The River Is Still There. The Fish Are Not.

There is a photograph that circulates occasionally among Thai fishing communities — grainy, analogue-era, shot somewhere on the Mekong near Chiang Khong in the early 1970s. It shows four men hauling a giant Mekong catfish (Pangasianodon gigas) onto a wooden boat. The fish is longer than two of the men standing end to end. By the time the image was taken, the species was already in trouble. By the time most people reading this were born, it was functionally gone from most of the river it once owned.

That photograph is a useful starting point. Not because it represents some pristine golden age — commercial fishing pressure was already intense by the 1970s — but because it captures the scale of what was once available and what has since been relinquished so incrementally that the loss barely registers in real time.

Thailand's wild fisheries are in serious trouble. That sentence requires no hedging.

What the Rivers Were

The Chao Phraya basin and its tributaries once constituted one of the most biodiverse freshwater systems in Southeast Asia. The Mekong, running along and through the north and northeast of the country, was even richer — a river so productive that its fishery has been compared, in terms of annual protein yield per kilometre of river, to the Amazon. Giant Siamese carp (Catlocarpio siamensis) — the world's largest cyprinid — moved through the main channels in schools. Pangasius sanitwongsei, the Chao Phraya giant catfish, reached documented weights exceeding 150 kilograms. Mahseer (Tor spp.) occupied the fast-flowing tributaries and headwaters of the north.

The giant freshwater stingray (Urogymnus polylepis), which can exceed 600 kilograms, inhabited the deep holes of the Mekong and Mae Klong systems. It still does, occasionally. That "occasionally" is the operative word.

These were not fringe species. They were the apex expressions of an ecosystem that had been evolving its productive capacity for millions of years, shaped by monsoon hydrology, Himalayan-origin sediment loads, and an extraordinary diversity of aquatic habitats — from the torpid lower Chao Phraya delta to the clear, oxygenated rivers descending from the Doi Inthanon massif.

What Happened

The degradation did not arrive in a single catastrophe. It came in layers, each compounding the last.

Damming is the most structurally significant factor. The cascade of hydroelectric dams on the upper Mekong — primarily built by China, but with Thai and Lao structures adding to the blockage — has fundamentally altered the river's hydrology. The seasonal flood pulse that once inundated vast floodplain nursery areas, triggering spawning runs and juvenile fish dispersal, has been muted. Sediment that once reached the lower river — and the nutrient web it supported — is now trapped behind concrete. The fish-passage infrastructure that exists is largely inadequate for the species that most need it: giant catfish do not negotiate fish ladders designed for salmon.

Closer to Bangkok, the Chao Phraya has been squeezed by urbanisation on both banks for most of its lower course. Water quality in the central river improved marginally after treatment plant investments in the 1990s and 2000s, but agricultural runoff from the central plains — fertiliser nitrogen, pesticide load — continues to drive periodic hypoxic events. The river does not smell as bad as it once did. The fish have not recovered proportionally.

Commercial netting operated with effectively no meaningful enforcement for decades across most of the Thai river system. While Thailand has nominal closed seasons and mesh-size regulations, the rural fishing communities along the Mekong, the Wang, and the Chi have historically operated outside formal oversight. The pressure was not malicious — it was subsistence and small-scale commercial fishing by communities with genuine protein needs. But aggregate, year-round, and applied across spawning aggregations, it was devastating.

Agricultural runoff from Thailand's intensively farmed central plains deserves its own chapter. The rice paddies, sugarcane fields, and cassava plantations that cover much of the Chao Phraya watershed are treated with volumes of chemical inputs that eventually find their way into drainage channels and ultimately the main river. Eutrophication — the process by which excess nutrients cause explosive algae growth, which then crashes and strips dissolved oxygen from the water — has made large sections of the lower Chao Phraya intermittently uninhabitable for large-bodied fish during certain seasons.

The cumulative effect of these pressures is not evenly distributed. Some river sections remain relatively productive. The Kwai Noi river system retains reasonable populations of mahseer and snakehead. Parts of the upper Ping still hold respectable Tor-family fish in the faster rocky reaches. The Mekong near Chiang Khong sees occasional giant stingray encounters that generate the kind of social-media documentation that speaks to the residual wildness of the system.

But the average is dramatically reduced. A fisherman working the Chao Phraya near Nakhon Sawan today encounters a fraction of the biomass his grandfather would have found in the same location.

The Numbers, Such As They Are

Precise data is difficult to come by. Thailand's inland fisheries monitoring has historically been under-resourced, and catch statistics from subsistence and informal commercial fishing are notoriously unreliable. What exists paints a grim picture.

