As the outside light fades on the outskirts of Phnom Penh, the flame of a small cookstove grows brighter. With help from her daughters, Sen Mary is cooking rice and freshly caught fish for the Math family’s dinner. While local food is plentiful for now, drastic change may be on the horizon.

“When I was 12, I caught a lot of fish, but now there are not as many. There may be no more fish in the future, depending on the current situation,” says Sen Mary’s son-in-law, Nan Sab Yi, who works as a fisherman to support the family. “We depend on fish and the river. If we don’t have that, we have nothing.”

The Tonle Sap, where Nan Sab Yi catches roughly 110 pounds of fish per day, is on the brink of radical transformation. A freshwater lake with an attached river, the Tonle Sap winds through the eastern part of Phnom Penh. The largest lake in Southeast Asia, it boasts a large volume of fish and is a tributary of the mighty Mekong River, which crosses five countries before reaching Cambodia.

With a handful of new hydropower dams completed — and more than 135 either under construction or forthcoming — the Mekong’s waterways will soon be altered dramatically. For Cambodians, who consume the most freshwater fish in the world and get up to 70 percent of their daily calorie intake from rice, this change may be devastating.

“Since the dams have been built, the amount of fish is decreasing,” says Nan Sab Yi. “Normally the river starts to rise up between June and July, and the fishing is good. Now the water is not high enough.”

With its location downstream from countries that are stakeholders in the hydropower business, Cambodia is especially vulnerable. Although hydropower development will bring low-cost, renewable electricity to its villages, the majority will be sold to countries up north. While receiving few of the benefits, the rural people of Cambodia will be among the most impacted by dam development.

With support from a National Science Foundation grant, UW researchers from the College of the Environment, College of Engineering and School of Public Health are racing to discover how changes to the Mekong will impact the future of fish and rice— and, ultimately, the Cambodian people.

At the bustling Prek Phnov Market in Phnom Penh, fresh fish arrive in the early morning. As locals fill their baskets and busily transport fish by scooter to restaurants, 18-year-old Pich Thann sells fish.

Pich learned the trade from her parents, who now stay home. Though their business — plying an expensive variety called krav fish — has been brisk, Pich is witnessing changes in the market firsthand.

“There are not as many fish from the river. I don’t know what the reason is,” she says. “I sold river fish before, but now I sell farmed fish.”

Although those who work in the industry have started to notice a decline from overfishing and the few completed dams, the average consumer still sees what appears to be an endless supply of fish.

The lower Mekong River Basin produces more than 2 million tons of fish annually, making it the largest freshwater fishery — and one of the most productive ecosystems — in the world, says Gordon Holtgrieve, professor of aquatic and fishery sciences at the UW. The Tonle Sap provides unusually fertile conditions for fish to grow and spawn.

Trey Riel

Henicorhynchus Siamensis

This migratory species makes up the vast majority of fish harvested from the Mekong and Tonle Sap rivers each year. Trey riel are crushed, dried and fermented to make prahok paste, a cornerstone of Cambodian cuisine. These fish are so important to Cambodians that even the national currency is named after the trey riel.

“Per square kilometer of lake, it produces a ridiculous amount of fish — more than the most productive ocean,” Holtgrieve says.

These conditions are the result of annual monsoon floods that cause a reversal of water flow in the Tonle Sap. When the Mekong floods, the water travels up the Tonle Sap River, increasing the footprint of the lake by up to six times. This swelling provides not only more space for fish, but also additional nutrients and minerals from the merging of the habitats.

“We call it a bonus source of energy for the whole system,” Holtgrieve says. “It makes the food web more productive, as it is getting energy from two places. This means more plants, more insects and ultimately more fish.”

With hydropower dams expected to disrupt the natural water flow, Holtgrieve is working to uncover how the nutritional quality and quantity of fish will be affected by different flooding cycles. To assess the first factor, he is analyzing tissue samples from more than 50 species, looking at both harmful elements, such as mercury, and beneficial nutrients, such as omega-3 fatty acids and vitamin A.

Because current data are sparse and collected using imprecise sampling methods, gathering information about quantity is not as simple. As a remedy, UW researchers are working with the local fisheries administration to introduce echosounders, an acoustic fish monitoring system. To demonstrate feasibility, aquatic and fishery sciences professor John Horne visited the region to oversee the installation of an echosounder at a fishery in January 2019. The researchers are now working to secure funding to support a network of sensors, which would be the first acoustic monitoring on the Tonle Sap.

“The overall goal is to monitor fish migration and fishing mortality along the Tonle Sap,” Horne says. “An acoustic network can provide near real-time fishery monitoring for management and be used for biological investigations.”

In the late afternoon, the rice fields of Kampong Thom province glow a bright green. Stretching for great distances, they make up one of several rice production powerhouses in Cambodia. On the eastern edge of Tonle Sap Lake, farmers grow rice primarily for export — because the same floods that create an ideal breeding ground for fish also fuel rice production.

