Carp farming in Saku is dated to the Tenmei era, when Usuda Tanemon, a draper from Sakurai, brought fish back from Osaka. In 1825 the lord of the Iwamurada domain, Naito Bungo-no-kami, carried yodogoi back from Osaka on his return journey and gave them to Namiki Shichizaemon, a wealthy merchant of Nozawa, and from there the farming stuck.

Raised in the cold running water of the Chikuma river, the fish grow slowly and take a long time to reach size. The extra time is the point: no muddiness, firm flesh, a moderate amount of fat.

After 1872, as mechanized silk reeling spread, silkworm pupae became available as feed and production could scale up.

Koikoku itself is unceremonious. The carp is cut straight across into thick rounds, bones and all, and simmered in miso for a long time. Whether sugar goes into the miso, and whether the scales come off, depends on the household. Eating it at new year is traced to the sixth day of the first month of 1746, when Shinozawa Sagoemon served it to a priest from Ise whom he had invited to his house. The wish attached to it is that the year should pass without illness. In a mountain province, this was also the fish for weddings and funerals.