Crimsons: Akai Koukaishatachi

From MangaHelpers:
Shintaro and his school of salmon have been living in a small lake for generations. Tired of being muscled out of food by the bigger fish, they decide to make a bold move and head for the “ocean” that they’ve heard about in stories where it is vastly large and food is beyond plentiful. As Shintaro leads his school of salmon out into the sea, they soon realize how protected they were in the lake and realize how dangerous it is.