How has the habitat degraded? It had "improved" several years ago when the deflectors were added.... perhaps the eradication of the Japanese knotweed had a little to do with it. The removal of streamside vegetation and and therefor cover puts fish at risk of predation.
This would be a good candidate for a residency study.....
but to do so you would have to stock fish in there.
I still say that the reason for the lack of fish inthe run is it is not stocked like it used to be.....lots were stocked, lots of people used the run, there was little opportunity for predators and poaching because of the traffic.
Now with less fish stocked, there are less people, more opportunity for poaching and predators and its a shadow of itself.
FI, what is the previous and present stocking numbers and by whom? Not looking for dates or conjecture, just want to know if the numbers have significantly decreased. I think this is where the answers lie.