Koi are a color variant of common carp (Cyprinus carpio). The distribution of carp includes temperate portions of Asia and Europe. It was widely introduced into North America by the U.S. Fish Commission in the mid 1800's. They occur in 16 of 32 watersheds in Massachusetts scattered throughout the state.

Carp do best in warm, shallow impoundments with abundant vegetation. They are tolerant of extremes in temperature and low dissolved oxygen. In addition, they fare well in areas of poor water quality. Carp feed on a wide variety of plant and animal matter. They are capable of taking food items floating at the surface, but generally root in soft substrate and pick out food items from the resultant cloud of silt. Spawning commences in early spring as water temperatures approach 63° F. Large congregations move to shallow, weedy waters then break up into smaller groups of several males to one female. A single female may produce up to 2,000,000 adhesive eggs which are broadcast and adhere to submerged vegetation. Eggs hatch within 3-6 days depending on water temperature.

Koi are cultured in private facilities for sale as an ornamental fish for stocking into private ponds of for the aquarium trade. Under culture conditions, optimal growth for young koi is between 81 and 93° F, while 68 to 82° F is optimal for adults. The pH should remain between 6.8 and 7.5, and while koi are capable of surviving in warm waters with low oxygen, dissolved oxygen levels should remain above 5.0 ppm for good growth.

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