Fatty 'katsuo' fish may foreshadow climate change, threat to Japan's sushi

Crew members on the Nakajomaru katsuo (skipjack tuna) fishing boat catch katsuo using traditional ipponzuri (single pole fishing method), in Tosa Bay, Kochi Prefecture, Japan, May 15, 2022. Locals and experts say fatty katsuo indicates climate change and a risk for the fish numbers already under threat due to growing demand and overfishing. It "must have something to do with the water temperature," captain Takeo Nakajo, 70, said. "I have a sense of urgency thinking what if katsuo doesn't come to the bay some day." REUTERS/Kim Kyung-Hoon

For half a century, Takeo Nakajo has been catching katsuo, or skipjack tuna - indispensable in Japanese cuisine whether eaten raw, dried or used as a base for the broth.

But he and other fishermen in Kure, in Kochi prefecture in southwest Japan, have seen something worrying in the past two years - an unprecedented number of unusually fatty katsuo.