This is the 3883rd most frequent Japanese word.
カツオ
Bonito fish.
Here, カツオ (Katsuo) is directly referred to as the fish species itself.
カツオは魚の一種です。
Katsuo is a type of fish.
カツオ (Katsuo) is used in the compound カツオ節 (Katsuobushi), referring to processed bonito flakes.
日本料理にはカツオ節がよく使われます。
In Japanese cuisine, bonito flakes are often used.
In this context, カツオ (Katsuo) refers to bonito prepared in the specific dish, カツオのタタキ (seared bonito).