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locate prey, primarily plankton. As these fish mature, the UV cones become less sensitive to UV and more sensitive to blue light, likely to aid in locating smaller fish and invertebrates which comprise most of their diet. Most anglers aren’ t targeting juvenile fish, so is there even a case for using UV while fishing? I think there is.
While many juvenile fish are hypersensitive to UV, there is considerable evidence that adult fish still retain the ability to see UV albeit highly reduced. Both humans and fish possess specialized cone shaped light sensors in our eyes that are tuned to particular light wavelengths. Humans have three cones that are sensitive in the red, green, and blue peaks. Fish possess similar cones, but these light sensitive cones also have a secondary“ beta” peak of sensitivity that allows them to detect UV light even in the absence of a UV sensitive cone. This sensitivity to UV light in the absence of specialized cones has been documented in both the salmon family, including trout, and in the sunfish family, including bass. The degree to which these fish are using UV in locating prey has not been studied, but if UV reflectance allows my flies to look more like natural prey or entice a curiosity bite, I am more than eager to have it.
The vast majority, 95 %, of the UV light reaching the planet’ s surface is UVA light. A smaller percentage is UVB. UVB is what gives you a sunburn, but also helps in the formation of vitamin D. However, UVA light is what fish are keying in on for visual cues. As mentioned earlier, UV light has a short wavelength. This property allows UV light to penetrate deeply into the water column like its corollary in the visible spectrum: blue. This is why the ocean appears blue, as it is the most reflected color from the water whereas colors such as red are absorbed in the top of the water column. This is why deep sea fish are often red in coloration as there is no red light available at depth, making the fish essentially invisible to predators. Water transmits light poorly so no matter what depth you are fishing, a UV lure is going to reflect a little more light, and that may just make the difference in low light situations whether fishing at depth, or at dusk or dawn.
Speaking of reflecting light, have you noticed how many UV products appear more vivid than their non UV counterparts? This is because many UV reflecting materials are also rich in phosphors. These structures absorb light energy( both visible and UV) and reemit it as visible light. This is why a black light, which emits UVA light, makes UV material shine so brightly. The phosphors are taking the UVA light that you cannot see and reemitting in a spectrum of light you can see, they are like little light factories, and that’ s the reason why phosphors are commonly added to many laundry detergents to make your whites whiter.
When looking for the maximum brightness, I always carry a small black light with me when inspecting flies and tying material. I am always going to select the material with the strongest fluorescence under a black light if I am going for brightness. Even if you don’ t believe fish are using UV light in locating prey, the brightness argument is a compelling and brilliant one.
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