I fished the Trent this week as well and it didn't come up thrumps for me. Clearly I can't have been in the right spot. Sometime, Jon, you'd better let me know where you caught them.
However, back to the thread. In my opinion, just like people, there will be two sides to a barbel's brain. One side will say pig-out in a flood and the other side may say it's too cold to feed. All fish need to respire, and must eat. Too cold though and cryogenic torpor will set-in and a fish in those conditions will not touch food. Temperature and water quality are key. Let's take these factors one at a time.
I remember swimming in a murky pond absolutely teeming with tadpoles. The visibility was a foot at most. I swallowd some of that water and a few tadpoles (true) with no ill-effects. If that water had been run-off from a farm/fields, it would have been like swimming in baby-bio and I suspect that I'd have felt sick and been sick. I have little doubt that barbel would not eat in those conditions.
Research was carried out on the effect of temperature on fish feeding related to temperature in Finland. The experiment was to put a load of ice in a pond of koi carp. The fish were reported to have huddled together in one corner of the pond and no amount of food could induce them to feed. Hot water was then added to counteract the ice and although the fish became very lively, they didn't eat either, so the experiment was inconclusive.
When I was guiding, I did experiments on the effects of temperature on river fish. A boffin friend of mine rigged up a telemetry station by the river and a digital probe recorded the temperature of the water and air every 5 minutes. The data was sent back to my house and fed into a computer model. The model could predict the river temperature two or three days in advance. This meant I could phone a client and suggest a day with a high probability of catching. My boffin friend then introduced moon phases into the program. The combined effects were staggering. We were about to launch this product when the telemetry station was washed away, my friend suddenly passed away with an unknown disease and his laptop was never found. I contacted the Police with my suspicions, but they were not interested as the hospital believed he had contacted ebola virus - bit surprising as he had never been to Africa and spent most of his time on the computer. It did me no favours when I sneeringly suggested that the virus might have been contracted through his computer. To cut a long story short, he was cremated before a post-mortem could be carried out because of the nature of the virus.
What this all suggested to me was that we need to be a lot more scientific with our fishing. Our chuck it and chance it is not good enough. Me and my boffin friend of mine were close to cracking a large part of the nut. In addition, it would not have been a great step to have been automatically sampling water quality every 5 minutes for nitrates, phosphates, and BOD. To factor in water quality into our model would not have been rocket science.
Maybe the project might be resurrected one day. I do feel that I was under surveillance for some time after my friend's demise. I also received some threatening phone calls from what sounded like an east European accent. The final straw was when my daughter found a dead cormorant in her sports bag.