The largest coral reef in Japan, located in Okinawa Prefecture, is currently undergoing an accident.
Coral bleaching is a problem, in which corals turn white over a wide area and become more likely to die.
Why did the change occur?
I will tell you in detail with NHK's underwater camera and expert commentary.
Japan's largest coral reef that supports a rich sea
Sekisei Lagoon, the largest coral reef in Japan, is located between Ishigaki Island and Iriomote Island.
Coral reefs stretch for about 20 kilometers from east to west and 15 kilometers from north to south.
Dive into the sea and you'll find more than 300 species of coral.
Corals are homes and breeding grounds for colorful fish, and play an important role in maintaining a rich ecosystem.
However, in this sea, the "bleaching phenomenon", in which corals turn white and become more likely to die, has become a problem.
Crisis six years ago again
At Sekisei Lagoon, a large-scale bleaching event occurred six years ago, bleaching 97% of the coral, of which 70% died.
After that, new corals were growing, but when I accompanied the national survey conducted in September and proceeded with the coverage, I found that the "bleaching phenomenon" was occurring at a level close to that of 6 years ago. .
The Ministry of the Environment's survey was conducted for six days from September 24, and NHK accompanied the survey on the first day.
In the survey, we actually dived into the sea and confirmed the progress of coral bleaching at 31 locations.
why do corals turn white
How does coral bleaching occur in the first place?
The color of coral is caused by phytoplankton called zooxanthellae.
They live in symbiosis with corals and provide them with nutrients.
The cause is “rise in seawater temperature”
However, if the seawater temperature continues to be high, the "zoxanthellae" will come out and the skeleton of the coral will look transparent and white.
Corals that have turned white will not be able to receive nutrients and will die if this condition continues.
"Sekisei Lagoon" where a large-scale bleaching phenomenon is occurring.
In August this year, the water temperature of the surrounding sea surface averaged 30.5 degrees, the second highest in recorded history.
Even in places where bleaching should be difficult
As the investigation progressed, serious damage became apparent one after another.
In the southern part of Kohama Island, most of the corals of all sizes on the seabed at a depth of about 5 meters have bleached.
Normally, it is said that corals do not bleach easily because the currents flow quickly and the seawater temperature does not rise easily, but the percentage of bleached corals was significantly higher than usual.
Not only bleaching, but also the point of extinction
This is the point where all the coral that can be seen has bleached.
However, when I dived into the sea, many corals were not pure white, but brown.
The bleached coral has already died and is covered with algae, giving it a brown appearance.
Expert “Lack of typhoon agitation is also an effect”
Associate Professor Takashi Nakamura of the University of the Ryukyus, a coral expert in charge of the survey, points out that this large-scale bleaching is occurring at a level close to the level six years ago when 97% of the coral bleached.
"The background is the rising trend of water temperature accompanying global warming. In addition to that, this year there was not much 'stirring' by typhoons, which stirs the surface seawater, which is usually warmed in summer, to lower the temperature.Therefore, the surrounding area A major reason for this is thought to be that the temperature in the sea area is getting higher and higher, creating the conditions that make coral bleaching easier.”
Even young coral bleaches
About 70% of the corals died after the massive bleaching six years ago.
In this survey, it was confirmed that corals with a diameter of 10 to 20 cm, which had newly grown after the damage six years ago, were also bleaching.
Expert “Ecosystem balance may collapse”
A young coral that has grown little by little over several years.
Associate Professor Nakamura points out that the death of these corals in the recent large-scale bleaching could lead to the decline of coral reefs.
“If the young corals that have newly grown die off, the speed at which the corals that have been damaged and reduced will recover will be delayed. is likely to collapse.”
Once coral is lost due to large-scale bleaching, it takes many years to return to its original state.
Associate Professor Nakamura points out that in addition to ecosystems, fisheries and tourism may also be affected.
The Ministry of the Environment will analyze the data obtained from this survey in detail and announce it by the end of October.