Bubblecar said:

Pickle juice now cooling. Ingredients include:

Apple cider vinegar
Water
Chopped onion
Bay leaves
Mustard seeds
Cloves
Freshly ground black pepper + whole peppercorns
A little vegetable oil
A little brown sugar

More chopped onion and some fresh dill will be added to the jar along with the fish, before pouring in the cooled pickle juice.

Has the fish been drying in the sun for a few days?