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.
It does preserve the food. My pickled onions keep for about a month in the fridge, which they wouldn’t if not pickled. But long life does not mean impossible to spoil.
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.