December 7, 2010


Initiatives to Restore the Tidal Flats of Ago Bay

Keywords: Newsletter 

JFS Newsletter No.99 (November 2010)

JFS/Initiatives to Restore the Tidal Flats of Ago Bay
Copyright Mie Prefecture Fisheries Research Institute

A tidal flat is a coastal wetland consisting of an area of sand and mud that is alternatively covered and exposed by the tide. With mixture of fresh water from rivers and seawater, tidal flats also receive abundant nutrients that flow down from the land. They also incorporate a great deal of oxygen and receive sunlight at low tide. These environmental factors help nurture a great many species, and tidal flats are often called the cradle of sea life.

A tidal flat is home to many families of species including crabs, shellfish, and lugworms. It is also a nursery ground for fish like gobies that live there as juveniles. For wild birds, it is a feeding ground as well as a stopover site. Thus, a tidal flat is a rich wildlife habitat.

Tidal flats have another role -- purifying water. By virtue of the ebb and flow of tides, dirty water is filtered through its sand grains. Microorganisms like bacteria living in the sand also help to dissolve organic material, which helps purify water. The sea is cleaned by creatures living in tidal flats, too. Substances floating in the water are taken in as food by sea creatures such as Japanese shortneck clams, which are harvested and eaten by wild birds and fish. The more creatures a tidal flat has, the stronger its purification effect is.

Moreover, a tidal flat also plays a role as a buffer between the sea and the land. If there is a tideland along a coast, it can reduce the power of storms and strong waves. For example, when Hurricane Katrina hit New Orleans in the US, it caused catastrophic damage. But it is said that if the tidal flats that originally lined the coast had not been reclaimed, the city would not have been so devastated.

In this way, tidal flats are precious both for wildlife and humans. They nurture many species, purify water, and protect lands from storms and waves. However, such tidal flats have been lost to reclamation worldwide. In Japan, 41 percent have disappeared since 1945. At the same time, there are also some efforts to restore tidal flats. This article introduces one of these initiatives in Ago Bay, which is famous for pearl cultivation, in the Ise area of Mie Prefecture in central Japan.

There used to be about 269 hectares of tidal flats equal to roughly 10 percent of the sea surface area in upper Ago Bay. These tidal flats, however, were reclaimed to create rice paddies in the late of Edo Period. As a result, some 70 percent of Ago Bay's tidal flat area was lost.

Looking back today, however, 85 percent of the rice paddies created by tidal flat reclamation were not used but lay fallow or were simply left to run to waste. The remaining tidal flats have been impacted by domestic wastewater and pollution from pearl cultivation in neighboring areas. As a result, upper Ago Bay's capacity for purifying seawater and nurturing species has deteriorated, resulting in chronic red tides and oxygen deficiency. This in turn damaged pearl oysters, one of the region's important products, and other bivalves. Coastal pollution has become a big issue for the local community.

As the reclaimed land is no longer used, people are now trying to re-convert the reclaimed land back to tidal flat and make Ago Bay a beautiful bay.

The leader of this effort is the Mie Prefecture Fisheries Research Institute. As a result of a study in which seawater flows were introduced into and out of a former tidal flat for three years starting 2006, many living things such as clams came back.

To deepen the study, a tidal flat restoration project was begun at a sewage-like site where rain and soil runoff had accumulated. This former tidal flat had dried out due to a dike that cut it off from the sea. This meant nutrients from the land did not flow into the sea, but accumulated together with rain water in the former tidal flat. While the polluted pond inside the dike was hyper-nutritious, the ocean outside was nutrient-poor. Neither had many kinds or populations of living things. They were barren.

This concrete dike had been built by the prefectural and/or national governmental authorities as a disaster prevention measure around 1960 to protect then-cultivated farm land from chloride damage in the wake of the Ise Bay Typhoon and the Chile Earthquake. The area was classified as "general marine area" without particular owners. After recent discussions with concerned parties, the area was chosen for a restoration project. The Ago Bay Nature Rehabilitation Committee is leading the effort together with the fishing and tourism industries and local municipalities.

No seawater had entered the area since the dike was built around 1960; rainwater and organic matter from the land had accumulated for over 50 years. In April 2010, a gate in the dike was opened to allow seawater come in and out in the hope that the area would be restored gradually as seawater decomposed the organic matter that had piled up for over 50 years.

