Fallen sweetfish - Ochiayu

...By eating limescale, ayu have a unique fragrance and have earned their status as a high-class river fish. In early autumn, the pituitary gland in the hypothalamus senses that the days are getting shorter, and as hormone secretion increases, they begin to mature sexually and move down to spawning grounds as ochi ayu or dashi ayu. When ayu are close to spawning, their bodies turn black and their abdomens turn red, and in males, small white protrusions called "oi-boshi" appear on the surface of their bodies, which feel rough to the touch. ...

*Some of the terminology that refers to "Ochi Ayu" is listed below.

Source | Heibonsha World Encyclopedia 2nd Edition | Information

…水あかを食べることによりアユは独特の香気をもち高級な川魚としての地位を得ている。初秋，日の長さが短くなるのを視床下部の脳下垂体が感じとり，ホルモンの分泌が盛んとなり性的に成熟し始めると落ちアユ，下りアユとなって産卵場所へと下る。産卵間近のアユは，体が黒ずみ腹部は赤く色づき雄では体表に〈追星（おいぼし）〉と呼ばれる白い小さな突起が生じ，手でさわるとざらざらした感じになる。…

※「落ちアユ」について言及している用語解説の一部を掲載しています。

出典｜株式会社平凡社世界大百科事典 第２版について | 情報