The FAO and regional fisheries bodies estimate that Mekong basin fish biomass has declined by 40 to 70 percent across various species groups since the 1970s, with the most catastrophic declines among large-bodied migratory species — exactly the fish that once defined the river's character. The giant Mekong catfish is listed as Critically Endangered on the IUCN Red List. The Mekong giant catfish has not been reliably documented in a Thai wild-catch context this century.

The giant Siamese carp, somewhat more adaptable, has fared marginally better, but its wild populations are a shadow of historical abundance. A species that once made up meaningful proportions of commercial catch in the upper Chao Phraya basin is now encountered occasionally, primarily in the lower Mekong tributaries of Laos and Cambodia.

The Accidental Preservation Mechanism

Here is where the story becomes genuinely complicated, and where the pay-lake industry enters the picture in a way that resists simple moral framing.

Thailand's pay-lake sector — the network of commercial fishing venues ranging from municipal ponds on Bangkok's outskirts to the enormous purpose-built fisheries like Bungsamran Lake and IT Lake Monsters — did not arise out of conservation consciousness. It arose out of economic opportunity. Urban Thais wanted to fish. Wild rivers were increasingly inaccessible or unproductive. Entrepreneurs built ponds, stocked them, and charged by the hour or the day.

But those operators needed fish. Large fish, specifically, because large fish are what draw paying customers. And large native species — giant Siamese carp, Chao Phraya catfish, freshwater stingray — are exactly what operators wanted in their lakes.

The breeding and grow-out operations that supply these fish have, inadvertently, created genetic reservoirs for species that are functionally absent from most of their native range. Bungsamran maintains populations of giant Siamese carp that dwarf what any angler is likely to encounter in the wild. The stock management required to keep these fish healthy and growing has generated institutional knowledge about their biology, breeding triggers, and habitat requirements that the formal scientific community simply does not have.

This is not a conservation solution. The pay-lake carp is not the wild Mekong carp, in terms of its behavioural ecology, its genetic diversity, or its ecological role. Releasing pay-lake fish into wild rivers — something that happens periodically through Buddhist merit-release ceremonies — may do more harm than good, introducing domestication-selected traits and potential pathogens into residual wild populations.

But it is something. At a moment when the alternative for some of these species is functional extinction from Thai waters, the pay-lake ecosystem represents a form of persistence, however imperfect.

What Remains

The picture is not uniformly bleak. There are reaches of river — particularly in the north and along the Myanmar border — where the combination of lower human population density, more difficult access, and residual forest cover has preserved something closer to functional wild fisheries.

The Mae Kok river in Chiang Rai province remains a working wild system. The Salween tributaries in Mae Hong Son province, where the river drains steep forested watershed before crossing into Myanmar, hold mahseer populations that receive almost no angling pressure. The upper Ping retains a character that distinguishes it from the degraded lower river.

For the wild-water angler willing to travel beyond the lakeside infrastructure of Bangkok and Chiang Mai's tourist circuit, Thailand still offers genuine river fishing. Snakehead, striped snakehead, barb species, catfish, and occasional mahseer are all catchable. The experience is different from a pay-lake — uncertain, physically demanding, dependent on local knowledge, offering no guarantees — but it is real fishing in real water, and it is available.

The mahseer question deserves particular attention. Thailand's mahseer — principally Tor tambroides and related species — occupy a specific ecological niche in fast, clear, rocky tributaries that have been somewhat insulated from the pressures most damaging to lowland species. Logging is the primary threat to their habitat, and in national park-protected headwaters, that pressure is moderated. Mahseer fishing in Thailand is not the product of a thriving wild population, but it is not the ecological illusion that lowland river fishing sometimes is.

An Honest Accounting

The honest accounting looks like this. Thailand's wild fisheries have been severely degraded by a combination of structural factors — damming, over-extraction, habitat loss, pollution — that were not unique to Thailand and were not managed better here than elsewhere in the developing world at comparable stages of economic growth. The people and institutions making decisions in the 1960s, 1970s, and 1980s were not uniquely negligent; they were operating under the assumptions of their era, which did not include meaningful concern for freshwater megafauna.

The consequences are now embedded in the ecology of the river systems. They are not easily reversible. Dam removal on the Mekong, which would be the single most effective intervention for migratory fish species, is not on any realistic political agenda. Agricultural practice change across the central plains watershed is generational work.

What can be said is that awareness is higher than it was. The DOF (Department of Fisheries) has expanded its protected species list and, in certain areas, increased enforcement. Academic partnerships with universities and international research bodies have improved the quality of freshwater monitoring data. Younger Thai conservationists are working the problem with more sophistication than previous generations.

And the fish that were saved by accident — in the ponds and lakes of the pay-lake industry, in the temple ponds where communities have historically protected fish on religious grounds, in the national park rivers where enforcement has kept nets out — represent a residual from which recovery, however partial, remains theoretically possible.

The river is still there. The question of what lives in it is more complicated than it looks from the bank.