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Fatty ‘katsuo’ fish and lack of wasabi are threats to Japan’s sushi

The effects of climate change are being felt in Japan where two ingredients essential to the country’s national dish, sushi, are undergoing massive changes.

July 12, 2022
By Kantaro Komiya and Irene Wang
12 July 2022

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.

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.

“The fatty katsuo must have something to do with the water temperature,” said the 70-year-old Nakajo. “I have a sense of urgency thinking what if katsuo doesn’t come to the bay some day.”

Noriaki Ito, the head chef at a century-old restaurant Tsukasa in Kochi City, said he too had “never seen such fatty katsuo during this season of the year”.

Takeo Nakajo, 70, captain of the Nakajomaru katsuo fishing boat, eats breakfast with his crew “I have a sense of urgency thinking what if katsuo doesn’t come to the bay some day.” (Kim Kyung-Hoon /Reuters)

This is worrying as changes in the sea and climate have already wiped out some other fish “including a shellfish called chambara-gai that used to be Kochi’s speciality”, Ito added.

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.

A participant in the katsuo matsuri festival eats a piece of katsuo no tataki (seared raw skipjack tuna), in Nakatosa Town, Kochi Prefecture. (Kim Kyung-Hoon /Reuters)

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.

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, and wasabi horseradish – an eye-watering condiment tucked under fish in a piece of sushi.

In Kure, a district in Nakatosa town, many fishermen have gone out of business in the past three decades, said Takahiro Tanaka, a fourth-generation owner of a fishmonger who calls himself a “katsuo sommelier”.

Takahiro Tanaka, 61, who runs a century-old fishmonger, cooks seared katsuo (skipjack tuna). “We can distinguish different tastes of katsuo, just like ordinary French farmers may savour subtleties of wine.” (Kim Kyung-Hoon /Reuters)

“We can distinguish different tastes of katsuo, just like ordinary French farmers may savour subtleties of wine … this place might be one of Japan’s last communities where katsuo is part of the daily culture,” he added.

“But without fishers, this won’t last,” Tanaka said.

Fisherman Nakajo also rued the ageing community and fewer successors. “I asked my grandson if he would take over, but he’s now studying to work at a government office,” Nakajo said.

Sushi culture at risk

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.

“We have observed a catastrophic decline in landings over the last 10 years or so,” said Ukeda.

A Shinto priest bows his head towards an altar with food offerings, before the katsuo matsuri festival in Nakatosa Town. (Kim Kyung-Hoon /Reuters)

“A growing number of people fear we may no longer be able to eat katsuo in the near future if things continue like this.”

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.

“I’m really unsure if we can continue this,” said Takeuchi.

Wasabi production faces challenges

Wasabi, the tangy horseradish that is an essential for Japanese food, especially sashimi and sushi, is facing similar production challenges.

Typhoons and rising temperatures have hurt production in Okutama, a mountainous area to the west of Tokyo, said Masahiro Hoshina, 72, head of the local wasabi growers’ association.

“I am extremely worried about the future of our farming,” Hoshina said.

The number of farmers in the area is down 75% from the 1950s due to depopulation, and unless something changes, some worry sushi itself could be endangered.

“The combination of raw fish and spice, as in katsuo and wasabi, is an art, and we must maintain both,” said Ukeda. “I never want to think about a future” without them, he added.

Hoshina, a Japanese farmer, starts worrying about typhoon season months before it begins, haunted by memories of the heavy rains and landslides that washed away wasabi farms during one 2019 storm.

Wasabi farmer, Masahiro Hoshina, 72, carries recently harvested wasabi plants. (Kim Kyung-Hoon /Reuters)

“Recently the power of typhoons feels totally different from before due to global warming. It’s getting stronger,” said the 70-year-old farmer in Okutama, west of downtown Tokyo.

“Since it’s happened once, there’s no guarantee it won’t happen again.”

Wasabi, the tangy Japanese horseradish that’s an essential part of sushi and dabbed onto slices of raw fish or into bowls of soba buckwheat noodle soup, is usually grown along streams in narrow valleys, leaving farms prone to disasters.

Typhoon Hagibis, which slammed into eastern Japan in 2019, slashed production in Okutama by nearly 70 per cent the next year. The need for replanting and careful tending meant it’s taken nearly three years for sushi farms there to recover. 

Newly harvested wasabi roots are kept in running water to keep them fresh. REUTERS/Kim Kyung-Hoon

Experts say global warming is affecting production not only by increasing the number and severity of storms, but with rising temperatures that threaten growth of the plants, which need to be in water a consistent 10-15 degrees C year-round.

A lack of wasabi could also endanger traditional Japanese foods such as sushi and sashimi, where the tang of the wasabi is used as a contrast with raw fish.

Weather isn’t the only obstacle wasabi farmers face. A drop in rural populations due to ageing means there are no successors. Because of the two factors, the output of wasabi grown in clear-flowing water, like at Hoshina’s farm, had fallen to half that of 2005, according to the Agriculture Ministry.

Norihito Onishi, head sales manager at a chain of soba buckwheat noodle restaurants called Sojibo, has seen his business directly affected by wasabi shortages and supply problems.

Noriaki Ito, 57, head chef of 106 year-old restaurant Tsukasa, prepares to cut raw katsuo (skipjack tuna), in Kochi, Kochi Prefecture, Japan, May 13, 2022. Ito affirms it is getting more common for this unusual fat to show in fish during spring. “Never seen such fatty katsuo during this season of the year,” he said. REUTERS/Kim Kyung-Hoon

The restaurants were long known for allowing customers to grind their own wasabi roots to produce the spicy paste used as a condiment for soba. But they’ve had to mostly give this up.

“In the past, we served all the cold soba noodles with a piece of raw wasabi, but now we can no longer do that,” Onishi said.

Though wasabi root was plentiful when the restaurant first opened 30 years ago, Onishi said over the last 5 to 10 years there have been times when he couldn’t get any at all. The precious root is now made available only for certain types of dishes.

“If this unstable supply of wasabi persists, due to many factors including global warming, we will face a situation where we need to come up with other ways to overcome the problem so we don’t end up not serving raw wasabi at all,” said Onishi.

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