Ocean Warming Threatens Marine Produce

[Kobe Shimbun] “The fish have never eaten so much of the seaweed before. The problem is getting worse and worse.”

In mid-March 2023, seaweed farmer Katō Kazufumi was stumped about how to deal with attacks on his seaweed farm by black porgy, or blackhead sea bream. This season, over half of Katō’s 1,200 seaweed nets were victims of browsing. While he would normally harvest seaweed until mid-April, he reeled in his nets earlier this year.

For a few years now, seaweed farms throughout the Harima Sea and Ōsaka Bay area have been browsed by these fish. While in the waters around Kōbe, Akashi, and the northern part of Awaji Island, this browsing subsides around New Year as water temperatures fall below 13 degrees, temperatures off the southern part of Awaji Island remain high all year, and black sea bream are active in that area throughout the seaweed season.

According to the Hyōgo Prefectural Fisheries Technology Institute, which is based in Akashi, lower levels of nutrients and food sources in the sea seem to be also a factor. In other prefectures, seaweed producers have been driven out of business entirely.

As Kyūshū’s Ariake Sea records record low seaweed yields, Hyōgo has the potential to become Japan’s largest seaweed producer. In recent years, however, a lack of nutrients in the water has caused seaweed to lose its characteristic black color, while ocean warming is affecting harvests and quality.

The farming of seaweed, which grows in cold winter waters, begins in autumn, as water temperatures drop. Farmers agree that 23 degrees Celsius is the optimum temperature for cultivating seeds, and 18 degrees the optimum temperature for beginning production. In recent years, however, the ocean is staying warmer longer, which tends to cause delays in production. The Fisheries Technology Institute says that the production season for seaweed is becoming later and shorter.

The later season does not only mean that the seaweed is no longer growing during the period when the sea contains the most nutrients. It also means that the season for picking the choicest seaweed, both around southern Awaji Island and around Kōbe and Akashi, coincides with the peak of black sea bream activity, meaning that the highly prized first seaweed of the season is sometimes eaten by fish.

Awaji Island is struggling to produce wakame seaweed as well due to rising summer sea temperatures. In summary, changes in the sea caused by global warming take various manifestations while threatening the future of fishing.

Conger Eel Catch Decimated

It is mid-March in the Kōbe fishing port of Tarumi. A fisherman who has just wrapped up work for the morning arrives with his catch of sea bream and flounder.

“Mid-March is supposed to be the best time for catching juvenile sand lances, but we’ve got used to these disappointing catches of fish like these instead,” says Maeda Katsuhiko, head of the Settsu Dragnet Fishing Association.

The juvenile sand lance catch has plummeted to a tenth of its former levels over the past half-century and remains at record lows. While the season once lasted for a month here, in Ōsaka Bay, it effectively ended after just four days.

“We can no longer rely on sand lances for income,” sighs Maeda.

The Hyōgo prefectural government blames the poor catch on decreases in the zooplankton that sand lances feed upon, as well as a lack of nitrogen, phosphorus, and other nutrient salts. Advances in sewage processing are said to have made the water “too clean.”

Meanwhile, in places like Sendai Bay on Miyagi Prefecture’s Pacific coast, the main cause of severely falling catches is thought to be ocean warming. Susceptible to high temperatures, sand lances burrow into the sand in summer and lie dormant. In recent years, however, persistently high temperatures have made this period of dormancy longer, and the fish may be becoming weaker, with the result that fewer adults are spawning.

The surface temperature of the waters around Japan has increased by 1.19 degrees over the last 100 years, and an increase of around 1 degree has also been observed in the Seto Inland Sea, but it is unclear whether there is any correlation between this and the falling sand lance population. Tanda Minoru, former director of the Settsu Dragnet Fishing Association, warns that if water temperatures continue to rise, sand lances may become unable to live in the area.

The conger eel fishing industry is also in dire straits, with catches of saltwater eels in Hyōgo down around 7% from 30 years before. Takatani Shigeki, director of the Iho Fishing Cooperative, based in Takasago, says, “we only catch two or three eels a day now. By the same token, we catch a lot more pike congers.”

Takatani says that while the rising temperatures mean less food for the juvenile sand lances that travel to the Seto Inland Sea, they might have also made the area a more suitable environment for pike congers, which prefer warmer water.

(Translated from Japanese. Banner photograph: Farmed seaweed is pulled up from the waters off Suma, Kōbe, in January 2022. Article by Ishizawa Nanako and Yokota Ryōhei; photograph by Suzuki Masayuki.)

[© The Kobe Shimbun]