1. What Tidal Flats Are: Places That Become Sea and Land Twice a Day
Seawater rises and falls about twice a day. This happens because of the gravitational pull of the moon and the sun. When the tide goes out, the bottom of the sea comes above the water. Wide, flat areas of mud or sand appear. This is a tidal flat. A few hours later, it sinks back under the sea. [1]
Few places on Earth are covered by the sea at high tide and exposed at low tide. Unique creatures live in tidal flats. These include clams like the Manila clam, crabs such as the fiddler crab, ragworms, fish like gobies, and birds that come to eat them. [1]
It seems like a difficult place to live. The water covers it or dries it out every few hours. It is hot in summer and cold in winter, and the salt levels change. Yet, a huge number of creatures gather there. The reason, as we will see in the next section, is that food is abundant. [1]
2. How Japan Reclaimed One-Third of Its Tidal Flats
Here is a less happy story. Japan’s tidal flats decreased significantly in the last 80 years. According to a survey by the Ministry of the Environment, there were 82,621 hectares of tidal flats in Japan in 1945. By 1979, about 35% of that area had been lost. The biggest cause was land reclamation, but it was not the only one. [1]
82,621 hectares is larger than the area of Tokyo’s 23 wards. More than one-third of this area disappeared in about 30 years. The decrease in Tokyo Bay’s tidal flats was even greater. In the late Meiji era, Tokyo Bay had about 136 square kilometers (about 52 square miles) of tidal flats. By 1983, this had shrunk to 10 square kilometers (about 4 square miles). That is less than one-tenth of the original area. [1]
Land reclamation means putting soil and sand into the sea to create land. Factories, ports, houses, and roads were built on this new land. It is not rare to walk along Tokyo Bay and realize, "This used to be sea." [1]
The author of this article lives in Chuo Ward, Tokyo. Much of the land in this ward was created by reclamation. This is why there are few natural water areas nearby, as mentioned in an article about toads. [1]
3. How Tidal Flats Help Clean the Water
The most famous function of a tidal flat is cleaning water. Clams like the Manila clam suck in water and eat the phytoplankton and small particles floating in it. Then they spit out the filtered water. [1]
The amount is significant. A parent clam about 3 cm (about 1.2 inches) long can filter about 1 liter (about 0.26 gallons) of seawater per hour under good conditions. That is the volume of a milk carton, per clam, per hour. A tidal flat can contain many clams. [1]
Another worker is the ragworm. As mentioned before, ragworms dig holes in the mud. Seawater and oxygen enter these holes. When oxygen reaches the mud, microorganisms become active and break down accumulated organic matter, such as dead creatures and waste. [1]
Together, these processes act as a purification system. Dirt from rivers enters the flat. Clams filter it, and creatures eat it. Microorganisms in the mud break it down. Birds and fish eat these creatures and carry the matter away. The dirt changes form through the bodies of living things and leaves the system. Reclaiming a tidal flat stops this purification device. [1]
4. Job 2 of Tidal Flats: How Tidal Flats Feed Migratory Birds
In spring and autumn, long-billed birds gather on tidal flats. These are sandpipers and plovers. They peck at the mud to find creatures inside. These birds do not stay there all year. They are migratory birds that travel between northern breeding grounds and southern wintering grounds every year. The distance is thousands of kilometers. [1]
To fly long distances, they must stop to eat and recover energy. Tidal flats are these stopover points. Japan’s tidal flats are on these migration routes. For example, the Ramsar Convention wetlands in Ariake Sea are described by the Ministry of the Environment as important stopover and wintering sites for sandpipers and plovers in East Asia from autumn to spring. [1]
Think of it like driving a car. You need gas to go far. If one gas station closes, you can drive to the next one. But if many close, you cannot reach your destination. For migratory birds, tidal flats are like gas stations. Reclaiming one spot affects the entire route. When Japan’s tidal flats decrease, it affects birds even when they are not in Japan. [1]
Creatures that cross borders cannot be protected by one country alone. The international agreements mentioned in the next section were created for this reason. [1]
5. Job 3 of Tidal Flats: A Cradle for Fish and a Place for People
Tidal flats have another important role. They are places where small fish grow. For baby fish, the biggest threat is large fish. However, tidal flats are shallow, so large fish have trouble entering. Also, food is abundant. Few enemies and plenty of food are the best conditions for young fish. [1]
Many fish, and some shrimp and crabs, spend their childhood in tidal flats or nearby shallow areas. They move to the open sea after growing. These places are sometimes called "cradles of the sea." [1]
Tidal flats are also places for human work. People catch clams and grow nori (seaweed). The Ministry of the Environment notes that in one tidal flat in Ariake Sea, many ragworms and shellfish live, and nori farming and clam fishing take place. Clam digging is also a game that is possible because of tidal flats. [1]
The value of tidal flats is not just that "nature is important." Fish grow, clams are caught, and water is cleaned. This connects directly to the lives of people who live there. [1]
6. Tidal Flats That Remain in Japan and Efforts for the Future
Although tidal flats have decreased, some places remain. In the inner part of Tokyo Bay, there is Sanbanze. It is a tidal flat and shallow area of about 1,800 hectares (about 4,449 acres) spreading along the coasts of Urayasu, Ichikawa, Funabashi, and Narashino cities. It is defined as areas shallower than 5 meters (about 16 feet) at low tide. It supports diverse life like clams, crabs, and flatfish, and has water purification functions. [1]
Sanbanze is a precious place that remains while Tokyo Bay’s tidal flats have shrunk to less than one-tenth. [1]
To protect wetlands, there is an international agreement called the Ramsar Convention. It was adopted on February 2, 1971, in the town of Ramsar, Iran. [1]
What is interesting about this convention is that it does not aim to preserve wetlands by keeping people out. The convention aims to promote wise use alongside conservation. This means protecting while using. It is an idea of finding ways to use the area without destroying it. [1]
Initially, the focus was on the role of wetlands as habitats for water birds. As countries worked on this, the various roles of wetland ecosystems became widely recognized. [1]
7. How to Visit a Tidal Flat: Checking the Tide Table
You can visit a tidal flat to observe it. Check the tide table. Search for "tide table" or "tide graph" to find the low tide time for that day. It is recommended to arrive 1 to 2 hours before low tide. [1]
Observe the surface of the mud. Look at the size and shape of holes, and the sand piled around them. The traces differ depending on who lives in the hole. [1]
Wait without moving. If people get close, crabs hide. From a safe spot with an adult, wait quietly for about 5 minutes; crabs may come out. [1]
Look at the birds. In spring and autumn, you might see sandpipers or plovers on their migration. The length of their bills differs because they target different depths. [1]
Go in boots and go with an adult. Always check the time when the tide will come in. Tidal flats are flat, so when the tide rises, water comes quickly to your feet. [1]
SAFETY NOTE: The best time for observation is 2 hours before and after low tide. Simultaneously check the time when the tide will turn to high tide; this is the key to safety. Wear boots or shoes that can get wet. [1]