All species in the world are born and evolve alongside a specific ecosystem that they have adapted to live in. In the Mekong River, the inevitable change that occurs every year is that the water level gradually decreases during the dry season and increases during the rainy season. If you look at old graphs, you will see a simple bell curve. If you place the dry season in the middle, both sides resemble a taut rope. The level gradually decreases until it reaches a low point, stays there for a few months, and then gradually rises to the point of overflowing. What happens at the lowest and highest points is crucial for the life cycles of all creatures.

The Low Point

At the lowest point, the water flows slowly, is shallow and clear, allowing sunlight to reach the rocks, gravel, and various materials beneath the Mekong. Algae will grow, and small creatures like diatoms, water insects, and snails will come to feed on the algae. Fish will also come to eat the algae and these small creatures. Important fish species in the Mekong basin, such as the Mekong giant catfish, snakehead fish, and others, all feed on these organisms. In July, if anyone visits the Mekong, especially in Loei province, locals will build pavilions in the shallow water to enjoy meals. Those who are not careful may easily slip on the underwater stones due to the biological slime.

Moreover, if you observe closely, this low water period is when small fish thrive. The slow water flow and shallow depths provide a refuge for small fish, where larger fish cannot follow. Many species, such as the rice fish and the mudfish, take this opportunity to spawn during this season, allowing their fry to grow in a safer environment. When the water rises, they can fend for themselves. If these small fish do not take care of themselves, they may become prey for larger fish.

Along the riverbanks during this time, various algae will also begin to grow, including sticky rice algae and other types that thrive on the sediment washed in during the flood season. These larger plants provide shelter for shrimp, crabs, and young fish, including small species like the dwarf catfish. Some fish even consume these algae as food. Edible algae, such as the gray algae, also flourish during this season, providing a direct food source for people. The sediment left on the sandy riverbanks serves as excellent fertilizer for locals to plant short-term vegetables to harvest during this season. Fish are in the water, vegetables are on the banks, and rice is in the fields.

Another group of fish that thrive in July are those that feed on the riverbed. These fish dig into the sediment for food. The slow water flow creates sedimentation areas for these fish to forage. Many small creatures feed on the sediment and live within it, becoming food for fish like the catfish and the snakehead fish, which are excellent examples of bottom-feeding fish that thrive in this ecosystem.

Additionally, the slow water flow during this season allows nutrients and fertilizers to accumulate in specific areas of the calm eddies. These nutrients are concentrated enough to support the growth of single-celled algae and plankton, which serve as food for both small and large fish. Filter-feeding animals like bivalves also thrive during this season, while fish that eat clams, such as the catfish and the tilapia, benefit as well.

This dry season is also a time when many trees and flowers reproduce. The drought stimulates these plants to bloom and bear fruit. The dry winds blow the petals of the water lily onto the water's surface, becoming food for the fish. The fallen seeds of the mango tree also provide food for the fish, as the water along the banks flows slower than in the middle of the river.

As the water recedes, numerous sandbanks and islands emerge in the Mekong River. Many bird species, such as swallows and sandpipers, use these sandy beaches to build nests, lay eggs, and raise their young. These birds create simple nests on the ground of the sandy beaches, where their flightless chicks find safety from predators on these small islands, which are abundant with insects and earthworms to eat. Occasionally, they may be startled by a giant soft-shelled turtle, the size of a table, dragging itself from the water to lay eggs. These turtles inhabit the sandy beaches where the water is not too deep, allowing them to bury themselves in the sand while still being able to breathe. The Mekong River is rich in fish, which lie in wait for passing fish to snap up, growing to sizes of several dozen kilograms. On the low riverbanks, small holes dug by fish during high water now serve as homes for families of kingfishers, with neighbors being the white-throated kingfisher.

What has been mentioned so far are all small fish, herbivorous fish, and sediment feeders. Alongside these small fish are predator fish. If there are tigers in the forest as large predators, in the Mekong River, there is the giant Mekong catfish, one of the largest and most graceful predator fish in the world. It has a streamlined shape reminiscent of a shark, with long, beautiful fins that evoke images of fighter jets. It is not much smaller than the giant catfish. This large predator can only survive in a complete ecosystem. Other large predator fish in the Mekong include the giant snakehead and the giant catfish, all weighing over 40 or 50 kilograms. Smaller species include the tilapia, the striped catfish, and the red tilapia, all of which are good-quality fish and excellent sources of protein, rich in DHA, making them beneficial for the brain.

