Strange you don't understand why otters are thought to be the contributors to stock crashes (and especially Barbel) JayD as there is plenty of literature online and on here about it.
Search for eating preferences (Barbel as prey) and then consider the size of fish that the forms of predators eat.
Also look for Severn seal videos.
As for fish restocking,doesn't tend to happen much on the Severn and yet the Lower fish stocks are improving every year.
I do understand why that's thought to be the case Lee, but that isn't the question I asked.
I'll rephrase it,
If, as it's often said, there is an increase in furry, and feathery predators, and if as you said, there's also an increase in 'fishy' preds, then why is it that there are those who say that the prey fish numbers appear to be also increasing?
I can only think of a few reasons why that might be happening,
The prey fish numbers are being increased by stocking. You've ruled that out.
The water quality, conditions, and appropriate food supply is such that there is a fantastic survival rate of the prey fish. It must be an exceptional stretch of water if that's the case.
The predators are mainly preying on their own young and juveniles. Possible, but unlikely if the pred numbers are increasing, as suggested.
It could be a temporary situation that as I said in my earlier post, is not sustainable, and at least one of the elements is likely to 'crash' eventually.
I'm only asking as an interested angler, a layman with no qualifications, just my experience, observations and reading a few books over the years to go on. I wanted someone with more knowledge than me to explain how this scenario works in a natural environment. I'd assume that an increase or decrease in one of the things mentioned, would impact on one or more of the others.