I manage to land 34 Small River barbel in June, not bad but none over 5lb in weight. The average size of barbel has never been high (as long as I've fished it), but the average is getting smaller; an average size of 5lb 0oz in 2016 down to 2lb 15oz last year. I have blogged about this before and I have a couple of theories:
Natural Cycle: All river go through a natural cycle, big fish die off and some of the small barbel will grow on to replace them - the new doubles. And in a few years the balance will have turned.
Pollution Effect: The river used to suffer pollution every 2-5 years. The pollution killed off 90% of the barbel, leaving the survivers with little competition to grow on. Perhaps this kept the old balance?
Percentage of Small River barbel over 5lb (last 8 seasons).
Percentage of Small River barbel over 7lb (source).