I witness distressed fish in keepnets in summer,on a very regular basis,usually as a result of the keepnet being positioned in shallow,non flowing water in the edge,and not staked out.
This applies to the Rivers primarily.
Modern Keepnets do not kill Barbel,and can help recovery if correctly used,however a badly placed one,or misused one can.
Many dedicated Barbel Anglers still insist on weighing and photographing every fish,regardless of size,or weather conditions,so it is unfair to blame the pleasure angler or Matchmen for all deaths.
As has been said earlier a released fish that appears to swim away strongly,is no guarantee that it wont turn belly up a short while later.
Pons