I recently read something on UV in mature trout eyes and apparently UV ability is present in fry, then lost. It is then regained at "maturity". Some speculation was that its role may be for navigation coupled with polarized light use. When I heard "maturation" I remembered research I'd read pertaining to the gain of red visual pigments at "maturation" in anadromuous salmon and steelhead, for use in shallow water and nuptial behavior. (I was interested in this research bc it explained the short wave length nuptial coloring in steelhead and all other salmonids.
Made me wonder if the reference to UV sensitivity in “mature” rainbow trout has a purpose other than feeding, and whether it is an annual event with “annual maturation” –associated with nuptial coloring, kype development, and nuptial behaviors. Would resident rainbow populations maintain this year round?
Mack's question of whether UV vision is useful is a valid question not just for fisherman but for fish too. Evolution tends to prune less than useful functions due of energy allocation pressures.
Interestingly, female largemouth bass have been described by some anglers as “lighting up” –taking on a very UV lavender “aura”. Doug Hannon talked about this in his writing. And I’ve actually seen this too, but so infrequently (despite looking at spawning bass closer than most) that I could say it’s always present. Maybe in situ lighting was involved?
Also, I’ve always noticed that post-spawn brown trout, in particular females as they recuperate (and a get “metallic” sheen again), have lots of bright UV-ish looking blue pigment on their bodies, especially on the gill plates and forward parts of the sides of the body.
And, come to think of it… is the very pretty pale blue pigment in resident stream brown trout –usually there’s a spot behind the eye (often associated with a black spot), and the blue halo’s around the black spots on the sides (esp fore)– UV reflective? If so, this would suggest social visual cues in the UV range amongst trout. Is this maintained year round? Is it more prominent in clear water, turbid water, shade, sun?
This one is lacking the black spot behind the eye, or maybe it's obscured. Anyway, it shows a lot of the blue pigment:
Anyway, all purely speculative. The simplest speculation is that UV vision and red vision in resident trout is permanent and applicable for visual cues and for feeding. Since UV reflectance is common in the terrestrial insect world it would make sense that rout might retain UV sensitivity. But I doubt seasonal changes in trout vision has been pursued very far.