Chub are paradoxical; they are, as Walton put it, the fearfullest of fishes, but they soon twig that anglers aren't really much to be afraid of, a minor inconvenience, as Sam Vimes just said, and become considerably easier to catch than unfished-for chub. The last time a chub was taken for food must have been around the General Strike, so they can't really see us as predators.
Chub that haven't seen an angler before, however, are up there with big roach in the uncatchability stakes.
At Croxley Moor, the river Gade runs into the Grand Union canal, and the only obstacle to chub is a stretch of shallows under a railway bridge, which they can easily run at night. It used to be a popular "alternative" match venue for clubs whose main river matches had been flooded out, the canal system buffering all but the most extreme floods, and the fishery being free.This meant a lot of food going in in the winter, and some pretty good chub-fishing...the lightly-fished canal, however, had little reputation as a chub water.
So smart they're simple - but they still give a good pull, just in case you're using cotton, and they still know where every twig that can be used as a disgorger sticks out of the river-bed.
And, bless them, they'll still take anything from tiny dry flies to huge lumps of crust and four-inch plugs.