Katsuo bonito or skipjack tuna, appear in January and February in the warm Ogasawara islands, riding the warm current up to the Sanriku-oki islands Aomori from August to September. In autumn they finally reach Southern Hokkaido. These are known as agari-gatsuo, or the 'climbing' bonito. The fish then turn back south towards the mainland of Honshu, and, grown large by constant feeding en route, are known as kudari-gatsuo 'descending bonito'
The classic way to serve agari-katsuo is as katsuo no tataki, with the fish charcoal grilled on the outside, using a searing technique that involves cooking them in high-temperature burning straw. The interior of the fish remains raw, and it is sliced served with grated ginger, garlic and a ponzu citrus dipping sauce. Kudari-gatsuo on the other hand is most likely eaten as sashimi, especially the rich, succulent toro.
Katsuo are caught in the largest numbers off Miyazaki, Miyagi, Tokyo, Shizuoka, Mie and Kochi Prefectures. The latter has become synonymous with bonito, and in Kochi street vendors grill katsuo for tataki right in front of you. It's great to watch and even better, of course, to eat.
Most of the catch, however, goes into making katsuobushi dried bonito flakes, for use in dashi. Katsuobushi is found in different grades depending on how long it had been dried and cured, and the quality of the bonito used. The resulting stock from these bonito - or other fish - flakes has different characteristics and suits different uses.
Arabushi is aromatic, and cured almost to the point where it goes moldy. It's dark brown, and rock solid, but when shaved it produces the delicate, light pink, aromatic hanakatsuo flakes used in fine cuisine. Hongarebushi is a lighter color, and when shaved produces rougher and thicker flakes generally sold for household use. My own personal favorite which I source from the wonderful Shimamoto Nori Kanbutsu is mejikabushi made with mejika a frigate tuna that can only be caught in Tosa Bay in Kochi Prefecture between the months of August and September.