Hatsu Katsuo and Modori Katsuo

All art and illustrations in this series and article have been contributed kindly by the lovely Linran, who can be found at https://linranjiang.info/

This article continues from 5. How to make Dashi/鰹節のプロがこっそり教えるだしの取り方

Skipjack tuna (Katsuwonus pelamis), commonly known as bonito, is a migratory fish. From April to June, bonito swim northwards from Kyushu to Hokkaido along the Kuroshio Current in search of food. It is called First Katsuo (初鰹 hatsu katsuo), since it heralds the beginning of spring. In fact, the appearance of this fish decorating boxes at sushi restaurants signals the beginning of spring. It is also known as Ascending Katsuo (のぼり鰹 nobori katsuo), since it swims northward.

As autumn approaches, the waters around Hokkaido cool, and the bonito begin their migration south in search of warmer waters to mate and spawn, fat off the rich northern seas. These migrating bonito as are known as the Returning Katsuo (戻り鰹 modori katsuo). These bonito are also sometimes called Toro Katsuo (トロ鰹), after the fattiest cut of tuna belly. The highest quality returning skipjack tuna could easily compete against the highest quality bluefin tuna for richness. The main difference between them is the stronger taste of skipjack tuna compared to bluefin tuna.

This migratory pattern means that bonito are in season twice a year in Japan, from April to June (First Katsuo) and September to October (Returning Katsuo).

Hatsu Katsuo, as with many foods which act as harbingers of their season, are associated with good luck in Japan. These first finds of the season, which also includes matsutake mushrooms, are known as hashiri (はしり).

The famous sushi chef Sukiyabashi Jiro likens the difference between Hatsu Katsuo and Modori Katsuo to fresh greenery and autumn foliage, or cherry blossoms and chrysanthemums. That is to say, each beautiful in their own way. But when it comes to making katsuobushi, Hatsu Katsuo is seen as far superior to Modori Katsuo. 

Fish used for sushi and sashimi is typically prized for its high fat content, but the richness of Modori Katsuo increases the risk of the fish going rancid (or the fat oxidising) during the process of making katsuobushi. This causes the resulting dashi to develop off flavors. Furthermore, this high fat content causes the katsuobushi flakes to break apart and become powdery when shaved. Shavings from Hatsu Katsuo are much lighter and fluffier. Most katsuobushi, however, is actually made from Modori Katsuo. 

Looking at the image above, you can see roughly timeline for making Hatsu Katsuobushi caught in May, at the peak of its season.

Date	Japanese	English
05/09	仕込開始 (shikomi kaishi)	Beginning of Process
05/10	焙乾開始 (baikan kaishi)	Beginning of Drying and Smoking
07/16	かび付開始 (kabitsuke kaishi)	Beginning of Inoculation
10/8	仕上がり (shiagari)	Completion

 After being caught, the first steps of the process – filleting and simmering the katsuobushi – starts on May 9th. The drying and smoking then begins the day after, on May 10th. As the smoking and drying is repeated so many times, it is not until two months later, on July 16th, that the fillet was inoculated. The mold is applied and dried repeatedly to slowly develop the flavour until October 10th, when the product is finally finished. This means preparing this fish took a whopping total of five months from start to finish.

Read our article on how Katsuobushi is made.

Nagamatsu Mai, the chef and owner of Katsuo Shokudou (かつお食堂), commented that Hatsu Katsuo has a uniquely distinct sweet taste compared to Modori, saying that as Modori Katsuo are about to spawn and die, their flavour is not as fresh as the younger fish.

The lower fat of Hatsu Katsuo also makes it more suitable for the long smoking and fermenting process required to make the highest quality katsuobushi. I have also heard anecdotally that katsuobushi made from Hatsu Katsuo are more likely to have been simmered from fresh, rather than frozen, fillets.

This article leads on to 7. Katsuobushi (鰹節) Storage and Care (保管方法).