Commercial pay-lake fishing in Thailand involves concentrating large fish at densities that would never occur in natural waterways. Bungsamran Lake, which holds arapaima exceeding 100 kilograms alongside stocks of giant Siamese carp, Mekong catfish, and various introduced species, is a masterpiece of controlled aquaculture management. It is also, like any high-density aquaculture system, an environment where fish disease can spread rapidly when conditions deteriorate.
Disease in Thai pay-lakes is a reality that operators prefer not to discuss publicly but that anglers — particularly those visiting frequently — will eventually encounter. Understanding what the major pathogens are, how venues manage them, and what anglers can do to avoid contributing to transmission is practical knowledge for anyone fishing Thailand's commercial lake circuit.
The Main Pathogens
Aeromonas: The Opportunist
Aeromonas hydrophila is the most common bacterial pathogen in Thai freshwater aquaculture and is present at low levels in virtually every freshwater environment in the country. It is an opportunistic pathogen: healthy fish with intact immune function coexist with Aeromonas without showing disease. Outbreaks occur when a stressor — thermal shock, transport, overcrowding, poor water quality, concurrent parasitism — compromises fish immunity and allows bacterial proliferation.
The disease presentation most associated with Aeromonas is motile aeromonad septicaemia (MAS), sometimes called red sore disease. Affected fish display haemorrhagic lesions on the skin, at the base of fins, and in severe cases around the mouth and abdomen. Internally, the liver is often pale and the spleen enlarged. Mortality rates in acute outbreaks can be substantial, particularly in stressed or recently transported fish.
Pay-lakes are most vulnerable to Aeromonas outbreaks during periods of stocking stress — when a new batch of fish has been transported from a hatchery and introduced to unfamiliar water chemistry — and during the hot season (March to May) when water temperatures in shallow lake sections can exceed 32°C and dissolved oxygen drops.
Columnaris: The Cotton-Wool Disease
Flavobacterium columnare is a gram-negative rod bacterium that causes columnaris disease, one of the most visually distinctive fish infections. It presents as grey or white cotton-wool patches on fins, the saddle area behind the dorsal fin, and the mouth — the latter giving rise to the alternative name "mouth fungus," though it is bacterial, not fungal.
The 2024 event at Bungsamran Lake involved columnaris infection in striped catfish (Pangasianodon hypophthalmus), a species known to be particularly susceptible. The outbreak, which occurred during a warm-weather period and was associated with a large batch stocking, prompted temporary closure of affected lake sections and emergency oxygen aeration deployment to maintain dissolved oxygen above critical thresholds.
Columnaris Misidentification
Columnaris is frequently misidentified as fungal infection (saprolegniasis) by lay observers because of its cotton-wool appearance. The distinction matters for treatment: columnaris responds to antibacterial agents while true fungal infections require antifungal treatments. Misdiagnosis leads to ineffective treatment and continued mortality. Pay-lake operators should use microscopic examination or PCR diagnosis before committing to treatment protocols.
F. columnare spreads via water contact and is transmitted most efficiently at water temperatures above 25°C and during periods of physical injury — including minor abrasions from netting, transport, and handling. Pay-lake fish that are regularly caught and released do experience minor stress and occasional scale loss that could in theory create entry points for columnaris infection, though this transmission pathway is considered low risk for well-managed venues with short fight times and good handling practice.
White-Spot Disease (Ich)
Ichthyophthirius multifiliis — ich or white-spot — is a protozoal parasite rather than a bacterium, and the most universally encountered fish pathogen in freshwater systems worldwide. It presents as small white cysts (typically 0.5–1 mm) visible on fins and body, resulting from the parasite's tomont stage beneath the skin epithelium.
In wild fish populations at natural stocking densities, ich outbreaks are self-limiting. In pay-lake environments with high fish density and frequent water contact between individuals, the parasite can cycle rapidly and produce high mortality, particularly in exotic species that lack evolutionary exposure to the organism — arapaima, alligator gar, and pacu have all experienced ich-related mortality events at Thai pay-lakes.
