Amphidromous fish

…Ayu reach their full growth in the middle or upper reaches of a river, begin to travel downstream in the fall and spawn in the lower reaches. The hatched young fish then flow down to the sea and spend the cold winter in the ocean near the shore, and when the water temperature in the river starts to warm up, they begin to travel upstream. In this way, ayu are quite different from anadromous fish such as eels, so they are called amphidromous fish. The above anadromous, anadromous, and amphidromous fish are collectively called diadromous fish, which is the opposite of potadromous fish, which are river migratory fish that only migrate in freshwater areas, and oceanodromous fish, which only migrate in the sea. …

*Some of the terminology that mentions "amphidromous fish" is listed below.

Source | Heibonsha World Encyclopedia 2nd Edition | Information

…アユは河川の中流ないし上流で十分な成長を遂げ，秋口から川を下り始めて下流域で産卵し，孵化（ふか）した稚魚は海に流れ下って寒い冬の間を岸近くの海洋中で過ごし，川の水温がぬるみ始めるころから川をさかのぼる。このように，アユの場合はウナギのような降河魚とはだいぶ違うので両側回遊魚amphidromous fishと呼ばれる。上記の遡河魚，降河魚および両側回遊魚を総称して通し回遊魚diadromous fishというが，これは淡水域だけを回遊する河川回遊魚potadromous fish，海の中だけを回遊する海洋回遊魚oceanodromous fishに対することばである。…

※「amphidromous fish」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