This is a traditional soup served in Japan over New Year's. The mochi cakes, kelp, bonito flakes and naruto can be found in Japanese shops.
If you can't find yuzu, an East Asian citrus fruit, use a mandarin or grapefruit. It is said that the flavour of the yuzu resembles both.
First I believe there is a typo that should be noted. Naruto is a fish "cake" formed as a log either in a plastic warp as a round tube shape or on a wooden stick but not as a "paste". Thank you for your share I finally got it right! I didn't realize sake was used, and that makes the flavor so much better. My mom isn't around to make this any more, but now I finally can make it at home for myself. Now I never have to miss this soup ever again. - 27 May 2012 (Review from Allrecipes US | Canada)