The fish ingredients are marinated between sheets of kelp and then left overnight in the refrigerator. The kelp flavor is imparted to the light-tasting white fish, making it even tastier. This process removes excess moisture from the fish and thus it keeps longer and brings out better texture. In the end, the white fish is the leading role here, so if the kelp flavor is too strong, it will counteract the delicate flavor of the fish.
To put this scientifically, combining the two umami components of ingredients with more glutamic acid and inosine acid, which is found in kelp and white fish, this umami gets dramatically stronger. This umami component Inosine acid, found in fish and meat, increases with time, so white fish is used after maturing a moderate amount.