It wasn't until the early 1800s that red and white mutations started occurring within the common carp, which had been kept and bred for food by Japanese rice farmers for centuries. In the Niigata prefecture, the well known heartland of koi, the farmers viewed these mutations as a curiosity and began breeding them together. Early Kohaku characteristics began emerging in some of these offspring - red heads, gill covers, and lips, or small patches of hi on the back and belly, though nothing worthy of being called a pattern.
Then, in 1888, a man named Kunizo Hiroi bred a red-headed female koi with one of his own males, whose markings resembled cherry blossoms. The fry resulting from that coupling were used to create the now extinct Gosuke bloodline. All of the well known Kohaku bloodlines established in the years that followed (Tomoin, Sensuke, Yagozen, Manzo) arose from Gosuke koi crossbred with other unrelated, yet promising, fish. These bloodlines were named after the breeders who worked diligently to refine their koi over many generations of careful selection. Today, the Tomoin and Yagozen are the two remaining major Kohaku bloodlines in Japan.
What to look for in a Kohaku...
Kohaku are white koi with hi markings that should be of an even intensity of color. The color of a koi can be improved and stabilized over the years by color feeding and constant attention to water quality. The main color should be snow white, with no yellowing and should exhibit a fine luster. The kiwa must be sharp. However, in young, unfinished Kohaku the scales are still kokesuke (semi-transparent); the pattern definition will stabilize as the koi matures.
Head hi is essential. On a "classic" Kohaku, this head hi should form a U-shape ending level with the eyes, but hanatsuki or kuchibeni are nowadays just as acceptable. Hi extending over one or both eyes is also acceptable. The fins are preferred to be snow white without any traces of hi. A Kohaku brandishing stand alone hi on the head is considered a Tancho and is judged in a separate class. Ideally, Tancho hi should be centered and circular in shape, resembling the sun pattern on the Japanese flag. If this type of marking is accompanied by hi elsewhere on the body of the koi, it is called "Maruten Kohaku."
Kohaku can be further described depending on the patterning of their hi. "Stepped" patterns are recognized by consecutive "islands" of hi on the koi's head and body. A Kohaku brandishing 2 "steps" is called nidan, literally meaning two-stepped in Japanese. Sandan and yondan are the respective terms for three and four step patterns.
For continuous head-to-tail hi patterns to be acceptable, they must be interesting. The best example is still the classic inazuma or lightening strike pattern of hi running from head to tail.
Body hi need not be symmetrical, or even conventionally balanced, as long as it is pleasing to the eye. But hi confined to one side of the koi when viewed from above, or making the fish head-heavy or tail-heavy, is a fault. The Japanese still prefer a break in hi at the base of the tail, although many top-grade Kohaku do not have this.
Today's trend is for Kohaku patterns to be imposing, and this should be taken into account when buying young fish. As the skin stretches, areas of hi move apart from one another, so avoid young koi that appear to be small replicas of finished adults. Conversely, large, apparently dull blocks of hi can "break" later in life to form intriguing patterns. Finished koi typically show equal depth of color on the head, whilst tategoi (young koi) typically have deeper color on the head in comparison of that on the body. Quite often on young Kohaku, the scales on the body will contain a small, inner diamond of darker color that matches the head. With good water quality the hi on the body should darken to match the head color and the color seen on the inner scales. This will continue each year until all the hi on the body is of a uniform color. This is when the koi would be considered finished.