Hanako was a scarlet koi fish that lived from c. 1751 to 7 July 1977. In that remarkable span, Hanako—which means “flower girl/maid” in Japanese—was owned by several individuals, the last of which being Dr. Komei Koshihara.
Her age was determined by examining her scales. According to The Guardian:
The age of a fish is calculated in much the same way as one works out the age of a tree by counting its rings; most fish have growth rings on their scales known as annuli. This technique was used to estimate the age of Hanako, meaning “flower maid”, the world’s oldest koi carp, who died in 1977 at the age of 226 years. [source]
At the time of Hanako’s scale examination in 1966, she weighed 7.5 kg (16.5 lbs) and was 70 cm (27.5 inches) long. She is the oldest known documented koi fish ever. [source]
Next to whales, tortoises, and tuataras, koi fish are one of the longest living vertebrae on Earth. According to Fish Laboratory:
It is quite common to witness a koi that is over a century old in Japan. This is one of the reasons why koi fish have gained so much admiration in Japan and the rest of the world as well.
Environmental factors that contribute to a long lifespan are clean water, nutritionally balanced fish feed, and long cold winters. The cold season will slow down the koi’s metabolism drastically. During this period, the fish may eat very little to none. [source]
Two scales from different parts of her body were taken off with tweezers. The individual growth rings on the scales were painstakingly analyzed over a period of two months by professor Masayoshi Hiro of Laboratory of Animal Science, Nagoya Women’s University.
In May 25 1966, Dr. Komei Koshihara made a broadcast to the whole Japanese nation through Nippon Hoso Kyokai Radio Station about the story of Hanako. He explained that the koi was passed down from his grandmother on his maternal side, who had inherited the fish from “olden times.” Dr. Koshihara described Hanako as his dearest friend. [source]