Feeding habits of barbel
It always surprises me just how little we know of the feeding habits of fish. All we seem to have is circumstantial evidence. If it were mammals or even Great Whites, it would be all well documented. There must be scope for some Ph.D studies. A relatively easy fish to start with would be a trout. They have feeding spells which often just stop, typically 2pm - nothing to do with water temperature on a particular day, possibly the level of light (probably not as it doen't seem to matter whether the day is sunny or cloudy), midge hatch slowing down - possibly. They may start to feed again in the evening, but not always, but if they do, it is almost certainly connected to a hatch. Another case, what triggers a pike feeding frenzy, high pressure, low pressure, moon phase etc or just a simple concentration of bait fish, for whatever reason?? In both cases, feeding spells could simply be the sudden availability of a food source. With barbel, a bit more tricky. There is less natural food available in the colder months but a fish's metabolic rate will slow hence needs less food anyway. But any animal (hibernators apart) will need a certain amount of food and when food sources become available, eg a flood, tuck in, or if an easy meal comes along likewise.