like anything mathematics do play a part on the effect of barbel populations, so think on this:
Male barbel out number female barbel between 7 and 10 to 1 on most rivers, dependent on the genetics and environment, Female barbel out live male barbel, male barbel live to about 7 years old and reach a maximum size of about 7 to 8 pounds and then they die, a female will live to 20 years plus, ( I wouldn't be suprised if the recent river Thames 20 pounder being 30 years old) a ripe fit female barbel of about 11 or 12 pounds will shed about 20,000 eggs per spawning cycle, so in a stretch of river that has 200 barbel of various sizes only 20 to 25 will be female, whilst we consider a 12 pound barbel to be big and strong, I truly believe that Otters see them as fat and cumbersome and easy prey and often living out solitary lives or in very small groups, without the protection of a shoal until spawning time, so very much an easy kill for the Otter.....when an otter takes a large female (10lb +) barbel we dont just lose a single big fish, we lose the potential of 20,000 fry per annum for the next 10 years or maybe more, so all the females get eaten, whats left are small male barbel and maybe some small none egg producing females, people think yippee the small fish are coming through, then all of a sudden through predation and natrual deaths you get a barbel population crash, very few barbel. Then the EA come along and surreptitiously stock fingerling and slightly grown on barbel, and everyone thinks yippee the barbel are back, trouble is the chub and perch populations have declined because of the decline in barbel, so the ever present and increased numbers of Otters shout Yippee!, the barbel are back and so the cycle continues, but its the end of big barbel fishing as we know it..... I apologise if I seem to be flippant on this very serious subject, I truly think that it's a disgrace and heart breaking, and tragically its coming to a river near you, if not already.