マグロのマリネ | Marinated tuna
It’s fitting that Zuke actually comes from the Japanese word for preserving, because that’s the reason for its existence as a popular Tane. Before the days of refrigeration, someone much smarter than me got the bright idea to use shoyu (soy sauce) to keep Tuna, a fish that turns brown quite quickly, lasting longer. That tradition continues today.
Recipes vary (as they always do), but the common thread among sushi chefs is that they will use some combination of Shoyu and Mirin Sake to produce a marinade that will keep the inside of the Tuna red, while the outside of the Tuna remains a darker colour.