What does grass carp eat?

A grass carp eats plants, in quantities that are difficult to believe until a lake has been stripped by a handful of them.

The adult is almost exclusively herbivorous, consuming submerged and emergent aquatic vegetation and taking terrestrial plant material that falls onto the water. The single verified relationship is aquatic plants, recorded as a food category and joined with `consumes` rather than `preys_on`, because there is no predation here at any stage of the adult's life.

The striking part is the volume, and it has a mechanical explanation. The digestive system extracts energy from plant material inefficiently — there is no specialised fermentation chamber of the kind a herbivorous mammal has — so the fish compensates by processing very large amounts. Daily intake is high relative to body weight, and most of what goes in comes out only partly digested.

That inefficiency is why the species works as a vegetation control agent and why it is dangerous as an introduction. A modest stocking can clear a water of macrophytes entirely, and removing the plants removes the spawning substrate, the invertebrate habitat and the shelter that everything else in the system depends on.

Juveniles take invertebrates before switching, which is the usual pattern in reverse: most fish in this catalogue start on invertebrates and become predators, and this one starts on invertebrates and becomes a herbivore.

The species rarely breeds outside its native range, requiring long river reaches and specific flow conditions to spawn, which is why introduced populations are usually maintained by stocking.

Grass carp are difficult to catch for the same structural reason mullet are: the fish is not looking for an item of food. Where they are fished for deliberately, the approaches that work present plant material or something resembling it, on or near the surface, and that reasoning is a direct inference from the diet.

The more common angling situation is incidental. Grass carp stocked for weed control are caught by accident by anglers fishing for other species, and the fish is large and powerful enough that this is a memorable accident.

The regulatory position is not interpretation and it is the most important thing on this page. Grass carp are stocked deliberately in some jurisdictions, prohibited in others, and in several places only sterile triploid fish may be introduced. Moving live grass carp between waters is restricted or illegal in a great many places. What applies where you are fishing is a question for the relevant fisheries or environmental authority, and it is worth asking before rather than after.

One food-category relationship, which in this case is close to a complete description of the adult diet rather than a fragment. It says nothing about the juvenile invertebrate phase. Consumption rates vary substantially with temperature, plant species and fish size, and nothing here supports a specific figure for how much any individual eats.