Where Do Fish Go When the Water Rises?

Once the dry season is over, the rain begins to fall, and the water starts to flow more vigorously, carrying various minerals with it, changing the chemistry and temperature of the water. Many fish species rely on these signals to begin their migration, hurrying to move before the large schools arrive, as it will be too difficult to travel then. The question is, where do they go and what do they do? Let's assume there are points A, B, and C, with water flowing from A to C, taking three days. Let's say point C is where the fish 'suwai' (a local name for the catfish) is most comfortable. The eggs of the suwai take about 24-36 hours to hatch. The fry, which rely on the yolk for food, are not good swimmers. It takes them another couple of days to be able to swim and take care of themselves. If the suwai wants its fry to stay at point C, what should it do? The eggs at point C will be swept down to point E. Therefore, if the suwai wants its fry to stay at point C, it must migrate upstream to point A, where the eggs are, and let the oxygen-rich and nutrient-filled water carry the fry along. In three days, they will reach point C, just in time for the fry to take care of themselves. Now, you might wonder what point B is for. Point B is where a selfish person has built a dam, preventing the suwai's parents from migrating. What will happen now? There are five possibilities. First, the eggs will be lost there, and the fry will not be in the right place. They will drift away. Second, they may not be able to breed due to stress and exhaustion, leading to infertility. Third, even though there are laws prohibiting fishing near the dam and sluice gates, in reality, no one enforces them. Fish migrating upstream often get trapped at the dam and are exploited. Fourth, the eggs may get stuck and die there.

Fifth, let's start a new paragraph because there is a dam that claims to be fish-friendly, investing in creating a passage for fish to cross, almost expecting gratitude from the fish and the public. Let's assume the suwai accidentally sees the fish ladder (which is actually referred to as a fish passage because it is very steep). In reality, most fish get caught in the water flowing through the dam's turbines because it flows faster there. But let's assume the suwai finds this passage. Below, there are giant catfish, snakehead fish, and other parent fish weighing fifty, a hundred, or two hundred kilograms, all wondering how to get through this fish passage. But our suwai is not interested in swimming up the ladder; it just takes the elevator, reaching a height equivalent to an eleven-story building, crossing the dam. The elevator opens, and it will be a bit confused. Normally, fish swim upstream, but the water flowing through the dam does not flow like river water. It does not flow uniformly; it has areas of slow and fast flow depending on how much they open the gates.

Let's assume our suwai encounters a current that guides both upstream until it reaches the point where they used to meet before the dam was built. They successfully spawn, and the eggs drift away. After a while, they encounter the dam's overflow, and they stop drifting and sink there, not being eaten by other fish. They will die there, unable to breathe at the bottom of the dam. How many will be lucky enough to catch the current and flow past the dam? Our local fish are not like the salmon from the countries that built fish ladders (which also do not help the salmon survive). Salmon swim upstream to spawn and die, not needing to migrate back. The fry live upstream until they grow to a certain size and swim back down, passing through the turbines, some surviving and some dying, but they can swim by then and are not too large. Our local fish, after spawning, flow back down as eggs and small fry that cannot fend for themselves. They cannot easily pass through the still water behind the dam. As for the parent fish, on their way back, they just hope to flow through the channels used for boats, avoiding the risk of passing through the turbines, which are said to be safe for fish, but there is always a chance of getting caught in the blades.

What Do Fish Eat When the Water Rises?

Let's assume we go back in time. Let's say we can drink bubble tea without gaining weight, and right now, there are no dams in the Mekong River. Let's say the water is flooding, and the Mekong is murky, full of minerals washed down from upstream (the color now resembles Thai tea with milk). The water is beginning to overflow its banks, flooding fields, wetlands, and small ponds everywhere, even flowing into rice paddies and agricultural areas. Fish of all sizes flow in with the water. The first thing fish will eat is various animals that did not escape the flood: ants, termites, earthworms, and other surface-dwelling creatures, as well as edible plant parts like leaves and young pods. Large animals that drown also become food for many aquatic creatures. After a while, algae will start to grow on leaves, grass, logs, and stones, attracting many fish and aquatic animals. As the flooding continues, little heavens begin to form. Plants start to decay, becoming food for bacteria, which in turn become food for small animals. These small animals become food for small fish, which then become food for medium-sized fish. Medium-sized fish that are not eaten by humans become food for larger animals. When terrestrial plants die, aquatic plants take their place, becoming food for aquatic animals.

