Southern corals take root in warming waters of Tokyo Bay

A coral species found mainly in western and southern Japan has spread into Tokyo Bay and is threatening to destroy ecosystems in the warming waters around the capital.

A variant of the Acroporaquarrosa coral, native to areas of the Kii Peninsula and Shikoku region, created a vibrant, tropical-like scene near the Boso Peninsula at the entrance to Tokyo Bay.

On September 9, a diver searched the Katsuyama area in Kyonan, Chiba Prefecture, where sea water temperatures reached 28.2 degrees. After a 10-minute boat ride from the port, the researcher dove into the water and noticed a 100-square-meter carpet of coral on the seabed.

A bright blue neon damsel and an orange sea goldfish swam above the corals.

Tateyama, Chiba Prefecture, located at the southern tip of the Boso Peninsula, was considered the northernmost area in Japan where coral reefs could form.

However, the diving point is 15 km north of Tateyama.

Jiro Uochi, 68, who runs the Kacchama diving service in Katsuyama, said corals began appearing there about 30 years ago.

Initially they were sparsely distributed along the seabed, but five or six years ago the species suddenly began to expand dramatically.

Changes in the marine environment are behind the coral invasion.

Global warming has prevented water temperatures from cooling, leaving rocky shores exposed and reducing the number of seaweed-eating abalones and turbans.

Coral species such as Acropora solitaryensis have taken root in the damaged areas. Table corals block sunlight and inhibit the growth of algae, causing serious damage to fisheries.

“Corals now exist everywhere, likely due to warmer ocean temperatures,” Wachee said. “While some may find it beneficial to replicate the Okinawa-like landscape here, it is causing changes in the types of fish caught. A serious problem arises for fishermen.”

Statistics from the Japan Meteorological Agency show that global warming has increased the average sea surface temperature worldwide by 0.61 degrees over the past century. The water level around Japan rose by 1.28 degrees.

In the first half of this year, sea surface temperatures off Japan were the warmest since 1982, and waters near Tateyama were 1 degree above normal as of Sept. 9.

“Living things change their environment in response to environmental changes to survive,” said Nina Yasuda, a professor of marine biology at the University of Tokyo. “Sea temperatures are expected to rise further over the next 100 years.”

Yasuda also noted that changes in the Kuroshio current are causing coral eggs and larvae to be transported into Tokyo Bay from the Shikoku offshore area and the Kii Peninsula.

Yasuda said a long meandering current since 2017 has created a warm distributary flow that has transported 40 to 50 species of coral to Tateyama.

Unlike Tokyo Bay, colonies in some southern waters are at risk of extinction due to rising ocean temperatures.

When exposed to warmer water, corals secrete photosynthetic cells called zooxanthellae, disrupting the symbiotic relationship. Corals cannot survive without zooxanthellae and become whitish.

The phenomenon, known as coral bleaching, was widely observed in August off Amami-Oshima Island in Kagoshima Prefecture.

The Amami Marine Life Research Association found that 90 percent of the corals were bleached in one of four locations marked by colonies.

The study results also showed that the 5m colony of Porites australiensis died out despite surviving the last round of bleaching.

“Sea temperatures did not fall this year because ocean water was not sufficiently disturbed by fewer typhoons,” said Katsuki Oki, chairman of the research association.

He said coral bleaching was reported to be increasing and monitoring needed to continue.

“One possibility is that the number of corals in Tokyo Bay may increase but will mostly disappear off Okinawa,” Yasuda said.

“Countermeasures, such as reducing carbon dioxide emissions, need to be taken to slow the rate of marine warming.”

(This article was written by Hiroyuki Kobayashi, Daiki Koga and Eiji Zakoda.)