Most cormorants do their damage just as light is breaking.
That's why a lot of anglers don't realise that they are responsible. If anglers are on the banks at dawn, the birds fly somewhere else. If nobody is there, it's slaughter time.
That's what happened on my local River Torne, though I had no idea at the time. It was a conversation with a former neighbour, who'd just returned after emigrating to Spain, that enlightened me. He walked his dogs alongside that river every morning at first light, regularly disturbing numbers of cormorants. Because the river is so close to my house, and the roach were so unpressured, there was never any need to be on the banks early or late.
Incidently, the roach made a bit of a comeback on this river this year. I was told by a few freinds of mine about the increasing size and numbers of fish in certain areas. It wasn't long before the black death was sighted causing havoc again though.
What this seems to suggest to me is that mother nature will see that the shoals of roach return, where the conditions are suitable, as long as the cormorant problem is tackled.
Regarding the Trent roach. I know there are a few very large roach in the tidal stretch, well over 2lbs, because they have been caught by barbel anglers using pellets. What I am sceptical about is that they are there in any numbers to be worth fishing for. There is the odd very localised area where you can get a bag of roach, ranging from ounces, to well over a pound, but you have to do a lot of searching on a lot of water to find those odd pegs. Mick Lomas is one of the few anglers who has caught a few good bags of roach from the tidal over the last few years, but he's worked very hard to find those said areas. Definately not a challenge for the faint hearted, or those with little time.
I don't know about other areas, but I'm almost certain otters don't have anything to do with declining numbers of good roach near to me.