating. Lake and river ice are relatively transparent to UV radiation because the organic matter that prevents UV rays from penetrating is largely excluded during the freeze-up process.* In clear Antarctic lakes, UV light has been shown to penetrate ice thicknesses of over 3 meters (about 10 feet).**
But solar radiation is highly reflected by snow cover, especially white fresh snow, and snow and white ice can severely limit penetration of UV rays.*** A snow removal experiment on Hudson Bay, Quebec, where the ice was about 3 feet thick with white ice at its surface and topped by 2 cm of snow (less than an inch), showed that the thin layer of snow reduced below-ice exposure to UV by a factor of three.
So UV lures have more utility, from a brightening standpoint, under clear ice with little to no snow. This also brings up questions about how deep UV light penetrates given the specific conditions at hand, and how much UV is necessary to brighten UV lures to a meaningful level. We also don’t know how sunlight passing through an open hole can affect this process. And as with open water, the angle of the sun has a large affect on how much light penetrates into water. At low sun angles, much of the light is reflected off the surface.
Lures with brighteners that fluoresce only under UV light are only part of a “fluorescence spectrum.” As mentioned above, other types of fluorescent paints found on many lures fluoresce when exposed to wavelengths into the visible portion of the light spectrum. So while a UV lure may not fluoresce at a certain depth where UV light doesn’t reach, another one painted fluorescent orange or yellow with fluorophores that are excited by visible light may fluoresce at that same depth or deeper because visible light can often penetrate deeper. For example, in a sample of Northeast U.S. lakes, the median depth at which UV decreased to 1 percent of its intensity at the surface was up to 0.92 meters (about 3 feet), while for “photsynthetically active radiation” (visible light) it was 3.27 meters (about 11 feet).****
We must also factor in glow or phosphorescence, which like fluorescence is a form of luminescence. The primary difference is that unlike fluorescence, phosphorescent materials continue to emit light for a period after the energy, or light source that excited the substance, is removed. Lures that glow also can be highlighted with fluorescent paints, and even coated with UV paints. In low light when there isn’t enough UV light to cause a lure to fluoresce, glow may take over as the visual trigger.
While we continue to research the UV topic, keep in mind that in the presentation process we shouldn’t focus solely or too heavily on fish vision, but also consider other presentational aspects including working depth, vibration, sound, scent, and retrieve patterns, which at times can me more important to a successful presentation.
*Belzile, C., J. A. E. Gibson, and W. F. Vincent. 2002. Colored dissolved organic matter and dissolved organic carbon exclusion from lake ice: implications for irradiance transmission and carbon cycling. Limnol. Oceanogr. 47:1283–1293.
**Lamare, M. D., and M. F. Barker. 2004. Transmission of ultraviolet radiation through the Antarctic annual sea ice and its biological effects on sea urchin embryos. Limnol. Oceanogr. 49:1957–1963.
***Wrona, F. J., T. D. Prowse, J. D. Reist, J. E. Hobbie, L. M.J. Le´vesque, R. W. Macdonald, and W. F. Vincent. 2006. Effects of ultraviolet radiation and contaminant-related stressors on Arctic freshwater ecosystems. Ambio 35: 388-401.
****Williamson, C. E., R. S. Stemberger, D. P. Morris, T. M. Frost, and S. G. Paulsen. 1996. Ultraviolet radiation in North American lakes: Attenuation estimates from DOC measurements and implications for plankton communities. Limnol. Oceanogr. 41:1024-1034.