The Tonle Sap Lake, the largest lake in Southeast Asia, is connected to the Mekong River by the Tonle Sap River. In terms of fish abundance, the lake is among the most productive in the world. For about eight months of each year, especially during the dry season, the river flows “down” from the lake, joining the Mekong in Phnom Penh. During the wet season, however, the vast quantities of water flowing down the Mekong push the Tonle Sap river “up” towards the lake, inundating floodplains and providing access to many fish species for breeding, rearing, and feeding. Such “reverse flows” generate enormous fish production, essential for Cambodian food security where fish is the main staple after rice. Between 1996 and 2005, the reverse flows averaged about 43 km3 in volume and last about 120 days until the water starts flowing back down the river into the Mekong.

In 2020, the reverse flows were significantly delayed – about two weeks later than 2019 and 40 days later than the average between 1997 and 2017. The first started on 7 July and ended on 15 July, with a volume of just 0.21 km3. A second occurred in late July with the major reversal not taking place until August. Reverse flows also occurred in late September and the third week of October, finally stopping in the last week of October.

At 18.89 km3, the volume for 2020 was only about 44 percent of the average and the lowest since 1997. As a result, the Tonle Sap Lake was suffering from extremely dry conditions by the end of October. This is when the annual fishing season usually starts with fish migrating down the Tonle Sap River, which has the largest commercial fishery in the Lower Mekong Basin. Catches from the fishery — located on the river in northern Phnom Penh and neighbouring Kandal province — usually peak in December or January.

Based on Article 6 of the Mekong Agreement between Cambodia, Laos, Thailand and Vietnam in 1995, the Mekong River Commission considered the reverse as “unstable” in 2020. Except in cases of historically severe floods or droughts, the article provides for cooperating to maintain Mekong flows from diversions, storage releases or other actions of a permanent nature “to enable the acceptable natural reverse flow of the Tonle Sap River to take place during the wet season.” In the case of 2020, the MRC concluded that the reverse flow volume appeared to be low due to the low rainfall between May and July as well as the late arrival of the annual monsoon rains, which lasted from August to October. This rainfall pattern was well captured by characteristics of the reverse flows into the Tonle Sap Lake.

Compared to long-term average volumes, low flows persisted in the Tonle Sap Lake in 2021. But the situation was nowhere near as critical as 2020. The reverse flow started in mid-June – about three weeks earlier than the previous year. By October, the flow volume had exceeded the 2019 volume and later rose to around the same as 2018. As of early November this year, in only one month (September) were volumes considered to be “critical”. That compared with five critical months in 2020 and two in 2019. But in all three years, volumes have been either low or critical – and not a single month has experienced “normal” conditions based on average volumes between 1997 and 2019.

With bigger water volumes in the Tonle Sap Lake in 2021, fisherman Path Tol said fishing was the best he’d seen in three years. “Catches are much bigger,” he told Catch and Culture - Environment in early November as Cambodia’s main fishing season was getting underway. Path Thol is village chief in Kampong Koh in Kampong Luong commune in Krakor district in Pursat province. Kampong Luong is located near two fish conservation areas and has many fishing grounds. The main fish habitats are the lake and flooded forest, which provide rich feeding grounds when the Tonle Sap flood plain is inundated every year.

Path Thol said his floating village had 97 fishing households. “Some fishers have only one career, which is fishing by net,” the village chief said, adding that most tend to be older. “All the young people have left to work at garment factories or construction sites.” He said fishers in his village fish only in the open lake, about 5-6 km away, using drift gillnets, known as mong bandet in Khmer. “We cannot fish in the flooded forest,” Path Thol said, explaining that the nets get snagged in the trees. He said fishers typically head out to the lake mid-afternoon to set the drift nets before returning home. They return in the early evening to collect their catch, taking the fish back to the village. They go back to the lake the next morning before dawn to bring in a second catch. The catches are dominated by Borneo river sprats (Clupeiodes borneensis), a tiny species used to make a Vietnamese-style fermented fish paste. The motorised boats used have at least three men to haul in the catches from the drift nets, which can be up to 2 km in length. “These fishers can catch 100-150 kg a day,” the village chief said.

