Nilgiri Mystus habitat and distribution

The Nilgiri Mystus (Hemibagrus punctatus), also known as the Porthole Bagrid, is a species of freshwater catfish belonging to the family Bagridae. Endemic to southern India, this species forms part of the rich ichthyofaunal diversity of the Western Ghats, a global biodiversity hotspot. It is among the rarest and most threatened freshwater fishes of India, with a highly restricted distribution and declining populations.

Geographical Distribution

The Nilgiri Mystus is endemic to the Western Ghats region of peninsular India, occurring primarily in east-flowing river systems that originate in the Nilgiri and adjoining hill ranges. Its known distribution centres around the Cauvery (Kaveri) River basin, one of southern India’s major river systems.
Within the Cauvery drainage, the species has been recorded from:

• The Bhavani River, a prominent tributary of the Cauvery flowing through Tamil Nadu and Kerala.
• The Kabini and Moyar rivers, which also rise in the Nilgiri ranges and join the main Cauvery system.
• Possibly some stretches of the Pambar and Noyyal rivers, though records from these regions are historical and require further confirmation.

There have been anecdotal and unverified reports of the species from the Bharatapuzha River, a west-flowing system in Kerala, but these are yet to be scientifically validated. Overall, the Nilgiri Mystus is restricted to a few fragmented river stretches, and its historical range has diminished over time due to habitat degradation and environmental pressures.

Habitat Characteristics

The Nilgiri Mystus inhabits clear, fast-flowing freshwater rivers and medium to large streams that run through hilly and submontane regions. It prefers clean, well-oxygenated waters with moderate to strong current and specific environmental features that support its ecological requirements.
Key habitat features include:

• Substrate: Rocky, gravelly, or sandy riverbeds with scattered boulders, submerged logs, or crevices providing shelter.
• Water Flow: Moderate to high current zones interspersed with deeper pools and runs that offer resting areas.
• Water Quality:
  • Temperature: 22°C to 28°C
  • pH: Approximately 6.5 to 7.2 (neutral to slightly acidic)
  • High dissolved oxygen and low turbidity levels.
• Vegetation: Riparian vegetation providing shade and organic matter, helping maintain cooler water temperatures and ecological balance.

The species typically avoids stagnant or heavily silted waters and depends on the dynamic flow of hill streams for feeding and reproduction. Seasonal fluctuations in flow, caused by monsoonal rainfall, are believed to influence its breeding patterns and habitat occupancy.

Ecological Role and Behaviour

Although detailed ecological studies on the Nilgiri Mystus are limited, general information about bagrid catfishes provides insight into its likely ecological role. The species is presumed to be benthic and nocturnal, dwelling near the riverbed and feeding on small aquatic organisms, including insect larvae, crustaceans, and detritus.
Its flattened body shape and strong pectoral fins are adaptations for life in fast-flowing waters, allowing it to resist strong currents and move efficiently along the substrate. The fish may exhibit seasonal movement within the river system, seeking deeper pools during dry seasons and dispersing during high-flow periods.

Present Status and Distribution Trends

The Nilgiri Mystus has undergone a severe decline in population and range over recent decades. Once known from multiple tributaries of the Cauvery system, it is now considered extremely rare, with very few confirmed sightings or collections in recent years.
Key observations include:

• The last confirmed specimen records date from the late 1990s.
• Occasional unverified reports in the 2010s suggest that small, isolated populations may persist in remote or less-disturbed stretches of rivers.
• Field surveys in recent years have often failed to locate the species, indicating that it may now survive only in limited microhabitats.

The restricted and fragmented distribution, coupled with ongoing habitat loss, has led to its classification as Critically Endangered, and some assessments have considered it possibly extinct in the wild.

Environmental Threats

Several anthropogenic and environmental factors threaten the survival of the Nilgiri Mystus:

1. Habitat Degradation: Large-scale deforestation, agricultural expansion, and construction activities in the Western Ghats have led to soil erosion and siltation of rivers, degrading water quality and altering natural flow regimes.
2. River Regulation and Dams: Hydroelectric and irrigation projects across the Cauvery and its tributaries have fragmented habitats, disrupted migratory pathways, and altered the hydrological cycle critical for the species’ breeding and feeding.
3. Water Pollution: Effluents from industries, domestic sewage, and agricultural runoff have reduced oxygen levels and introduced harmful chemicals into freshwater systems.
4. Overfishing: Although not a target species, incidental capture in fishing nets and the use of destructive fishing methods (such as explosives or poisons) contribute to population decline.
5. Climate Change: Rising temperatures, altered rainfall patterns, and seasonal droughts threaten the delicate ecological balance of the river systems supporting the species.

Conservation and Research Needs

Given its precarious conservation status, urgent measures are required to prevent the extinction of the Nilgiri Mystus. Key conservation and management strategies include:

• Habitat Protection: Safeguarding critical river stretches within the Cauvery basin and regulating activities that cause pollution, deforestation, or flow alteration.
• Scientific Surveys: Conducting systematic field studies to locate remaining populations and assess population size, distribution, and genetic diversity.
• Ex-situ Conservation: Establishing captive breeding and rearing programmes to maintain a genetic reservoir for potential reintroduction.
• Policy and Awareness: Integrating fish conservation into river management plans and raising awareness among local communities about sustainable fishing and river stewardship.
• Restoration Projects: Rehabilitating degraded river sections and maintaining natural hydrological regimes to support aquatic biodiversity.

Ecological and Conservation Significance

The Nilgiri Mystus is not only a unique component of India’s freshwater biodiversity but also an important indicator species for the ecological health of the Western Ghats river systems. Its decline mirrors broader environmental issues affecting freshwater habitats across the region. Protecting this species thus contributes to the conservation of an entire ecosystem that supports numerous endemic and economically valuable aquatic organisms.