The humpbacked mahseer, a critically endangered fish found in the Cauvery River basin, is disappearing from much of its historic range as altered river flows, poaching, pollution and introduced species reshape the freshwater system. Its decline is not only a conservation concern. It is also evidence of how decisions made for agriculture, drinking water and infrastructure can change the ecological conditions on which river species depend.
The fish, scientifically known as Tor remadevii, is endemic to the Cauvery basin in the Western Ghats. Historically, it was recorded across the Cauvery and tributaries including the Kabini, Bhavani, Moyar and Pambar, covering parts of Karnataka, Kerala and Tamil Nadu. There is no exact census of the remaining wild population, but experts cited in the report say the species is locally extinct across much of its former range and now survives in a small number of isolated breeding populations, primarily within protected areas such as the Cauvery Wildlife Sanctuary.
That fragmentation is significant because the humpbacked mahseer is adapted to a particular kind of river. It evolved in clean, highly oxygenated and fast-flowing water. Its breeding behaviour is linked to rain and sudden floods, which prompt the fish to swim upstream and spawn. Sandeep Menon, secretary of the Wildlife Association of South India, says the management of Cauvery flows during the dry season can create irregular water events that confuse these patterns.
The issue is not simply that less water is available. It is that the timing, speed and location of water movement are changing. The report describes how flows are managed for agriculture and drinking water, while dams and other interventions have altered the river’s natural behaviour. For a migratory species that responds to hydrological signals, these changes can affect movement, breeding and survival even when water continues to move through the river channel.
A tagged female fish offers a particularly clear illustration of the risk. In 2024, conservationists followed a 24-kilogram female with stable and predictable movement patterns for six months. During a drought year, amid water-related tensions between Karnataka and Tamil Nadu, a large volume of water was released into Tamil Nadu on May 15. The fish migrated from a deeper section of the river. Its signal disappeared, then returned two weeks later from a shallow pool about six kilometres above its summer range. The tag was recovered there, and Naren Sreenivasan, who was involved in tracking the fish, said the evidence indicated that it had likely become stranded and died.
The incident shows how water-release decisions can have consequences beyond immediate human demand. The report does not establish that one release alone caused the fish’s death, but it records the connection between a sudden change in river conditions, the fish’s movement and its subsequent disappearance. For conservationists, such cases make clear why river management cannot be assessed only through the volume of water delivered downstream.
The humpbacked mahseer also occupies an important position in the river’s food web. Naren Sreenivasan describes it as a large-bodied migratory predator. Adult fish can be prey for crocodiles, while juveniles are consumed by other fish and birds. The loss of a predator can produce cascading effects, disturbing relationships among species and changing how the river ecosystem maintains equilibrium. This is why the fish is treated as a flagship species, comparable in conservation symbolism to large land animals such as tigers, elephants and rhinos.
Its decline is being driven by several pressures at once. Overfishing, poaching and dynamiting directly reduce fish numbers. Pollution damages water quality, while dams and altered flows fragment habitat. Competition from introduced species adds another pressure, according to the report. These threats are interconnected: a population already divided into isolated breeding groups has less resilience when illegal fishing, poor water quality or an extreme flow event affects one of its remaining habitats.
The absence of a precise wild-population census makes the problem harder to administer. Authorities and conservationists can identify threatened stretches, record catches and track individual fish, but the supplied report does not provide a complete estimate of how many humpbacked mahseer remain. That gap limits the ability to measure recovery across the entire basin and makes long-term monitoring especially important.
The conservation response has therefore relied on a combination of government action, scientific tracking and local participation. The Wildlife Association of South India has maintained angling records since 1976, creating a historical dataset on catches. Its members use catch-and-release practices, and radio-tagging has helped researchers understand the movement of large mahseer. The organisation also hires guards to prevent poaching and dynamiting along three stretches of the Cauvery that it protects in collaboration with the Fisheries Department.
This arrangement reveals an unusual institutional structure. Anglers, a non-profit organisation, local communities and government departments are contributing to the protection of a species that is listed as Critically Endangered on the IUCN Red List but is not protected under Indian laws, according to the report. The conservation model depends not on one agency alone, but on the combination of data collection, enforcement, community knowledge and access to river stretches.
Local participation is central to that model. Community members are trained to radio-tag and track large mahseer, while guards are hired from communities living around protected stretches. This creates a direct link between conservation and local livelihoods, although the report does not quantify the number of people involved or the economic value of the programme. What it does show is that enforcement is being organised at the riverbank, where poaching and dynamiting actually occur, rather than relying only on distant administrative oversight.
The Karnataka government is also working with the Wildlife Association of South India and the Forest Department to breed the species in captivity at Harangi. Fisheries officials collect mature male and female brood fish, separate them and use hormone injections to induce breeding. Eggs are then fertilised, and the resulting fingerlings are fed a diet rich in fats and proteins until a release site is selected.
Captive breeding can support recovery, but the report also makes clear that releasing fish is only one part of conservation. If river flows remain irregular, habitat stays polluted or illegal fishing continues, released fingerlings will face the same pressures as wild fish. The breeding programme therefore sits within a wider question: whether the river system can provide suitable conditions for a species whose life cycle depends on clean, oxygenated and connected flows.
The fish’s legal and administrative position adds another layer of difficulty. Its international conservation status signals a high level of threat, but the report says it is not protected under Indian laws. In practice, its survival depends on the Fisheries Department, the Forest Department, conservation groups and local enforcement. Each institution has a different responsibility, while the river itself crosses state boundaries and supports competing demands in Karnataka and Tamil Nadu, as well as parts of Kerala.
That geography makes the humpbacked mahseer a test of basin-scale governance. A species does not recognise departmental or state borders, and its movement can connect deep pools, tributaries, breeding grounds and downstream stretches. Decisions made in one part of the basin can affect a fish far away, particularly during droughts or sudden releases. The supplied evidence does not establish a basin-wide management framework for the species, but it demonstrates why isolated interventions may not be enough.
The history of the fish also shows how public attitudes can change. Once prized by anglers, the humpbacked mahseer is now caught only for photographs and released unharmed by conservation-minded fishers. Officials cited in the report say awareness has improved and that electricity, poison and dynamite are less common than before, although illegal practices have not disappeared. Those changes suggest that enforcement and education can reduce direct harm, but they do not resolve the effects of altered flows and habitat fragmentation.
For cities and towns connected to the Cauvery, the larger lesson is that water infrastructure has ecological effects beyond reservoirs, canals and treatment systems. Agriculture and drinking water remain essential demands, but the river also carries biological processes that depend on seasonal variation. Managing water only as a supply quantity can overlook the flow patterns required by fish, wetlands and downstream ecosystems.
The evidence confirms that the humpbacked mahseer is under pressure from a combination of human activities, while conservation efforts are generating better data and some local protection. It does not establish the size of the remaining population or prove that any single intervention will secure the species. The developments that require continued attention are the results of the Harangi breeding programme, enforcement against poaching and dynamiting, the protection of remaining breeding populations, and how future Cauvery flow decisions account for ecological movement as well as human demand.