Known as trey bondol ampeou (“sugarcane stalk fish”) in Khmer, the Borneo river sprat is native to Cambodia, Laos, Thailand and Vietnam in addition to Borneo. It grows to about 7 cm – compared with common sizes of about 10-12 cm for small species of mud carp known as trey riel in Khmer (Gymnostumus siamensis and Gymnostumus lobatus). Such short-lived carp species are among those that drive fish catches across the Lower Mekong Basin (carps account for more than 45 percent of the basin’s fish abundance and biomass).

Phat Thol said the river sprat catches are either exported to Thailand on ice where such small species can fetch wholesale prices of almost US$5 per kg. They are also used locally for processing into fish cakes or Vietnamese-style fermented fish paste known as mam. Other uses are as feed for local farms raising snakeheads and walking catfish in cages and ponds.

“We had a lot of fish in 2017 but catches started to collapse in 2018,” said Dy Ith. Four years ago, she said “women could easily catch 10-20 kg an hour” from motorised boats with three gill nets about 60 m in length. The target species were mud carps used to make prahok, the fermented fish paste that serves as a staple food in Cambodia. From 2018 onwards, however, she said catches dwindled to only 2-3 kg an hour even though the women used longer nets of up to 1,200 m.

Unable to earn an income from fishing, Dy Ith said she turned to rearing snakeheads (Channa spp) in cages instead. But receding floodwaters flowing into the village were polluted and she had to frequently move the cages. “It was too difficult so I stopped ... Now I’m raising walking catfish," she said, referring to fish from the Clarias genus of air-breathing catfishes known as trey andeng in Khmer. "They can adapt more easily to bad water.” Dy Ith said she had also started trading catches of other species unloaded in the floating village, taking them to sell on land.

Notwithstanding the improved catches this year compared with recent years when water levels were exceptionally low, Dy Ith said mud carps were nowhere near abundant as two decades ago. “I used to be able to row around the canals in the village as the floodwaters receded and I could easily catch mud carps by slapping the oars on the surface. The fish would jump out of the water and I’d have enough food to eat for three days,” she said.

“In those days, the village could catch tonnes of fish every day from arrow traps,” Dy Ith said, referring to the large-scale commercial fishing gear operated by groups of fishers. "There were times when we’d have three or four tonnes a day and have to throw a tonne back into the lake because the buyers said they didn’t want so much.”

Kouk Sokonn – a young fisherman who settled in the village a few years ago with his wife – said the peak fishing period between November and January was the fifth to eighth day of the waxing moon. “I'm now catching 80-100 kg a day,” he said. “But we have to spend more time and set longer gill nets than before.” Kou Sokonn said he offloaded his daily catches to traders based in the floating village who then sold the fish to land-based traders based in Kampong Luong.

MRC monitoring of fish abundance and diversity at five stations around the Tonle Sap Lake showed increasing catch trends at four stations including one located in Kampong Luong between 2007 and 2018. Part of a broader study at 25 stations in all four countries of the Lower Mekong Basin, the monitoring found that fish species composition was “quite stable” around the Tonle Sap (the study – the first of its type – found 617 species across the basin).

Published earlier this year, the MRC study recommended that fish abundance and diversity be protected to sustain food supplies for millions of people living across the basin. “Fisheries are under stressed conditions due to overfishing and habitat degradation induced by human population growth and economic development, and environmental changes (e.g. flow change) including climate change,” it said. The study also recommended setting up more fish monitoring stations close to hydropower dam projects on the Mekong mainstream. “This will enable hydropower projects in the Mekong mainstream to be assessed in terms of their impacts.” Among other recommendations were fish protected areas or conservation zones in transboundary areas and a study to assess the impact of sand mining on fish populations.

First published in the December edition of Catch and Culture - Environment, the fisheries and environment newsletter of the Mekong River Commission

Source: Agency Kampuchea Press