The Reversal Is the Fishery
Once a year the Tonlé Sap river stops, turns around and runs backwards for four months, and the lake it feeds grows about six times larger. That reversal is what produces one of the largest freshwater fisheries on earth, and it has been arriving late, low and short.
At Phnom Penh four channels meet in one place the Khmer call Chaktomuk, four faces. The Mekong comes in from the north, leaves to the south, throws off the Bassac on the way, and the fourth arm is the Tonlé Sap river, 120 kilometres of it running up to the lake of the same name. For most of the year that river drains the lake southwards into the Mekong, which is what a tributary is supposed to do. Then, some time around late May or June, the monsoon flood coming down from Yunnan and Laos raises the Mekong at Phnom Penh higher than the lake behind it, the gradient inverts, and the river turns round and flows north for about four months.
The lake goes from roughly 2,500 square kilometres to something like 15,000, and from about a metre deep to nine or ten. Water spreads kilometres into the forest around it. In October it drains back out, and the national holiday, Bon Om Touk, is set to that — a water festival timed to a hydraulic event, which is not a metaphor anybody had to invent.
The flood is the crop
What the reversal does is put a forest under water for a season. Fish move out of the river channel into the inundated woodland to spawn and feed; the young grow there through the wet months with cover and food and no current; and when the water falls they are pushed back out down the Tonlé Sap river in one enormous migration between October and March. That out-migration is the fishery. Cambodia's inland catch is among the largest in the world and fish supplies the majority of the animal protein in the Cambodian diet, in a country where a very large share of rural households fish at least part-time. None of it is farmed and none of it is planted. It is produced by the timing of a flood.
The longest catch record on the river
The out-migration is measured, and unusually well. Strung across the Tonlé Sap river below Phnom Penh are the dai — stationary bagnets held open by the current, licensed in numbered rows, fished only during the falling-water months. Because the licences are fixed and the gear has barely changed, the catch has been monitored since 1995 by Cambodian fisheries researchers with the Mekong River Commission, which gives the basin its longest continuous catch series. It is not an encouraging one. Total catch per unit of effort has fallen heavily over the series, the largest species have thinned out first, and the seasons since 2019 have been among the worst recorded — in 2019 and again in 2020 the reversal came weeks late and the flood that followed was one of the lowest on record.
A dam does not have to take the water away to end a fishery. It only has to change when the water arrives.
What abolishing the lots did
The domestic half of the argument is about who is allowed to fish. From the French protectorate onwards Cambodia auctioned exclusive commercial concessions on the lake — the fishing lots — to operators who paid for a stretch of water and everything in it. They were resented for a century and reasonably so. More than half the lot area was released to community fisheries in 2000 and 2001, and in March 2012 Hun Sen abolished the remaining lots outright, converting them into public fishing ground and conservation areas. It was popular and it was defensible. It also removed, overnight, the only people who had ever had a financial reason to police that water. The Fisheries Administration was given the job and not the budget; several hundred community fisheries were created and mostly left unfunded. What followed was more fine-mesh netting, more electrofishing, and more pumping-out of floodplain pools in the dry season, which takes the brood stock rather than the surplus.
Upstream
The other half is not Cambodia's to decide. Eleven dams now sit on the Lancang, the Mekong's upper course through China, and two more have been built on the mainstream in Laos — Xayaburi, in commercial operation since October 2019, and Don Sahong. Hydropower does not necessarily reduce the volume of water in a river over a year. What it does is hold water back in the wet season and release it in the dry one, because that is when electricity is worth most, and the result is a flatter hydrograph: less flood, more baseflow, and a smaller difference between the two. For a system whose entire productivity depends on the size and timing of an annual pulse, that is precisely the wrong shape. Industrial sand mining from the bed, which has deepened channels through the delta and up towards Phnom Penh, works the same way from below.
Consultation, not consent
The 1995 Mekong Agreement created the Mekong River Commission and a procedure of prior consultation for mainstream projects. Consultation is the operative word: the process obliges a member state to notify and discuss, and it does not give the others a veto. Laos ran the procedure for Xayaburi, the objections were entered, and the dam was built. China is not a member of the commission at all but a dialogue partner, and until 2020 it released outflow figures from its Lancang dams only for the wet season; the dry-season numbers, which are the ones that would show water being held back, have been on the table for barely five years. The institution that exists to manage the river was designed to produce agreement between four governments with different interests, and it does what it was designed to do, which is not the same as protecting a fishery.
What a visitor actually sees
Very little of this, honestly, and that is worth saying. The floating villages on the lake still float; the stilt houses at Kampong Phluk still stand on their six-metre legs; the boat from Siem Reap to Battambang still runs when there is water under it, which is roughly August to January and no longer than that. What has changed is not visible from a boat over the course of an afternoon. It shows up in a catch series, in the size of the fish in a market at Kampong Chhnang, and in the fact that a village that used to be a kilometre from the shore in March is now further. If you go out on the lake, go from Kampong Phluk or Kampong Khleang rather than Chong Kneas, ask what the water did last year, and take seriously that the answer will be about weeks and metres. Those are the units this place is measured in.