Fatty katsuo fish may foreshadow climate change, threat to beloved Japanese food
For half a century, Takeo Nakajo has been catching katsuo, or skipjack tuna -- indispensable in Japanese cuisine. But he and other fishermen in Kure, in Kochi prefecture in southwest Japan, have seen something worrying recently - an unprecedented number of unusually fatty katsuo.
"Before, the fish were lean with no fat, but this year, the fish have changed, they are very fat," said Nakajo, 70, who set sail from the port town of Kure.
While heavier katsuo means more money, locals and experts say it indicates climate change and a risk for katsuo numbers already under threat due to growing demand and overfishing.
Originally from tropical waters, some Pacific katsuo migrate northward on a warm ocean current every spring, making Kochi's arc-shaped bay a fertile fishing ground.
The average surface temperature of the bay in winter has risen by 2 degrees Celsius in the four decades to 2015, local fisheries lab data shows, and the fatter katsuo may be due to ample prey in the warmer sea.
But longer-term, this warming may prevent mineral-rich water from rising to the surface, resulting in a drop in plankton and smaller fish to feed on, leading to fewer katsuo, said Hideyuki Ukeda, an agroscientist and vice president of Kochi University.
Overfishing has already hit catch numbers and dealt a blow to the fishermen in Kochi who have stuck to traditional single-pole fishing methods versus large-scale seine fishing across the western Pacific.
Government data shows catch numbers in Kochi are only at a quarter of their 1980s peak.
Production of katsuobushi, dried and fermented katsuo, often used as a shaved condiment over traditional Japanese dishes or as a broth base, is already suffering.
The number of katsuobushi manufacturers in Kochi has plunged from dozens some forty years ago to only a few, said Taichi Takeuchi, who runs one in the town of Usa.
That has added pain for a century-old restaurant "Tsukasa" in Kochi.
The restaurant serves katsuo only caught locally through the traditional single-pole fishing methods that keep the skin intact and ensure the best taste for sashimi or raw fish dishes.
"At the moment, we are not yet at a stage where we don't have katsuo and cannot provide meals. But it is something that might likely happen in the near future. If we leave the situation as it is, we can anticipate that katsuo will disappear,” said restaurant manager Kosuke Kitamura.
