Climate warning behind Japan's unusually large tuna

Kochi fishermen still catch tuna with traditional hooks

For half a century, Mr. Takeo Nakajo has been fishing for katsuo, an indispensable ingredient in Japanese cuisine that can be prepared in a variety of ways, from raw, dried, or used to make broth.

However, he and other fishermen in Kure (Kochi Prefecture, southwestern Japan) have noticed a worrying phenomenon over the past two years - an unprecedented amount of fatty tuna.

While heavier tuna means more profit, locals and experts say the unusual weight of the tuna also reflects climate change and the threat of extinction to the species due to increasing demand and overfishing.

“The tuna are getting fatter because of the water temperature,” said Mr. Nakajo, 70. “I’m worried that one day the skipjack tuna will disappear from the bay and we won’t know what to do.”

Noriaki Ito, head chef of the century-old Tsukasa restaurant in Kochi city, said he had never seen tuna this fatty.

This is particularly worrying because climate and sea level change have already wiped out several fish species, including the chambara-gai, which used to be a Kochi delicacy. Native to tropical waters, Pacific bluefin tuna migrate northwards each spring with the warm waters.

According to data from the local fisheries laboratory, the bay’s surface temperature in winter has increased by 2 degrees Celsius in 40 years. It is likely that the large size of the tuna stems from the warmer seas, which have allowed more species to thrive, thus increasing the food source for the fish.

However, in the long term, warming temperatures could prevent mineral-rich water from rising to the surface, leading to reduced plankton and small fish, and therefore fewer tuna.

The situation comes as Japan's aging population threatens the sustainability of local fishing and related businesses such as tuna production.

“We can distinguish the different tastes of skipjack tuna, just like French farmers can appreciate the subtleties of wine,” said Takahiro Tanaka, a fourth-generation fisherman from a long-standing fishing family in Kure. “This may be one of the last communities in Japan where skipjack tuna is part of the everyday culture. But without the fishermen, the culture would disappear.”

Overfishing has also hit fishermen in Kochi hard, as they still rely on traditional pole-and-line fishing instead of the large-scale purse seine fishing that has been common across the Western Pacific.

Government data shows that catches in Kochi are just a quarter of their peak in the 1980s.

“We have seen a serious decline in skipjack tuna stocks over the past 10 years. More and more people are worried that they may not be able to eat skipjack tuna in the near future if this trend continues,” said Hiroyuki Ukeda, vice president of Kochi University.