Research urged on Ariake Sea shellfish

Research urged on Ariake Sea shellfish
Nets to cultivate nori cover the surface of the Ariake Sea, as seen from a Yomiuri Shimbun helicopter.

JAPAN - Shellfish catches continue to be extremely poor in the Ariake Sea, which is surrounded by four prefectures in the Kyushu region, while the cultivation of nori seaweed is thriving.

While many people say the poor hauls of shellfish-about a 10th of the volume in the latter half of the 1970s-is due to the effects of a state-run reclamation project in Nagasaki Prefecture in which dike gates have been used to partially seal off Isahaya Bay, which is part of the Ariake Sea, experts also point to other diverse factors.

A systematic verification, therefore, must be carried out to examine the cause of the poor shellfish catches.

Michigoshi fishing port in Tara, Saga Prefecture, a town about 10 kilometers from the dike, is particularly worried.

The port was once prosperous bringing up an abundance of tairagi shellfish, a high-grade species of bivalve, but in the past 10 years, fishermen have struggled with poor catches. In December, fishermen based at the port decided to suspend tairagi fishing for the second consecutive season.

Katsuzo Akagi, 69, chairman of the management committee of Oura branch of Saga Prefecture Ariake Sea fishery cooperative federation, said: "Even if we dive into the sea, there are no young shellfish. Many fishermen have given up on fishing and make their living working at a new port construction site."

According to Akagi, what triggered the poor catches was the closure of the dike gates, which local fishermen called the "guillotine," in 1997. Observing that the sea currents have become slower since then, he said he no longer feels exhausted, even after long dives, because he takes fewer breaks aboard his boat.

"I think it is because currents stopped flowing into the bay. Authorities need to thoroughly examine the effects by opening the drainage gates," he said.

The Ariake Sea was once dubbed "a sea of treasures" due to its abundant fishery resources. According to fisheries statistics, its total shellfish catches peaked in fiscal 1976 at 111,308 tons. Catches of asari accounted for more than 50 per cent of the nation's total.

But in fiscal 1984, the catches suddenly halved, and figures have been on the decline ever since. But the fall had already begun even before the dike gates were closed, so the mystery could not be explained solely in terms of the reclamation project.

Gunji Aramaki, a professor emeritus of Saga University, said, "During the time when [the Ariake Sea] was known as a sea of treasures something strange started happening."

During the period, a large volume of household waste water started to flow into the sea, and there was a rapid population increase in coastal areas, causing eutrophication to occur. The sudden nutrient enrichment led to a great increase in plankton, and the number of asari clams feeding on them rapidly increased.

On the other hand, refrigerating technologies saw advances in the 1970s, making it possible to transport the clams to larger markets.

As a result, asari clams were over-harvested and plankton that had not been consumed by clams turned into sludge on the seabed, causing environmental conditions in the sea to deteriorate.

Other experts have pointed out that the cultivation of nori seaweed, which began at about the same time, also has some deleterious effects.

Yoji Esashi, a professor emeritus of Tohoku University, noted that acid treatment agents were used in nori cultivation.

The acidic agents contain nutrient salts, which cause seawater to eutrophy, thereby increasing the plankton population, which can cause red tides.

"The reason for the increase of nori production to current levels is that onigiri rice balls sold in convenience stores have become popular," Esashi said. "The environmental deterioration in the sea has something to do with consumers in big cities."

The reason the reclamation project came to be seen as the main cause of poor fishery catches is because of extremely poor harvests of nori that occurred in 2000 and 2001 just after the closure of the dike gates.

In recent years, nori harvests have recovered. But the central government and local residents have been embroiled in endless court battles over the opening of the dike's gates, with no solution in sight.

Ariake-kai Saisei Kiko, a Saga-based organisation that conducts research on the Ariake Sea to restore its previous marine environment, proposed in December last year, "Concerned parties should stop battling in court and create a venue for responsible discussion."

The proposal said the opening of the dike gates alone would not revive the Ariake Sea. All possible causes of the adverse effects on the sea should be debated, including the widespread use of acidic agents for nori cultivation, decrease of water inflows from the Chikugogawa river due to increased water intake from the river, and construction of a new port off the coast of Kumamoto Prefecture.

Yoshiyuki Kawakami, an adviser for the organisation, said: "Many of the factors that have already been mentioned have not been sufficiently examined. Unless we start discussions without blaming one another, the Ariake Sea will have no future."

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