Originally borrowed from Ainuサㇰイベ(sak ibe, “salmon”, literally “summer + eat”, in reference to how the fish would be caught and eaten in the summer),[1] or possible alternative unattested compound *サㇰエㇷ゚(sak ep, literally “summer + food”).[2]
As with many terms that name organisms, this term is often spelled in katakana, especially in biological contexts (where katakana is customary), as サケ.
Probably a sound shift from the sake reading, possibly to help differentiate from 酒(sake, “alcoholic beverage”). Alternatively, might be a borrowing from a different dialect of Ainu.
↑ 1.01.1“さけ 【鮭・鮏】”, in 日本国語大辞典(Nihon Kokugo Daijiten, “Nihon Kokugo Daijiten”)[1] (in Japanese), 2nd edition, Tōkyō: Shogakukan, 2000, released online 2007, →ISBN, concise edition entry available here (Note: Dialectal meanings, etymological theories, pronunciation including modern, dialectal, and historical information, Jōdai Tokushu Kanazukai, historical dictionaries containing this word, and the kanji spellings in those dictionaries have been omitted.)