JFS/Initiatives to Restore the Tidal Flats of Ago Bay
Copyright Mie Prefecture Fisheries Research Institute

Before seawater was introduced, the pond was in extremely bad shape with scarcely any living creatures except chironomids (non-biting midges). The number of living species was six. A survey in June 2010, three months after the introduction of seawater, revealed that this number had grown to 14 species. In just three months, the mudflat had begun showing signs of restoration.

How is a tidal flat restored? First the number of opportunist species with short life-cycles such as lugworms and small shellfish drastically increases. In a couple of years or so, other creatures from the upper food chain that feed on these species begin to appear. These creatures live long and grow big. It is thought that the situation settles into something similar to that of the former tidal flat in about three years.

JFS/Initiatives to Restore the Tidal Flats of Ago Bay
Copyright Mie Prefecture Fisheries Research Institute

The area of this tidal flat restoration project is about two hectares. Seawater is introduced into the area at the replacement rate of 70 or 80 percent. It is surprising to learn that seawater flows into the entire 2-hectare area just by opening a gate sized 1.8 x 2 meters located at the center of the dike. How many more species will be identified in the next study? We find this project very exciting.

We have learned that there are 500 such "ex-tidal flats" totaling as much as 185 hectares in Ago Bay that were once developed but are not used any more. The Department of Agriculture, Fisheries, Commerce and Industry of Mie Prefecture studied all these unused tidal flats and created a database with information on owners, area and status of utilization. With this database, the prefecture can identify places that can be turned back into tidal flats.

So, what are challenges when we try to restore tidal flats? They say that one of the biggest hurdles is to gain the understanding of land owners. Fortunately, the site presently being restored has no owner. However, most of Ago Bay's "ex-tidal flats" are owned by individuals or businesses. The upcoming challenge is to convince these owners how recoverable their lands are and how beneficial such restoration would be to them.

The other challenge is to achieve collaboration among the municipal, prefectural, and national governments. Such collaboration will become much more important, because the government of Shima City, where the presently targeted area is located, is promoting the initiative, while the management of the land is entrusted to Mie Prefecture by the Japanese Ministry of Agriculture, Forestry and Fisheries (MAFF). When we stand on the dike dividing the tidal flats and the sea and look around, we see a beautiful scene of the sea, mountains and rice paddies, but at the same time we can also see a virtual walled maze of vertically divided administrative systems standing in our way. The sea is managed by the Japanese Ministry of Land, Infrastructure and Transport; the place itself is designated as a national park and therefore under the jurisdiction of the Ministry of the Environment; and the dikes themselves are managed by the MAFF as part of their agriculture operations.

Seeking for the understanding and involvement of local people, project managers have pursued activities with citizens and schools nearby, for example by hosting monthly observation tours and having volunteers plant seaweed in the restoration site. Another plan is to place seed clams on the site and have a clam dig in future.

JFS/Initiatives to Restore the Tidal Flats of Ago Bay JFS/Initiatives to Restore the Tidal Flats of Ago Bay
Copyright Mie Prefecture Fisheries Research Institute

In Japan, we have initiatives to create artificial tidal flats, but not many tidal flat restoration projects. According to a simulation calculation based on numerical models, if we increase the area of tidal flats by restoring all of Ago Bay's "ex-tidal flats" back to what they used be, the current 84 hectares of tidal flats will increase to 266 hectares, greatly increasing the bay's ability to eliminate organic matter up to 126 tons from the present 16 tons per year and suppressing the occurrence of red tides and oxygen-deficient water incidents greatly.

JFS/Initiatives to Restore the Tidal Flats of Ago Bay
Copyright Mie Prefecture Fisheries Research Institute
(click to view large image)

In addition to initiatives by prefectural marine institutes, local administrations, citizens and non-government organizations, corporate socially responsible activities and public awareness projects, a program to put environmental labels on pearls will also be important. I for one will be keeping an eye on efforts to clean up Ago Bay by restoring tidal flats to see if it will also lead to restoring the declining production and sales of pearls, the prefecture's local specialty, as well. I hope that good news of the results of restoring tidal flats in Ago Bay will spread to other places, too.

Written by Junko Edahiro


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