Grayling dont tolerate poor water quality, so would agree.Yes I have a water temperature sensor main river side of the bridge below the falls on the marteg, so i go there regularly (one on the Dernol too).
So if the grayling are there it must mean that the water quality is pretty good? So maybe not 'in river' problems for the salmon just purely low numbers of spawning adults?
Its probably 15 years since decent numbers of parr have been encountered. At the turn of the century you'd regularly have 2 or even 3 on a team of flies in shallow runs. I even stopped fishing some bits to avoid them, for their benefit not mine. Its been some years since I caught a single one.
I have had salmon in july up there but only when my visits have coincided with prolonged high water. 7 or 8 years since the last one. I did hook one when grayling fishing half a mile or so up the elan a few years ago on a size 12 black pennell. We parted company after 3 or 4 minutes. An 8.5 foot #4 is not the right tool really.