STORY: What has fatty fish got to do with climate change?
For half a century, fisherman Takeo Nakajo has been catching katsuo fish in Japan’s Kochi prefecture.
But he and his fellow fishermen are getting concerned about the future.
"The katsuo we caught this spring are fatty. Before, the fish were lean with no fat, but this year, the fish have changed, they are very fat."
While heavier fish might sound like a good thing when it comes to selling it, locals and experts say it’s a side effect of climate change…
that in the long run might reduce katsuo numbers – which are 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 over 35 degrees Fahrenheit in the four decades to 2015, according to local fisheries lab data.
Ample prey in the warmer sea could be responsible for the plumper fish.
But in the 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.
Hiroyuki Ukeda, an agroscientist and vice president of Kochi University, says it’s not just warmer water that’s having an effect.
"It is an undeniable fact that essential nutrients for phytoplankton in coastal areas are running dry. As a result, we can say global warming is certainly a reason why fish catch is declining because katsuo are not migrating to this area anymore.The impact of overfishing and global warming combined is creating the current critical situation."
Overfishing has already dealt a blow to the fishermen in Kochi who have stuck to traditional single pole fishing methods.
Government data shows catch numbers in Kochi are only at a quarter of their 1980s peak.
This comes as Japan's ageing population is threatening the sustainability of local fishing and related businesses such as the production of dried and fermented katsuo - katsuobushi.
Production of that, often used as a shaved condiment over traditional Japanese dishes or as a broth base, is already suffering….
with the number of manufacturers plunging.
It’s not just the fishermen who are concerned about what’s next.
It’s also worrying for those at the other end of the chain – like Kosuke Kitamura, the manager of century-old restaurant ‘Tsukasa’ in Kochi, which only serves locally-caught katsuo.
"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. Japanese cuisine is built on broth made using katsuobushi. Without katsuo, it would be very difficult to serve Japanese food, so I’m very worried."