The fish that put back in the Trent from the Severn came from the Trent in the first place!!!
Explain how Mark? As it's well document that fish stocked in the Severn by Angling Times circa 1960s came from the Hamps Avon.
It's also documented that Calverton took their stock from the Severn in the early days circa late 1970s. The Trent being one of the first rivers to receive the newly perfected technique of breeding barbel.
And if Ron is right, those Avon fish that went in the Severn came from the Thames and Kennet, then they are not of the Trent provenance. They are of Thames/Kennet provenance and there will be a slight genetic difference.
Yes Ron the Dove did have sustainable barbel stocks in it (not sure the Derwent did as it ran in those days through the heartland of Darby and it's industrial base), but the Upper River Trent at the Dove confluence (Clay Mills) was polluted and had very little fish stocks in it in the mid 1970s suffering periodic bouts of pollution until the mid 80s (Stoke area industrial runoff) and why Calverton stocked Severn/ Avon/Thames/kennet barbel prodigy in the mid river sections. The reason for this being the Trent is a big river and it diluted and dispersed the upper reaches pollution by the mid section. Strange but true the mid river was cleaner than the upper and able to support more fish life.
Yep Peter they're Southern Ditch origins.
Ron you may think it matters not about the provenance of the fish in the Trent but the EA now do when they considering stocking riverine fish species. It's all about stocking local provenance stock as they are more adapted to those local conditions and are less likely to have that slight genetic difference.
As to your question on roach believe it or not, roach are quite pollution tolerant and usually the first species, if they get in, to make a comeback on polluted rivers. On the Manchester Ship Canal and the Mersey they were the first fish to start to appear when those waterways came back from the very biologically dead state they were in.