A few months ago, I visited Miyakojima in Okinawa, and it had some of the most breathtaking beaches I’ve ever seen—the crystal-clear blue waters and soft white sand were unforgettable. But what stood out the most was how incredibly warm the sea water was. It felt like I was taking a bath! I found this fascinating and started wondering why the water there was so warm. During the trip, my husband and I went snorkeling. When you snorkel, you get to see Nemo-like fish and a whole range of colorful marine life living among the corals. The corals were surreal, almost different world. I felt like I spent hours lost in their vibrant ecosystem, completely forgetting about the world outside.

The corals came in so many different shapes and colors, and some even seemed to move, almost like a car-wash system, with fish swimming between them as if enjoying a cool breeze. It felt like a tiny underwater forest for all the sea creatures living there.

One thing I learned, though, that I didn’t know before, is that warmer water temperatures can lead to coral bleaching. When the water gets too warm, corals expel the algae (zooxanthellae) living in their tissues, which causes them to turn completely white, therefore 'coral bleaching'. While corals can survive a bleaching event, they’re put under severe stress and are more vulnerable to dying(Source)
If the stress that causes coral bleaching isn't too severe, corals can recover.

However, if the algae loss is prolonged and the stress continues, the coral eventually dies. Unfortunately, more than 90% of the coral in the Sekisei Lagoon, located between Iriomote and Ishigaki islands in Okinawa Prefecture, has already bleached, according to the Environment Ministry. This lagoon is home to over 360 species of coral and supports a rich variety of marine life, including reef manta rays and endangered species of sea turtles, such as green turtles, loggerheads, and hawksbill turtles.

Why the temperature has increased?

The rise in ocean temperatures, which has contributed to widespread coral bleaching, can be attributed to several factors. The unusually low number of typhoons, which typically help cool the sea, and consistently warm sea temperatures throughout the year have played a significant role. In the case of Sekisei Lagoon, located in Okinawa Prefecture, these conditions have led to 92.8% of the coral bleaching, with more than half of the reef dying. This is the worst coral bleaching event since 2016, when 97% of the coral was affected, and more than half perished.

The Environment Ministry, which has been conducting annual surveys of Sekisei Lagoon since 2005, reported that in its most recent survey of 31 locations, 17.7% of the coral colonies had completely bleached and died, while 32.3% had bleached but survived. When partially bleached colonies are included, nearly all (92.8%) of the coral in the lagoon has suffered bleaching, with only 7.2% showing no signs of damage.

Coral reefs around the Okinawa archipelago support rich marine ecosystems, act as natural seawalls, and absorb significant amounts of carbon dioxide. Of the world’s approximately 800 species of coral, about 415 are found in Okinawa. These reefs are vital not just for marine life but for local communities. For instance, Onna "Coral" Village on the west coast of Okinawa’s main island has a 30-kilometer-long reef that has supported local seafood, such as Onna mozuku, a type of seaweed traditionally eaten in Okinawa, for centuries.

The degradation of these reefs poses serious ecological and economic threats, highlighting the urgent need for action to protect and restore these vital ecosystems.

Healthy Symbiosis:

Corals and zooxanthellae share a mutually beneficial relationship. Corals provide a safe home for the zooxanthellae, while the zooxanthellae, through photosynthesis, produce oxygen and organic compounds such as carbohydrates and proteins that corals use as essential nutrients. Corals, in turn, release some of this organic matter in the form of a viscous liquid, which feeds tiny marine organisms, helping to sustain the rich and diverse ecosystems surrounding coral reefs.

The ideal ocean temperature for corals to thrive is between 25-28°C. However, in recent years, summer water temperatures have consistently exceeded 30°C, with some areas within coral reefs reaching over 32°C. These rising temperatures put significant stress on corals, leading to widespread coral bleaching and threatening the balance of this crucial symbiotic relationship.

Solution from Startup: Sea Seed

In 1998, local fisherman Koji Kinjo discovered that the coral reefs off the coast of Yomitan had turned white. Nearly 90% of the coral around Okinawa’s main island perished that year. Shocked by this loss, Kinjo, along with other fishermen from the Onna Village Fisheries Cooperative, took action by becoming coral farmers.

By 2003, they successfully transplanted farm-raised coral back into the sea, a significant milestone in their efforts to restore the reef. Over time, their initiative gained the support of marine researchers. Today, the fishermen collaborate with the Okinawa Institute of Science and Technology (OIST) to cultivate Acropora coral on iron posts off the coasts of Onna and Maeganeku.

So far, they have transplanted over 100,000 coral seedlings. Kinjo also founded Sango Batake, a land-based coral farm where the water is kept four to five degrees cooler than the ocean, allowing him to grow coral in a controlled environment. There, he discovered a new type of coral that can survive in shallow waters, making it accessible for children to experience. Visitors can observe 120 different species of coral at the farm, and anyone can create their own coral seedling, which is nurtured until it’s ready for transplant into the ocean to help revive the endangered reefs.

In 2015, a breakthrough occurred when Kinjo noticed a colony of Acropora gracilis, a delicate and bleach-prone species, was showing resilience to high seawater temperatures. The following summer of 2016 saw record-high water temperatures off Okinawa’s coast, with 48 days exceeding 30°C and peaking at 32.8°C. While other corals around it bleached, this colony remained unaffected. Researchers from OIST took notice, and together they continued studying the heat-tolerant coral to find solutions for coral restoration.

Since its founding in 2004, the startup initiative "Sea Seed" has transplanted more than 150,000 coral seedlings into the waters of Okinawa as of September 2020. At their Coral Farm, they’ve created a land-based underwater ecosystem. Instead of “managing and raising creatures,” they focus on building an environment where marine life can thrive naturally, promoting biodiversity. Traditional aquariums often rely heavily on artificial seawater, pumps, filtration systems, and coolers, which consume large amounts of energy. In contrast, Sea Seed has developed a sustainable ecosystem using the power of bacteria and the natural heat of vaporization, minimizing the need for energy.

References:

• Mitsubishi Corporation. "Activities to Protect Coral Reefs and Reforestation." Link

• Sea Seed. "Developing Coral Reefs through Farming and Sustainable Methods." Link

• Sea Seed. "The Future of Coral Restoration." Link

• Ministry of the Environment, Japan. "Coral Reefs in Japan." PDF

• Japan Coral Reef Society. "Studies on Coral Bleaching and Conservation." Link

• Visit Okinawa Japan. "Okinawa Sustainability and Responsible Travel." Link

• Sea Seed. "Coral Farm Initiative." Link

• Marine Research (Japan). "Okinawa Dive Points and Coral Conservation." Link

• Ministry of the Environment, Japan. "Coral Reef Conservation and Rehabilitation in Japan." PDF