The traditional fermented sushi from the Lake Biwa region made with crucian carp is funazushi. It is a specialty of Shiga Prefecture and one of Japan’s best-known examples of narezushi, an older form of sushi preserved through fermentation.
Authentic funazushi traditionally uses nigorobuna, or round crucian carp, a fish associated with Lake Biwa. The fish is salted, packed in cooked rice, pressed under a weight, and left to ferment for months. Lactic-acid fermentation develops the dish’s strong aroma and sour, savory flavor while helping soften the bones so the fish can be eaten nearly whole.
Unlike modern sushi, funazushi does not rely primarily on vinegar-seasoned rice. The rice acts as part of the fermentation process and is generally discarded before serving. The finished fish is commonly sliced thinly and eaten as a snack with sake; it may also be served in chazuke with tea poured over rice.
Narezushi is a broader category, not the specific Lake Biwa dish. Hayazushi and oshizushi describe other sushi traditions, while funazushi identifies this regional fermented crucian-carp preparation.