Multiple studies done in recent years usually balance conjecture formed because of the results of any one study. Here for example, is another study:
The distribution of corner (putative ultraviolet-sensitive) cones in the retina of Atlantic salmon was examined from the small
juvenile (parr) stage to the adult stage (approaching sexual maturation). Small parr weighing 5 g lacked corner cones everywhere
except, mainly, near the dorsal periphery. Large fish (5 kg) approaching sexual maturation showed corner cones in other areas of
the dorsal retina besides the periphery. These areas, characterized by low resolving power, had similar corner cone densities to
analogous areas in the smolt retina, suggesting that corner cones are formed in the periphery and incorporated into the dorsal retina
of the Atlantic salmon sometime during the smolt stage. This incorporation is partial both in numbers of cones and in location (only
the dorsal retina is affected). These findings contrast with the situation in rainbow trout where corner cones from existing mosaics
are only partially lost from the ventral retina, if at all, and where production and incorporation of these cones into the dorsal retina
occurs throughout life. Thus, in salmonids, there are at least two different strategies that determine retinal corner cone distributions.
from Partial re-incorporation of corner cones in the retina of the
Atlantic salmon (Salmo salar)
Inigo Novales Flamarique
Vision Research 42 (2002) 2737–2745
Gradual and partial loss of corner cone-occupied area in the
Recently, Martens (2000) has shown that smolt rainbow
trout retain corner cones throughout most of the dorso-temporal retina. This result challenges the notion of a functional loss in UV (corner cone-driven) sensitivity in smolt fish, as concluded from behavioural studies
(Hawryshyn et al., 1989; Browman & Hawryshyn,
1992). It also raises the question of whether the loss of
corner cones in this non-anadromous species is different
from that in anadromous (sea-going) salmonids where
corner cones seem to disappear almost entirely around
the time of smoltification (Kunz, 1987; Kunz, Wildenburg,
Goodrich, & Callaghan, 1994; Novales Flamarique,
retina of rainbow trout
Inigo Novales Flamarique
Vision Research 41 (2001) 3073–3082
They conclude is that some UV cones remain in the dorsal retina after smoltification. One would need to examine older wild fish at 2 years to see if that is true in 2 year old wild fish since they have not proven that thyroxin is equal to maturation in the wild.
See other studies above.
Secondly, even if this were true in wild fish, the visual field of the dorsal retina is that area of vision below the horizontal midline of the field of vision. So if we are to be accurate and accept the persistence of direct UV vision in adult wild fish, it would be approximately 50% of the visual field at a reduced level of UV cones, and would exclude dry flies that are seen by the ventral retina.
No, your "direct UV vision" (what is "indirect vision") is a strawman. Photons in the range 340nm to 400nm striking any of the cones - red, green, blue, or UV - will be recorded. At dawn and dusk, most of the vision would be through UV photons; the same is true with the rods after dark. The higher energy of the UV photons vs. visible light, is also significant.
Your statement "and would exclude dry flies that are seen by the ventral retina" is a red herring.
Fish are not inextricably held in a plane parallel to the stream bottom. Trout tip down to examine things on the bottom, and tilt backwards to examine floating curiosities. When we speak of rising trout we don't mean that they rise like a submarine blowing all its ballast. Trout rise at an angle and will often follow a floating fly for yards, drifting with their bodies at a 70+ degree angle to the streambed.
Add your previous post that stated the even with the "persistence" of some of the UV cones in the dorsal retina, the blue cones were responsible for most of the UV sensitivity in adult salmonids, I think they perceive UV as more blue than UV.
Well, you think you know how trout perceive UV. That is interesting. However, as I stated in my last post, it doesn't matter how they perceive the UV, what is important is that they do perceive the UV; so it behooves us to dress our flies with the UV reflectances of the trout's prey.