Crimsons - The Scarlet Navigators of the Ocean is a Hot-Blooded shounen adventure manga about fish by Kanno Takanori.
Yes, fish. It's Better Than It Sounds.
The protagonist is Shintarou, a feisty Sockeye Salmon whose school lives on the fringes of a freshwater lake. Life is tough in the shallows and food is scarce; the interior and its bounty are ruled by a tough and greedy gang of fish delinquents called "The Fearsome Fins". Remembering a rumour he heard when he was but a fry, Shintarou leads his school up-river to the ocean in search of a paradise to call their own. Shintarou grows into a brave and resourceful leader as he guides his school on their quest, fighting for survival against deadly predators such as sharks and whales. Along the way, they are studied by a marine biologist and his Canadian exchange student, Kris. She has a crush on her professor, who secretly reciprocates but must reject her advances to protect his career and reputation.
Though the premise is unusual, it's surprisingly accessible and captivating, as the migratory life cycle of the Sockeye Salmon easily lends itself to such a narrative. Written under the editorial supervision of a professor of ichthyology at the Tokyo University of Marine Science and Technology, it contains a healthy dose of educational content about marine biology, though some artistic license is taken.
Compare: Ginga: Nagareboshi Gin and Clan Apis.
The manga provides examples of the following:
- Anyone Can Die To the point that in the last chapter it's heavily implied that every character of the story so far had died. Even the human ones.
- Couch Gag: Once a chapter, a daphnia gets to monologue, just to be killed shortly after finishing.
- Foregone Conclusion: Those who know about the salmon life cycle know what's gonna happen once the salmon return to their birthplace. Poor Yuuka and her dreams of having a family.
- Hope Spot: After the school is stymied while trying to get back up the waterfall, Touya makes the jump! They're gonna make it! And then, bears.
- Once per Episode: Every chapter opens with a water flea waxing philosophical about life, who is then almost always killed as the chapter begins proper; also, the Professor explains something about marine biology relevant to the chapter, and Kris makes a move on him.
- Time Skip: After the school decides to return to their home lake to raise the next generation, two years pass.
- Two Lines, No Waiting: There's an ongoing side story about the Canadian exchange student Kris and her attempts to romance her university instructor, who usually says something that just happens to be related to what's currently happening to Shintaro's school.