Niijima is a volcanic island with abundant groundwater, so fish could be washed repeatedly in clean running water. That is the first condition.

The second is that rice could not be grown there. What the island produced instead was salt, and that salt was paid to the shogunate as tax. Salt was therefore too valuable to waste, and the brine used for soaking fish was not thrown away. Water and salt were topped up and the same liquid was used again, and again, for years.

Over that time the protein from the fish dissolved into it, microorganisms established themselves, and the brine fermented into something else entirely. Kusaya-eki, the kusaya liquid, is what eventually came out of a straightforward refusal to pour anything away. Some of the brines in use today are said to be very old indeed.

Muro aji is the classic fish for it, though flying fish, shark, and even moray eel all work perfectly well in the same liquid. Before the Meiji era the product was called shiochiruboshi, brine-dried. The smell during production is famous and not exaggerated. Grilled, though, what reaches you first is not the smell but something toasted and deep, and then it is too late to be squeamish.