With more than 50 years of experience as a scallop fisherman, Hideto Takada has seen his fair share of oddities connected to the ocean depths.

But in recent years, he has experienced something totally new that threatens to wipe out the entire industry.

Rising seawater temperatures off the Japanese archipelago are destroying indigenous fish species, including more than 50 percent of juvenile scallops in autumn 2023 at Mutsu Bay in Aomori Prefecture.

Takada, 76, a scallop fisherman at the Yomogita fishery port in Yomogita on the east side of the Tsugaru Peninsula in Aomori Prefecture, said 30 percent of his scallop fry under a year old died in the record hot seawater temperatures last year.

“A minimum of 20 percent of the fry seem to have died this year,” he said. “Nothing like this has ever happened before.”

Water temperatures this summer were not as extreme as in summer 2023, but they were higher than usual and forced Takada to make adjustments.

He transferred his scallop fry from a cultivation basket to another on Oct. 18, a process postponed by nearly three weeks for the scallops to grow healthy enough in the colder waters.

Aomori Prefecture is the second largest scallop producing area in Japan, after Hokkaido. Scallops from Mutsu Bay are thick-fleshed, fragrant and in heavy demand in markets.

Takada has a dire prediction if water temperatures keep rising.

“Scallops in Mutsu Bay might disappear several decades in the future,” he said.

OVER 23 DEGREES FOR 50 DAYS

According to the Aomori Prefectural Industrial Technology Research Center’s Fisheries Institute, scallop fry growth slows when water temperatures exceed 23 degrees. They are at great risk of emaciation and death in waters above 26 degrees.

Temperatures topped 23 degrees at a depth of 15 meters in Mutsu Bay off Aomori city on Aug. 3, and daily highs surpassed that level for 50 days until Sept. 24.

The maximum water temperature was 25.3 degrees, 2.4 degrees higher than the average of 22.9 degrees.

“Data from the Japan Meteorological Agency shows that water temperatures will inevitably rise in Mutsu Bay in the future,” said Kyosei Noro, a senior research specialist at the Fisheries Institute. “Countermeasures are indispensable for the increased temperatures.”

Some steps have already proved fruitless.

Last year, the scallop fry’s mortality rate reached 90 percent in the area of the Aomori city fisheries cooperative association, compared with over 50 percent across Mutsu Bay.

The shallow depths of the cooperative association’s service zone are vulnerable to temperature rises.

Aomori Prefecture and the Fisheries Institute asked fishermen in the zone to sink their baskets with fry close to the seafloor.

But the request was not enough to overcome the challenge.

The damage to the fry reduced shipments of the Aomori city fisheries cooperative association by 70 percent in spring this year from standard levels.

“If this situation lasts, scallop fishermen will not be able to continue business,” said Tsugumichi Karouji, head of the association’s guidance department.

Karouji said he has received complaints from multiple fishermen about their dramatically reduced incomes, as they “are struggling to find a way to earn a living.”

“Whether temperatures will reach such highs on a continual basis is unknown,” researcher Noro said. “Water temperatures can also be influenced by typhoons, prevailing westerly winds and other factors.”

PEARL FARMING EYED IN MIYAGI

Rising seawater temperatures have affected a range of regions and marine products.

In Miyagi Prefecture, catches of ascidians and oysters are declining, causing a growing sense of crisis among regional fishermen.

The prefecture will embark on a culturing test for pearls this fiscal year so that pearl oysters can replace ascidians and other products in the local fisheries industry at some point.

The Miyagi prefectural government will dispatch personnel to Mie Prefecture and other advanced pearl-producing locations for advice.

“We must consider approaches with the possibility that water temperatures may not decrease,” said a representative of Miyagi Prefecture’s fisheries infrastructure development department.

PROLONGED MARINE HEATWAVES

The Copernicus Marine Environment Monitoring Service (CMEMS), which is based in Europe and provides information on seawater temperatures and other environmental elements, noted significant mercury surges in waters off the Sanriku area and other sites in Japan’s northeastern Tohoku region.

The water temperature there was more than 10 degrees higher than usual in late April this year.

The chilly Oyashio current normally flows to waters off Sanriku from the north. However, the warmer Kuroshio current from the south is exerting a stronger effect, leading to prolonged periods of extremely hot sea surface temperatures associated with marine heatwaves.

“The water temperature in areas around Japan is rising over twice as fast as the global average,” said Shusaku Sugimoto, an associate professor of marine physics at Tohoku University’s Graduate School of Science. “Higher readings projected for seas worldwide by the end of this century are already being observed in Japanese waters ahead of other regions.”