With all due respect Fred, Barbel are studied probably more than any other species, especially in Europe where they are considered to be the indicator species of many European rivers due to their susceptibility to chemical and heavy metal pollution, people think the reason why barbel were stocked in the Severn were for the anglers benefit, yes it was plus point, but barbel were stocked in the severn in the 1950's, for the same reasons, as a long term indicator species, something the severn hadn't got at that time and bearing in mind the severn like other rivers had suffered heavy industry use and some pollution at that time as it was not long after the war.
In the early 70's myself and a some friends (as barbel anglers) got involved with Dr Peter Hunt of Liverpool University in a barbel population study on the river severn ( his paper is still available) Peter told us the true reasons for the Barbels stocking at that time, one of Peters main investigations was to asses the infestation of the Barbel by the parasite Pomphorhynchus Laevis (P.laevis), as this would indicate the heavy metal and chemical pollution levels on the Severn. Brown Trout and Chub suffer from the same infestation from this vile parasite, but not to the extent of Barbel, we were all horrified when Peter dissected the first Barbel in front of us and then witnessed the hundreds yellow grub like creature spew out onto the dissecting table in front of us from the Barbels stomach. Whilst Barbel appear to be a good host to P.Laevis and show no external visual signs of infestation, therefore in the short term they seem live with each other, the parasite can and does in some cases lead to the Barbels death if it burrows its way into the stomach wall and once again Bournemouth Uni are currently conducting a study on the parasite and its effect as most barbel in the country are infested with this creature. Even as we speak, there are various research programs going on by various universities, freshwater biologists and environmental groups in Europe monitoring the Barbels health and its environment.
Be in no doubt, if there had been a collapse in the Barbels population in any of the European rivers like some rivers have here, all manner of alarm bells would have sounded. Barbel in Europe are not stocked in their rivers so anglers can catch them, they are stocked for research and are stringently monitored.