Fatless fish like cod can be dried and will keep good (including nutrients) for years. This is what the vikings used for their long sea journeys, and the method is still in active use today. The vital aspects were low-fat fish and temperature just a little over 0C. Fat fish like salmon or mackerel will not last since the fat will get bad.
The most valued cod type is the Arcto-Norwegian cod ("skrei" in Norwegian) that spawns every year just outside the Lofoten islands in Northern Norway. The fish is easy to catch and they are huge in size, well over 10kg.
The oldest method here is stockfish. The fish is gutted and the head removed. Then they are just left hanging in the cold winter winds for 3 months maybe. After that they are hard as rock and very easy to transport and store for years. To eat a stockfish, you first need to keep it in water for 3-5 days. After that, it's used as regular fresh fish. The taste is good, a bit stronger and the meat is a bit firmer.
A more expensive method is salting and drying, "klippfisk". The fish is gutted and the spine is removed and the fish is "opened" as a book. A generous amount of salt is poured over the filet, the filets are stacked and there's a weight over them. The fish is kept like this for weeks, and the wet salt is changed once. Once ready, the filets are nailed on boathouse walls for further drying. When almost completely dried, they can be cut into smaller parts and put in freezer. Or kept further drying and preserved as-is. Before using, the fish needs to be (de-salted) in water for 2-3 days.
Read more about preserving cod:
I've did both of these methods this year and I just think it's amazing how a delicate fresh fish becomes this rock-solid thing that you can just toss around :)
Cheers from Tromsø, Northern Norway,