TOKYO -- The fishing industry sustained serious damage when waters around Japan became overly enriched with nutrients from industrial wastewater expelled during the nation's rapid economic growth period between around 1955 to 1973. But now, improved water quality brought about by draining regulations and other measures has made west Japan's Seto Inland Sea and other areas "too clean," and aggravated damage to fisheries.
Mikawa Bay in central Japan's Aichi Prefecture is famous for its plump, sweet clams consumed throughout the country. The Aichi Prefectural Government's Fisheries Administration Division says 16,063 metric tons of the clams were caught in its borders in 2013, accounting for about 70% of the nationwide catch. But in 2017, the total fell dramatically to 1,635 metric tons, or around 10% of the peak period.
Juvenile shellfish that emerge on Mikawa Bay's eastern tidal flats are transported and raised on its western side. But, Yoshiki Inagaki, head of the Nishimikawa fisheries cooperative in Nishio, Aichi Prefecture, said, "There are times now when, no matter how many juvenile clams we put in, nearly all of them die."
Clams feed on diatom and other types of plankton. It has been suggested that one factor behind their deaths is lower concentrations of nutrients necessary for plankton, such as nitrogen and phosphorus, leading to a decrease in food sources. Nitrogen and phosphorus are present in wastewater from factories and other places.
According to Inagaki, the water near the fishing port is now so transparent that the seabed is visible at a depth of 5 meters. He said, "Progress has been made in sewage disposal, and it means nutrients stopped flowing from land to sea. Like in field cultivation, it is necessary to foster an environment allowing living things to grow in the ocean."
Issues thought to stem from sea nutrient shortages have been occurring in Mikawa Bay, the Seto Inland Sea and other "closed sea zones" where water is seldom replaced by water from the open sea. Many of these zones were excessively enriched with nutrients, a process called eutrophication, from industrial wastewater dumped during Japan's rapid economic growth period. The situation gave rise to frequent occurrences of red tide.
As a result of advancements in measures to tackle eutrophication, including regulations limiting total drainage amounts, the problem now is a severe case of oligotrophication -- the depletion of nutrients for marine life.
In 2017, the Aichi Prefectural Government responded to requests from local fishing cooperatives including the Nishimikawa fisheries cooperative by easing regulations on nutrient concentrations including phosphorus in wastewater flowing into Mikawa Bay from two of the prefecture's sewage plants. Although the specified concentration is normally 0.3 to 0.4 milligrams per liter, it has been raised to 1 milligram per liter -- which falls within the standards set by the national government -- in autumn and winter, when nutrient concentrations tend to drop.
Mikawa Bay is showing signs of recovery following the changes, and in 2019 its catch of clams went up to 3,880 metric tons.
The city of Omuta in southwest Japan's Fukuoka Prefecture faces the Ariake Sea; in 2004 it became the first municipality nationwide to ease sewage plant drainage regulations. Although there had been severe discoloration of farmed seaweed, cultivated seaweed had better color when grown near the mouths of rivers where wastewater is released. Now this method has spread to municipalities on the Seto Inland Sea coast.
According to the Ministry of Land, Infrastructure, Transport and Tourism, as of March 2019 wastewater regulations for seaweed farming have been relaxed at 30 facilities in 22 local municipalities, including the prefectural capital of Fukuoka on the Hakata Bay coast facing the Sea of Japan.
Yoichi Nogami runs a seaweed farm at Hakata Bay. He told the Mainichi Shimbun, "Seaweed without nutrition becomes brown and its flavor fades. Its commercial value also falls remarkably. Seaweed from Hakata Bay costs around 16 yen (about 15 cents) a sheet, if it's good quality. If it gets discolored, they're 2 to 3 yen (several cents) per sheet, putting us in the red. We cannot measure the sea's 'richness' just by having it become clean."
In response to endeavors at sewage plants across Japan, in January the land ministry established an expert panel to discuss effective measures. Yoshio Kawamura, specially appointed professor of algae aquaculture at Saga University and a member of the expert panel, said, "For seaweed, which is often grown near river mouths, it is reasonable to ease wastewater restrictions and supplement them with the ocean's nutrients."
Furthermore, the Ministry of the Environment has shifted from a water quality management policy fully committed to enforcing regulations, to one focused on the appropriate management of substances that are nutrient sources for marine life. On June 3, the revised Act on Special Measures concerning Conservation of the Environment of the Seto Inland Sea was passed. It enables prefectural governors near coastal areas to make plans for sea nutrient management using measures including easing treated wastewater standards.
A senior environment ministry official said, "Decreasing nutrients in closed sea zones is a nationwide trend. There may come a time where even Tokyo Bay, which you hear being referred to as 'clean' more often these days, will be seen as 'too clean' if water quality keeps improving."
(Japanese original by Toshiyuki Suzuki, Science & Environment News Department)