There is such a bewildering variety of fresh fish available in stores in Japan these days that it's hard to imagine a time when things were different. But before refrigeration became widespread in the late 1950s, fresh, unprocessed fish was only available to the well-to-do or people living on the coasts. Most of the fish consumed by regular folk living inland was salted, dried, marinated or otherwise preserved.
Of these preserved fish, salted salmon — called shiojake, shiozake or simply jake — is arguably the most popular. It's one of the most common fillings for onigiri (rice balls); the star of sakeben, a standard bentō variety sold at convenience stores; and makes a frequent appearance as part of a traditional Japanese breakfast.
My grandfather's family was originally from a coastal village in Niigata Prefecture, and when he was still healthy his relatives would send him a batch of whole, gutted fresh salmon on ice every winter. My grandmother and aunt would then turn them into shiobiki, a highly salted type of shiojake that's traditional to that part of Niigata. The salmon was heavily salted inside and out, then hung under the eaves for a couple of weeks to allow the moisture to slowly dry off. I still have memories of gazing up at the open mouths of the salmon as they hung upside down.