“What is often not fully appreciated in the United States is how important rice is in Cambodia,” says Yasmine Farhat, a graduate student in civil and environmental engineering at the UW. “Most Cambodians eat rice three times per day.”

Rice requires not only a lot of water to grow, but a lot of water at just the right time. To determine how Cambodia’s most important crop will be impacted by the introduction of hydropower dams, Farhat and civil and environmental engineering associate professor Rebecca Neumann are investigating the nutritional quality and yield of rice in two of Cambodia’s top growing areas, including Kampong Thom province.

Although they’re interested in the research underway in their fields, the farmers are not yet vocally concerned about the dams — they care more about how the researchers’ findings can inform their day-to-day work.

“I want to know about the fertility of the soil: if it’s good or bad, and how to fix it,” says Lam Heang, who has been growing rice for more than 10 years.

In collecting rice and soil samples from the fields, Farhat is measuring both contaminants and beneficial nutrients. To help evaluate rice yield, UW environmental and forest sciences professor Soo-Hyung Kim and UW graduate student Manuel Marcaida are using data gathered from 19 rice fields to estimate productivity and determine optimal planting dates. They are also working to predict possible location shifts for rice farmers in response to lower lake levels.

“Once we figure out what key variables are important, we can assess whether those will be impacted by the flood duration and timing and make predictions,” Farhat says. “Now is a good time to start studying, before all the dams are built.”

As soon as they are weaned, Cambodian children are given rice mixed with fermented fish paste, called prahok, which is a signature ingredient in much of the local cuisine.

Yet because of insufficient intake of essential vitamins and minerals in what is still one of the world’s poorest countries, about a third of Cambodian children under the age of five suffer from chronic malnutrition. Any disruption to the food system could have devastating consequences.

In rice-centric countries, pairing this diet staple with a quality protein can help prevent nutritional deficiencies. To determine which varieties of fish and rice can best sustain the country if the local food system is compromised, UW epidemiology professor Adam Drewnowski is evaluating the nutrient profile of staple foods.

The nutritional value of fish varies, depending on factors such as species, habitat and food. Changes to the natural river flow from hydropower dams may cause some fish to become more dominant and easy to catch, which will in turn affect what nutrients are available to people.

“Tonle Sap Lake is the major source of freshwater fish and high-quality protein for that area of southeast Asia,” Drewnowski says. “If the upper stem of the Mekong River is dammed, there may be nutritional consequences downstream.”

Dams are nothing new to the Mekong River Basin — it hosts 315 of them across six countries. But many of the planned new dams, in addition to being larger, will have prominent downstream locations along the main river, rather than in tributaries.

One dam alone — the Lower Sesan 2 dam in northeastern Cambodia, which opened in 2017 — is predicted to lead to the extinction of more than 50 species of fish and cause a 9 percent decline in fish supply.

“Can you manage the dams in a way that makes it more favorable for fish downstream?” asks Matthew Bonnema, a civil and environmental engineering graduate student at the UW. “That’s the million-dollar question.”

Existing dams in the Mekong River Basin provide clues about how future dams may impact the region. To leverage this insight, Bonnema, along with UW civil and environmental engineering faculty Faisal Hossain and Bart Nijssen, are using satellite data to study 20 existing dams, some of which have been operating since the late 1960s. The researchers have identified a distinct change in water temperature downstream from hydropower dams that may result in less productive fisheries.

Evaluating existing dam management strategies, such as water release and holding patterns, will provide further insight that the researchers will use to develop recommendations for optimal dam operations, with the intent of supporting nutrient-rich and high-yielding fish and rice crops.

New dam construction is largely uncoordinated, placing Cambodia in a precarious position. To complicate matters, the region is also still recovering from the devastation of the Khmer Rouge regime and the subsequent civil war.

In lieu of a concerted basin-wide dam management effort among the countries involved, UW researchers anticipate providing recommendations to individual countries to inform the collective operation of hydropower dams.

To encourage the region to focus on shared goals, researchers at Arizona State University, working in conjunction with the UW team, are developing a modeling tool to help government officials understand competing needs, manage tradeoffs and see how cooperation may enable improved access to food and livelihoods for their citizens. This kind of collaboration with decisionmakers in order to advance the health of people aligns with UW’s Population Health Initiative and EarthLab. The researchers are optimistic that hydropower operations can be programmed to meet power-generation objectives while satisfying food production.

“This is something we have leverage over,” Holtgrieve says. “It may be possible to change how we release water to produce more fish and rice with only slightly less power.”

In the coming years, the impact of the researchers’ work will be most apparent in the daily ritual of preparing dinner. It remains to be seen whether the Cambodian people — including Sen Mary, Nan Sab Yi and the rest of the Math family — can continue to rely on the same foods that have sustained them for generations.