Certainly not claiming to be an out and out barbel angler, but I think there has been a massive change in the distribution or population of my two "home" rivers (Hants Avon and Dorset Stour).
30/40 years ago I used to catch barbel fairly regularly by accident when trotting for dace, chub, roach or whatever. Now I can't remember the last accidental capture, other than when feedering for chub. It seems I have to fish for them by design before I hook one (sometimes!). I also recall many days in the long distant past where I have bagged up on barbel, catching many in a session. Multiple catches now seem very rare indeed.
The fish are obviously getting bigger and I've had a few from both rivers which 30 years ago would have been national news. However their distribution seems to be more localised than ever.
There may be smaller populations now and perhaps they are more pressurised and harder to catch (or maybe it's us making them harder to catch - I keep saying I'm going to give the "several gallons of hemp and caster a go again - but never have). There's also bound to be lots of other interlinked factors involved as well as cycles of good years and bad years. But it's not all bad news!
A couple of years ago I filmed barbel spawning on Throop (in late June!). I have never seen so many barbel of all proportions before. Apart from being spellbound, I couldn't help wondering where the hell did they all come from and where have they gone now??
Over the last couple of early seasons, Throop has occassionally produced a fair number of very small barbel, a good sign for the future. The lower Avon, particularly The Royalty has a big head of barbel of all sizes but that population has received additions via several stocking initiatives.
Two and three years ago, I had more barbel from a few trips to the Severn than all my sessions on Stour and Avon put together (thanks ****y!).