Where have all the fish gone?

The mighty Mekong is drying up - and so is the river's rich harvest. Vast new dams in China could be to blame. Fred Pearce reports from Cambodia
Once, the world's rivers teemed with fish. No longer. Around the globe, dams and other river engineering projects have drastically reduced most inland
fisheries. But on one mighty river, the Mekong in South-east Asia, half a century of warfare had kept the dam-builders away. As a result, even the
poorest people in countries such as Cambodia can still dine regularly on wild river fish.
But now the engineers have moved on to the river, and the effects are already being felt. Scientists blame new Chinese hydroelectric dams for the
record low levels of the river this spring, for weird fluctuations in river flows, and above all for a collapse in fish catches. Is this the end for one of the world's last great untamed rivers?
The Mekong flows for 4,500km (2,800 miles) out of the Tibetan ice fields and through the mountains of Yunnan in southern China, before tumbling into the
flood plains of Thailand, Laos and Cambodia and entering the sea through its delta in Vietnam. Its flows are highly seasonal, with 30 times more water in
the river during the monsoon than in the dry season.
So great is the disparity that, in Cambodia, the swollen river forces one of its major tributaries, the Tonle Sap, to go into reverse from June to September. This unique spectacle takes place right in front of the Royal Palace in Phnom Penh, from where the Tonle Sap flows upstream for 200km, forming a great lake near the ancient ruins of Angkor Wat. The floodwaters then spread out, engulfing the surrounding rainforest.
The waters carry fertile sediment, fish larvae and fingerlings into the forest, which turns into a vast fecund nursery ground for fish - the source of one of the world's biggest inland fisheries. Here you will find the last of the Mekong catfish, the largest freshwater fish, which grows to three metres long and can weigh more than a cow. There's also the striped snakehead, which lives among tree roots and in lakes and swamps, and is known for its ability to slither overland between pools. Of greater value to millions of Cambodians is the fact that the flooded forest is also the breeding ground for the trey riel (Henicorhynchus siamensis), a sardine-like fish found in almost every net on the river.
As the forest slowly drains each autumn, the fattened fish migrate throughout the Mekong river system, where local fishermen, many living in floating villages, know almost to the hour when the fish will pass by. The peak moment of the annual flood on the Tonle Sap is precisely 10 days before the January full moon.
The intensity of fishing on the Tonle Sap in particular is extraordinary. Nets stretch for miles around the edge of the flooded forest. And near Phnom
Penh, small wicker "bags" lowered into the river can catch half a ton of fish in 20 minutes. About 50 million people in the river's lower basin - in Cambodia, Vietnam, Thailand and Laos - depend on it for food and income. Cambodians alone catch about two million tons of fish a year and are more dependent on wild protein than almost any nation on earth.
Yet scientists agree that, while the river is heavily fished, it is probably not overfished. The current levels of fishing are sustainable provided that the annual flood "pulse" is maintained. But that is increasingly in doubt.
In November, I toured the Tonle Sap with Eric Baran, who is investigating the Mekong for the WorldFish Centre, an international organisation that researches food security and poverty eradication. He says the river is in crisis, with dwindling flows triggering declining fish yields. The fishermen we met told the same story. Their catches had never been so poor, and they all blamed the low river-flows that began last summer. One, heading home to his floating village with empty nets, told us that "when the water is shallow in front of the Royal Palace, there are no fish in the river".
Nguyn Van Xia, a riverside buyer for the main fish wholesale market for Phnom Penh, said that with catches at such low levels, prices were three times higher than usual. The Mekong River Commission, an intergovernmental scientific body based in Phnom Penh, reported this year that fish catches
between November and March were only half the usual levels. And, as prices soared, the poor went hungry.
The crisis is evident everywhere along the river. For three months, low river levels have been exposing hidden sandbars, leaving ferries beached. The Mekong population of the rare Irrawaddy dolphin has been marooned in shrinking pools.
Poor monsoon rains last year take some of the blame. But the Mekong River Commission, which China has refused to join, has little doubt that Chinese
dams upstream play a role. After an emergency meeting last month, it called on China to release critical information on the operation of its dams.
China has so far built two giant dams on the main stem of the Mekong. The first, at Manwan, was completed 11 years ago. The subsequent filling of its
reservoir coincided with a period of unusually low flows downstream. The second dam was completed last year at Dachaoshan. "There is an assumption
that the two dams are the cause of the situation," says Surachai Sasisuwan, the director of water resources at the Commission.
Some say the dams cannot be blamed because only about one-fifth of the river's annual flow comes from China. But during the dry season, this proportion rises to between 50 and 70 per cent. And the dams provide their own unambiguous effect on day-to-day river flows. The dams are all providing hydroelectricity to power China's economic boom. As the turbines are switched on and off to meet hourly changes in demand, their reservoirs empty and fill and the river downstream sees fluctuations in water levels of up to a metre a day. "Since the dams began operating, river levels have gone up and down much faster," says Hans Guttman, a scientist working for the commission.
Perhaps even more important for the future of the river's fisheries is its load of fertile sediment. According to Matti Kummu of the Helsinki University of Technology, who is modelling the river's hydrology, as much as half of the river's natural annual sediment load comes from China, and an increasing amount is being captured behind the new dams. He believes that the sediment, which is carried mostly in the monsoon floods, is critical to the fertility of the Mekong's fisheries, and especially to those in the Tonle Sap.
There is some irony in this concern, as extreme flood levels are usually perceived to be a problem. But, even after the reservoir of the latest dam is filled, the day-to-day operation of the dams is likely to have continuing effects. Chinese engineers forecast that it will reduce the annual flood and increase the dry-season flow. And it is this diminution of the flood pulse, the lifeblood of the river's ecosystem, that is of most concern to fisheries scientists. "Engineers think of flood extremes as bad news, but in Asia they are a good thing because they drive the natural ecosystem on which millions depend for their food," Baran says. Guttman agrees. "There is a strong relationship between flood flows and fish migration," he says. "Flattening the flood peaks would have a severe effect on the river's ecosystem."
A nightmare scenario would be for the flood to subside so much that the Tonle Sap stops reversing its flow during the monsoon. That would dry up the
river's major nursery for fish.
The battle for the Mekong is gathering pace. Engineers see the Mekong is one of the world's last great unexploited sources of hydroelectricity and a
potential powerhouse for the industrialisation of South-east Asia. Two more Chinese dams are already under construction, and at least four more are
planned. The biggest, at Xiaowan, began construction in 2002. It will dwarf the others, rising 300 metres above the river and creating a reservoir 150km
long.
Chris Barlow, the manager of the commission's fisheries research, sees this policy as very short-sighted. "Fish are permanent, if we manage them
wisely," he says. "But a dam has a short life; 30 years or less. Even when the dam is dismantled, the fishery may never come back." Governments, he says, are easily seduced by the prospect of abundant energy. But fish on the Mekong are a vital source of food for millions of the region's people and an essential source of income for its poorest citizens. "The only alternative they have, if the fisheries disappear, is working all hours in a textiles factory in Phnom Penh," Barlow says. Full nets or crowded factories: that is the choice.