Originally Posted by overmywaders
Where do you get that information? Most fisheries biologists acknowledge that the dorsal temporal zone of the trout retina still retains UV specific cones after smoltification. "THE MAIN SENSORS OF THE UV LIGHT WILL NOW BE THE BLUE CONES"
You asked the source of my information that trout lose their UV receptors after two years. It is from Trout University and Sexyloops. They both agree that UV vision in trout regresses in adulthood. Some scientist believe that it returns during the spawning and that somehow it helps returinging fish locate their native waters (see citations below).
Color Vision in Trout Eyes | Trout University
"The fourth is outside the band of wavelengths visible to humans and is referred to as "ultra-violet". However, the fourth class of cones disappears by the time a trout is two years old."
"Most research indicates that salmonids have cones to detect UV light when small, but as the fish grow these cones gradually disappear. Their diet in their early period of life consists of zooplankton and other small creatures that reflect UV light, but as the fish get larger they can no longer filter such food with their gillrakers. This is given as the main reason why no UV receptors are found in fish above 2 years old.
Other studies, however, have shown that new temporary UV receptors are created annually to coincide with the spawning migration and that these are used to detect polarised light as a navigational aid.
This would mean that returning seatrout and salmon do have some ability to see UV but I have yet to see any evidence that would lead me to believe that this is used for prey detection. It would seem unlikely, both because this is the time when their appetite is suppressed and, if it was a useful tool for finding food, it is logical that they would retain it throughout their life."
Now I enter from science into philosophy and science. Let us assume that trout can see weakly into the ultraviolet via stimulation of the blue receptor at 440nm. A stimulation of the blue receptor is "interpreted" by the brain as indicating a specific color, in this case blue and not UV. So does the fish "see" blue or ultraviolet?
The fish does not "see" ultraviolet. The fish sees the color blue, because the blue receptor is what is stimulated as you quoted; "The main sensors of the UV light will be the blue cones." The trout does not see what it sees when the UV receptor at 355nm is stimulated.
If that be so, then the blue appears slightly brighter due to the slightly extra stimulation from the UV.
Allow me to give an example of an analogous phenomena that we have all experienced, the doppler shift of a moving train whistle. The whistle sounds different approaching us and going away from us than it does when standing still. The true sound is when it is standing still. But when it is moving, there is a doppler shift to either a higher frequency (train moving toward us) or lower frequency (train moving away) Our brain can only tell us how our our ear drum has been stimulated and not the true frequency of the train whistle.