This specialty of Lake Biwa is prepared from crucian carp that are fat with roe. The fish are cleaned (apart from the ovaries) and salted and left for at least a month, or as long as a year. The body cavities are then filled with a mixture of cooked rice and salt, packed into a vat and left to pickle for several months. The sour flavor of this preparation is considered the origin of modern-day sushi, which is made with vinegared rice.