In the flooded fields, not only is food easy to find, but there are also advantages. The slow water flow allows fry to survive, providing plenty of hiding places for them. At the same time, it serves as an ambush point for predator fish. Fry and parent fish feed in these flooded fields. When the water recedes, they flow back down with the current. The fry that grew up in the flooded fields, both those whose parents migrated upstream to spawn and those that briefly entered to lay eggs, are now big enough to take care of themselves. The white fish are now about one or two inches long and can live in the river.

As for the agricultural areas that were flooded, they now receive fertilizer from the sediment washed in. During the flooding, organic matter in the soil decomposes, turning into fertilizer. Salts accumulated from using groundwater or fertilizers are washed away, leading to better crop yields.

Plants, Plants, and More Plants

You will notice that in every season, I mention how plants grow, where they grow, and who eats them. Because in every ecosystem, the primary producers are plants. A healthy aquatic ecosystem is one where plants grow and interact appropriately with the seasons. Whenever the shallow parts of water bodies with plants are dug deep enough that sunlight cannot reach the bottom and there are no large plants to grow above the water, whenever the water is murky, and/or flows rapidly, and/or remains at a high level all the time, plants cannot grow. Whenever the water is held back and does not overflow its banks, fish cannot benefit from terrestrial plants. Whenever the sediment that would fall and serve as fertilizer for plants accumulates in the deep pools behind dams, plants cannot grow. Whenever the water levels do not rise and fall according to the seasons, plants cannot grow. At that point, the ecosystem collapses.

The Present with Dams

Water levels do not rise and fall according to the seasons. There is water in the dry season and no water in the rainy season. Animals cannot adapt quickly enough. Aquatic plants cannot grow, and riverside plants are dying. Some drown before they can bloom or bear fruit. Birds nesting on sandy beaches during the dry season are suddenly flooded when the dam releases water. Turtle eggs are swept away, and the turtles themselves are confused about where to bury themselves because the water levels are not natural. People trying to plant crops along the banks cannot do so because sometimes the vegetables do not grow before the dam releases water. Even if they can plant, the yields are poor because some of the sediment that serves as fertilizer is lying still behind the dam.

Blocking fish from migrating upstream to spawn, many species cannot pass through. For those that can, the fry have a lower chance of survival because the eggs and fry sink behind the dam. Of course, there will be an agency responsible for breeding non-native fish to release, further complicating the situation.

Water does not flood the fields during the flood season because the dam must retain water for electricity generation during the dry season.

Not to mention the loss of land and food production areas.

Water that rises and falls slowly in nature allows the moisture levels in the riverbank soil to gradually decrease, making the soil lighter and more stable, maintaining its structure. However, rapidly rising and falling water causes the moisture in the riverbank soil to dry out too quickly, making the soil heavy and prone to erosion. As the soil erodes into the Mekong River, Thailand loses land, and along with it, the water lily and fig trees that fish rely on for food. Some dams see water levels rising and falling daily or hourly. We will have to wait and see what happens.

At the mouth of the Mekong River in Vietnam, there are increasingly severe coastal erosion problems because the sediment from the Mekong does not reach the river's mouth. This area is the most important rice-growing region in Vietnam, feeding the entire country and exporting to many others. Therefore, it is not surprising that the Philippines expresses concern over dam construction in the Mekong basin, even though it is an island nation and does not border the Mekong River. The Philippines also consumes rice grown from the fertile lands of the Mekong Delta.

Is This the End?

Fish are a high-quality protein source for people, and vegetables grown along the banks are important food sources. Those who depend on these foods are primarily low-income groups, numbering in the hundreds of millions in the Mekong basin, some of whom still face malnutrition issues. Building dams to enrich a few families, claiming energy security without considering the broader impacts, especially on ecosystems, which directly affect food security and biodiversity, should not happen.

The question I would like Thai people to ask themselves today is: In the end, how can a society that continually allows the rich to get richer by seizing resources from marginalized people be peaceful and harmonious?

SOURCE: www.thaipublica.org