The salting evidently takes care of that. You neatly line everything up in a container, salt covering all empty space between fishes and surrounding every one of it, like you'd do with bricks and mortar. Then you hard press it with weight from above so there's no air gap, compressing fish in salt, and then you come back to it every couple of days to take care of juices coming out. The process is about 3 weeks.
Edit : the hard part, the trick is in the lining up of the fish layers, to make sure the unwanted fluids really drained off and didn't end up suspended somewhere. The pattern is where grandmas recipes are going to vary.
