Some Koi can get to be a meter long which is 100 centimeters, 3.28 feet or 39.37 inches. However, modern Koi average closer to 80 centimeters (31.5 inches), which is still a long way from the 1/16th inch of the newly hatched Koi.
Using the Ludwig von Bertalanffy growth equations (1938) it can be shown that generally Koi reach about 50% of their final adult length at 24 months, 95% at 10 years, and 99% at 14 years.
Obviously their size depends on the environmental conditions such as pond size, oxygen concentration, water temperatures, water quality, amount and type of food, and length of the growing season. Vitamins, exercise, and lack of stress may also play an important role.
In the mid 1980's in Lake Biwa in Japan they netted a 6.5 foot Cyprinus carpio, but it died in a public aquarium in Kyushu. There are other examples of carp reaching enormous sizes in large bodies of water. The mahseer, a member of the carp family Cyprinidae, genus Barbus, typically reaches 6.5 feet and a weight of 200 pounds. It is found in Southeastern Asia. Although that is a big fish, it is not as big as the wels, also called WALLER (species Silurus glanis), a large, voracious catfish of the family Siluridae, native to large rivers and lakes from central Europe to western Asia. One of the largest catfishes, as well as one of the largest of European freshwater fishes, the wels attains a length of about 4.5 m (15 feet) and a weight of 300 kg (660 pounds).
Back to our favorite Koi. It appears that depending on their age and under optimum conditions Koi can grow over 2 centimeters (almost an inch) per month or faster, remember there are 2.54 cm in an inch. Check out the growth rates in the following graphs.
Just click on the charts to expand them - then click on your browser's back arrow to return.
Graph C is of a Ludwig von Bertalanffy (1938) growth equation that shows length in centimeters and inches versus age in months and years. Again the Koi data from the literature are represented by the blue dots. This assumes a constant growing season in a temperate climate. In cold climates there would be steps notched into the upward curve during the winter non-growing season.
The true rate of growth in centimeters per month as a function of the Koi's age is probably somewhere between Graphs B & D, both of which show fish need to be fed well particularly during their prime growth period, when under 2 years of age.
Following is a table based on the Ludwig von Bertalanffy equation:
We would appreciate any comments and age versus length data you can send us to make these graphs more accurate.
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