What does chub eat?

The chub is the species in this catalogue that most clearly changes what it is over a lifetime, and the change is visible in what it eats.

A small chub is an invertebrate feeder taking insect larvae from the bed and the drift. A large chub is a predator, taking small fish and crayfish, and it will do so with a decisiveness that surprises anglers who think of the species as a coarse fish.

The two verified relationships bracket that transition: aquatic insect larvae for the early phase, freshwater crayfish for the later one. The crayfish relationship carries a further complication worth stating — the spread of introduced crayfish across European rivers has measurably changed what large chub eat, so this is a diet that is still moving.

Vision dominates throughout, and it is acute. Chub feed heavily on items falling from the bank and therefore watch upwards and outwards, which is unusual for a river cyprinid and explains the species' reputation for being difficult to approach: the same attention that intercepts a beetle off an overhanging branch registers an angler on the skyline.

The practical consequence of the whole picture is that chub occupy an awkward position in angling, targeted as a coarse fish by one person and as a predator by another on the same stretch on the same day. Both are correct, and which one is correct depends on the size of the fish.

The upward-watching habit is the reasoning behind the approach conventions the species is known for — staying off the skyline, working upstream, keeping low — and it is convention derived from a documented sensory orientation.

The ontogenetic shift is the reason two quite different angling approaches both work, and anglers use it deliberately: an approach aimed at invertebrate feeders and one aimed at a predator select for different sizes of fish on the same water. That is practitioner reasoning rather than a tested selection effect.

The willingness to take items falling from the bank underlies the long tradition of using terrestrial baits for chub, which is a practice evidenced separately from the diet fact that motivates it.

Both relationships are group-level. The diet is broadly omnivorous and includes plant material, terrestrial insects and fish, none of which is fully represented. The balance shifts continuously with fish size, and the crayfish component depends on whether introduced crayfish are present, which varies river by river and is changing.