The fish dried and shaved to make katsuobushi is skipjack tuna.
Katsuobushi is traditionally produced from skipjack tuna, also known as bonito in many English-language food contexts. The fish is filleted, simmered, repeatedly smoked, dried, and, for traditional karebushi, fermented with mold before it becomes extremely hard and ready to shave.
The thin shavings are a foundational ingredient in dashi, the stock used in foods such as miso soup and many sauces. They are also sprinkled over dishes including okonomiyaki, takoyaki, cold tofu, noodles, and rice. Their savory character comes partly from inosinic acid, a compound associated with umami.
A common mix-up is treating every packaged bonito flake as fully traditional katsuobushi. Some products are smoked and dried but skip the final mold-fermentation stage. The fish is also related to sushi culture through katsuo, the Japanese name for skipjack tuna served raw or seared.