What is Kaihori (Sea Moat)?

　　The Seto Inland Sea surrounding Awaji Island is experiencing a decline in the catch of sand eels, sardines, and other fish. It’s said to be about half of its peak. Seaweed farming was also popular off the coast of Awaji Island, but in recent years, the farming scene has declined and the color of the seaweed has deteriorated. This phenomenon is thought to be due to the ocean becoming cleaner and the lack of nutrients that fish feed on. This is called oligotrophic seawater. In fact, the levels of BOD and COD (both of which are indicators of ocean nutrients; the higher the levels, the more nutritious the water is and the more cloudy it becomes) are about one-third of what they were in the 1970s, when the seawater was at its most polluted. It could even be said that the seawater has become three times cleaner. This is due to the fact that, since the 1970s, when pollution was at its height, laws have been enacted prohibiting the direct discharge of domestic wastewater into the sea, and progress has been made in treating sewage in coastal cities and purifying industrial wastewater. However, this has had a serious impact on fishing, and now efforts are being made to manage the discharge of phosphorus and nitrogen from sewage into the ocean. Amid this trend, a traditional event called kaihori (sea digging) is gaining attention.

　 Kaihori refers to the cleaning of Awaji Island’s more than 10,000 reservoirs. It involves a series of tasks, including draining the water, cleaning the bottom of the reservoir, and drying it, to remove and clean the sludge from the reservoir. The event involves the involvement of the prefecture and city, and serves as a platform for local people and businesses who use the reservoir water to contribute to society. The sludge that is peeked into the water contains a lot of phosphorus and nitrogen, and is then released into the sea, preventing nutrient deficiency. It’s a rare sight, so we’d like to share some photos of the event. Similar work is being carried out not only on Awaji Island, but in other areas bordering the Seto Inland Sea as well, contributing to the revitalization of the ocean.

   Below is a glimpse of the kaihori (pictures taken by members of PASONA group for Social contribution), which took place for the first time in eight years on October 23, 2025, at Sichijin Pond in Kuruma, Awaji City. The pond is a round one, about 100 meters across, located on a hill a few hundred meters from the sea, and supplies water to the rice paddies below a dammed bank. Dozens of local people participated in the work, which involved draining the water, washing away the mud, removing discarded rubbish, flushing water to wash the bottom, allowing it to dry, and then filling the pond with water again. Washing water is drained to the sea, leading to enrichment of the sea.

Kaihori of Sichijin pond in Awaji City Kuruma area on Oct 23 rd 2025（Pictures taken by the members of PASONA Corp. group for social contribution.

Sichijin pond、a reservoir for water supply to rice fields. Usually the pond is full of water.

Sichijin pond is covered by a high bank and the water is flown to the small brook through the bank, leading to rice fileds and sea.

Under the bank rice fields wait for water supply from the pond.

People in the area stand by on the bank for cleaning the pond.

Now the plag of the pond is open to release the water out of the pond.

People are washing the bottom of the pond with a rubber pant.

People is struggling with the mud to remove trashes.

To remove garbages even an excavator is used.

The bottom of pond is cleaned by fresh water.

The cleaned bottom of pond waits for making dry and new water supply again.