Recipe:
Brine sardines in a 10% salt solution for 48 hours. Shorter than this and the fish turn out mushy, longer and they’re too salty.
Rinse brine off, put fish in a jar with ~65% vinegar solution and whatever spices you like. I use bay, onion, pepper and junpier.
Wait 4 days before eating. Obviously it needs to be kept in the fridge.
Are you not sealing the jar? All the canning and pickling I’ve ever seen makes things keep for years.
no, home canning of meat is really not a good idea due to risk of botulism.