red-spotted masu salmon; sea-run masu trout
Refers specifically to the sea-run (anadromous) form of the cherry salmon subspecies Oncorhynchus masou ishikawae, known as アマゴ (amago) when landlocked. The term is used in fisheries and regional cuisine.
See also: アマゴ
The satsukimasu is an amago that has gone down to the sea.
In this river, you can catch satsukimasu in spring.
Katakana spelling is standard for this fish name; usually written in kana.
Kanji spelling is rare and mainly used in specialized contexts; the word is usually written in kana.
The name サツキマス literally means 'May trout' (皐月 is an old name for May, 鱒 means trout), referring to the season when it returns to rivers. The exact historical derivation is uncertain.