One thing I remember someone saying on a different forum, was that they experimented with maggot and hemp for a while, and found that when pellet was introduced, the fish instantly zero'd in on the pellet and ignored the maggot hookbait. This was on the middle severn where they see a lot of pellet..
Fred Crouch mentioned barbel becoming very single minded regards food in 'Understanding Barbel' (still a great read). Rather than, as with humans, seeking out variety (reference multi coloured sweets, for example), they actually tune in to a food source. I think Fred wrote that his theory was the fish were 'programmed' to make use of a food source while it was available.
Food for thought perhaps...
Matthew,
As mentioned earlier, I'm not great with particles and barbel, but no, the river doesn't have to be clear for them to work. It's just that other baits such as meat, pellet and boilie seem more effective in these conditions - probably easier to find.
The different bites is caused by the different feeding response. When using large baits such as pellet, the fish take a couple of mouthfuls then swim downstream to chew. With particles such as maggot, they will stay for much longer periods - often passing the food out the back as they are eating it at the front. Because they are confidently feeding without moving off downstream, bites are far more subtle than the 3footer...