Treatment is well-established: salt baths, formalin, and in severe cases temperature manipulation (raising temperature accelerates the parasite's life cycle and increases effectiveness of treatments timed to the free-swimming theront stage). Most operators managing ich outbreaks can resolve them within two to three weeks using available approved treatments.
Saprolegnia: The Fungal Opportunist
Saprolegnia and related oomycete species cause the true cotton-mould infections that Flavobacterium is often confused with. Saprolegniasis typically follows physical injury — a bite wound, a transport abrasion, or hook injury — and presents as white or grey cotton-like growth spreading from a wound site.
It is common in catfish species after capture and hook removal and is one reason that proper hook removal technique matters in pay-lakes. Crushing or deeply embedded hooks that require excessive tearing to remove create large wound surfaces that are vulnerable to saprolegnia colonisation. Barbless or debarbed hooks, which the best-run pay-lakes in Thailand now require, reduce this risk.
How Venues Manage Disease
The management protocols at Thailand's largest commercial pay-lakes have become significantly more sophisticated over the past decade, driven by high fish asset values and the competitive reputation concerns that come with public disease events.
Water quality monitoring — pH, dissolved oxygen, temperature, ammonia, nitrite — is now conducted daily or continuously at Bungsamran, IT Lake Monsters, and other major venues. Deteriorating water quality metrics trigger prophylactic responses: aeration enhancement, partial water exchange, and temporary suspension of stocking before incoming fish are introduced to already-stressed water.
Quarantine systems separate newly arrived stock for observation periods of five to fourteen days before they are introduced to the main lake. This practice, borrowed from marine aquaculture, reduces the risk of introducing disease from hatchery or transport-stressed fish directly into a lake with a large established fish population. Not all venues maintain adequate quarantine capacity — the capital cost of dedicated quarantine tanks at the volumes required for large-lake stocking is substantial.
The Stocking-Disease Cycle
The highest disease risk period at commercial pay-lakes corresponds to maximum stocking frequency. Operators who stock very large quantities of fish over short periods — to build up fish density for peak revenue periods around Thai national holidays — create the exact conditions (crowding, transport stress, novel water chemistry exposure) that favour opportunistic pathogen outbreaks. Anglers visiting immediately after large stocking events may encounter less healthy-looking fish.
Veterinary consultation is available through the Department of Livestock Development, which licenses aquatic animal veterinarians and maintains diagnostic capacity for fish disease identification. Major venue operators have standing relationships with these veterinarians and can access rapid diagnostic services when outbreak signs appear. Smaller provincial pay-lakes often rely on empirical treatment approaches — adding salt, adjusting aerators, consulting experienced staff — rather than formal veterinary diagnosis.
Biosecurity Etiquette for Anglers
Anglers can inadvertently transfer pathogens between venues. This is particularly relevant for parasites with environmental stages — ich theronts can survive on wet equipment — and for viral pathogens that persist on damp surfaces.
Practical biosecurity for pay-lake anglers is straightforward. Allow equipment — nets, keepnets, hooks and rigs — to dry completely between venues before reuse. Ich theronts survive approximately 15–30 minutes in air at tropical temperatures; complete drying eliminates them. More resistant pathogens may survive longer, making wet storage of nets between venue visits a genuine transmission risk.
Do not use the same landing net, keepnet, or bucket between venues on the same day without disinfection. A 10-minute soak in a dilute chlorine solution (one teaspoon of domestic bleach per litre of water) followed by rinsing removes most bacterial and parasitic threats. Some competitive pay-lake circuits require incoming anglers to declare their previous fishing locations precisely because of this biosecurity concern.
If you observe fish at a venue showing clear disease signs — visible lesions, cotton-wool growths, haemorrhage at fin bases, fish floating or swimming erratically near the surface — inform venue management rather than simply fishing around the problem. Operators who are unaware of an early outbreak may welcome the alert; those managing a known event will appreciate confirmation of its distribution.
The fish at Thailand's pay-lakes are expensive, carefully maintained assets. The disease management challenge they present is a consequence of the aquaculture model that makes trophy freshwater fishing accessible to visiting anglers. Treating that challenge with the same seriousness that a livestock farmer applies to herd health — with biosecurity discipline, respect for management protocols, and communication when something looks wrong — is the minimum that the system requires from its users.
