Most fish have a vision that is similar to ours, but with better low light and contrast detection, and many species can see light in a lower spectrum than we can. The lower range that we cannot see, but some fish can, is called the Ultra Violet (UV) range. Sockeye salmon have been show to have UV spectrum perception which is thought to be linked to enabling them to have better prey contrast detection. Recently many manufacturers have started to produce materials and lures that are labeled “UV”; is there benefit to having an added UV range? And do the fish think so?
Anybody who has ever done some scuba diving knows that as we go deeper under the water the light changes colour. This is due to the absorption of certain wave lengths by the water. Reds and oranges are some of the first to ranges to disappear. These are the longer wavelengths. If you cut yourself underwater at a depth of over 3 meters or so, your blood will look black! All the wavelengths of the red light have been absorbed by the water and black is what remains. The shorter wavelengths like blue and indigo are visible at a much deeper level, which is why the underwater landscape changes to a blue tinted moonscape when we dive deeper; all the other wavelengths have been absorbed by the water. Most colours, including UV, will have disappeared in clear water of 200 meter depth. In relatively clear water encountered in clear lakes and rivers, UV light will penetrate approximately 50 meters. For us fishermen the upper water column is most interesting. Roughly we could say that in clear water the first 10 meters still contain some yellows, reds and orange frequencies. But in water deeper then that these colours will disappear and the midrange and high frequency waves are the only ones that penetrate to that level. For us, or the fish, to see them these frequencies have to be reflected off an object (most importantly our fly!).
Fish species that are able to detect UV light will be able to see fish or objects that reflect UV light. In conditions where large aspects of the light spectrum are absorbed, UV detection will make the fish or object stand out more. This would typically be the case in low light conditions or at depths over 10 meters, but UV could also be a colour that is attractive to certain species within the full spectrum.
Fluorescent or “hot” dye pigments will respond better to high frequency light, such as UV, and therefore will more visible. Research also indicates that lures with fluorescent properties are more effective then lures without.
So it appears likely that some fish, including salmon, have adapted their vision to include the UV spectrum. Without trying to scientifically prove the effectiveness of UV materials, it seems logical that the UV sensitivity of some fish is there for a good reason. Most likely UV detection is important in behavioural aspects such as reproductive behaviour, inter-species recognition and predation.
With that said, here are a few images of how a fly might look under different circumstances from a human or fishes perspective.
