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Part II: Koi diet

Carp are benthic omnivores. They do not have stomach and therefore, feed for long periods of time. Carp eat benthic organisms, swimming insects, micro-crustaceans, detritus and seeds. Hume et al. (1983a) found that carp in aquaria preferred to feed on chironomids, and only ate plant material such as pieces of plant tissue, seeds and filamentous green algae in the absence of chironomids.

Seeds contain carbohydrates and carp feeding on seeds may be looking for carbohydrate-rich high-energy food that helps them in fast-swimming (to escape from predators and anglers). But they have a preference for chironomid larvae (small worm-like larvae of midges) present in lakes and ponds. Swimming insects such as beetles, corixids (water boatman), some mollusks (snails) and terrestrial insects are also eaten when they are available. For example, micro-crustaceans are available in water in spring and summer. Mollusks are eaten when they are found in large numbers.

In Australia, researchers found the diet of young carp are different from those of adults. The diet contained small crustaceans (cladocerans, copepods, ostracods, decapods), aquatic insect larvae (chironomids, corixids) and seeds. While copepods and cladocerans seem to be the favorite of small carp, benthic insects, especially chironomids, dominated the diets of carp between 6” - 15” or 150–400 millimetres. In Lake Alexandrina in South Australia, Hall (1981) found zoo-plankton formed a large proportion of the diet of small carp.

Until recently, carp were thought to be bottom feeders. However, plankton samples revealed many types of micro -crustaceans were present in the mid-water and carp were consuming them. Cladocerans Daphnia spp. was a common food item in most sizes of carp, and was apparently selected when regular food is in short supply. In this study corixids were absent in the diet even when abundant in the water, whereas large amounts of detritus occurred in gut contents (Hall 1981) suggesting the carp were feeding in mid-water and from the bottom, but not swimming after fast-moving insects.

A study showed that dry pellets may be the cause of impacted gall bladders in salmon. The gall bladders returned to normal when their dry pellet diet was changed to a moist pellet. Too much starch (carbohydrates) or diet containing saturated fats (mammalian meat like beef) causes degeneration of the liver (liver becomes large, yellow and will have white blotches).

Bacteria present in the gut aid in digestion. Koi can digest 83% of wheat, 64% barley, rye, oats and corn, 79% of peas, 81% of soybean seeds (heat processed). Cooking, as in extrusion processing of feeds, increases digestibility of starch for fish.

Guziur, J. and Wielgosz, S. (1975) The influence of various numbers of carp stock (Cyprinus carpio L.) on the distribution of macrobenthos in Lake Klawoj. Acta.Hydrobiologie 17: 53–69.

Hall, D. (1981) The feeding ecology of the european carp (Cyprinus carpio L.) in Lake Alexandrina and the lower River Murray, South Australia, Bachelor Science Honours Thesis, University Adelaide.

Kugler, J. and Chen, H. (1968) The Distribution of Chironomid Larvae in Lake Tiberias (Kinneret) and their Occurrence in the Food of Fish in the Lake. Israel Journal of Zoology 17: 97-115.

Vaas, K.F. and Vaas-Van Oven, A. (1959) Studies of the production and utilization of natural food in Indonesian carp ponds. Hydrobiologia 12: 308–392.

Matlak, J. and Matlak, O. (1976) The natural food of carp fry (Cyprinus carpio L.) Acta Hydrobiologica 18: 203–208.

Hume, D.J., Fletcher, A. R. and Morison, A.K. (1983a) Interspecific hybridization between carp (Cyprinus carpio L.) and goldfish (Carassius auratus) from Victorian waters. Australian Journal, Marine Freshwater Research 34: 915–919.

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