Nilgiri Mystus 

The Nilgiri Mystus (Hemibagrus nilgiriensis) is a species of freshwater catfish endemic to the Western Ghats of southern India. It belongs to the family Bagridae, a group commonly known as bagrid catfishes, which are distributed widely across Asia and Africa. The Nilgiri Mystus is a lesser-known yet ecologically significant species that inhabits clear, fast-flowing streams and rivers in the hilly regions of Tamil Nadu and Kerala, particularly around the Nilgiri Hills, from which it derives its name.

Taxonomy and Classification

• Kingdom: Animalia
• Phylum: Chordata
• Class: Actinopterygii (Ray-finned fishes)
• Order: Siluriformes (Catfishes)
• Family: Bagridae
• Genus: Hemibagrus
• Species: Hemibagrus nilgiriensis

The species was first scientifically described by Silas in 1951. It was initially placed under the genus Mystus, hence its common name “Nilgiri Mystus”, but was later reassigned to the genus Hemibagrus based on morphological and taxonomic revisions.

Physical Characteristics

The Nilgiri Mystus is a medium-sized bagrid catfish, typically growing to a length of about 20 to 30 centimetres, though some specimens may attain larger sizes. It displays typical catfish features such as a flattened head, wide mouth, and prominent barbels (whisker-like sensory organs) around the mouth, which aid in detecting food in turbid water.
Distinctive features include:

• A streamlined body adapted to life in fast-flowing rivers.
• Brownish or greyish coloration with a lighter underside, providing camouflage against the rocky riverbed.
• Dorsal and pectoral fin spines used for defence.
• Well-developed caudal fin that enables strong swimming in current.

The species lacks scales, like most catfishes, and is covered with a mucus layer that protects it from infections and assists in locomotion.

Distribution and Habitat

The Nilgiri Mystus is endemic to peninsular India, primarily confined to the Nilgiri Biosphere Reserve and adjacent parts of the Western Ghats. Its known range includes:

• Rivers and streams originating in the Nilgiri Hills (Tamil Nadu and Kerala).
• Tributaries of major river systems such as the Bhavani, Kabini, and Moyar rivers.

It prefers clear, well-oxygenated, fast-flowing hill streams with rocky or gravelly substrates. The water in these habitats is typically cool, a characteristic of upland tropical river ecosystems. The fish shelters under rocks and boulders and is active mainly during dusk and night.

Behaviour and Ecology

The Nilgiri Mystus is a benthic (bottom-dwelling) predator and scavenger, feeding on a variety of aquatic organisms such as:

• Small fishes and invertebrates,
• Crustaceans,
• Insect larvae, and
• Organic detritus.

Its barbels are highly sensitive, allowing it to locate prey in low-visibility environments. Like other Hemibagrus species, it plays an important role in maintaining the ecological balance of freshwater systems by controlling invertebrate populations and recycling nutrients.
Reproduction occurs during the monsoon season, when rising water levels and favourable conditions stimulate breeding activity. The species lays adhesive eggs in sheltered areas, such as under rocks or vegetation, where they are protected from strong currents.

Conservation Status

The International Union for Conservation of Nature (IUCN) currently lists Hemibagrus nilgiriensis as a species of Least Concern, but this status may mask localised threats. Being endemic to a limited region, its populations are highly sensitive to habitat alterations.
Major threats include:

• Habitat degradation due to deforestation, agriculture, and construction.
• Water pollution from pesticides, sewage, and industrial effluents.
• River damming and hydropower projects altering natural flow regimes.
• Overfishing for local consumption.
• Competition from exotic species, such as tilapia and carps introduced in upland waters.

The Nilgiri Mystus is also vulnerable to the fragmentation of river systems, which restricts gene flow between populations and affects long-term survival.

Ecological Significance

As a native hill-stream predator, the Nilgiri Mystus contributes to the health and stability of freshwater ecosystems. It:

• Regulates populations of smaller aquatic organisms.
• Serves as prey for larger fish and birds.
• Acts as an indicator of river ecosystem health, since it thrives only in clean, oxygen-rich environments.

Protecting this species indirectly ensures the conservation of other freshwater fauna in the Western Ghats, one of the world’s recognised biodiversity hotspots.

Research and Identification Challenges

The taxonomy of Hemibagrus nilgiriensis has been historically debated, as several similar-looking species occur in southern India. Early researchers often confused it with Mystus montanus and Hemibagrus menoda, but detailed morphological and meristic studies have confirmed its distinct identity.
Further research using molecular genetics and ecological surveys is needed to clarify its phylogenetic relationships and population structure across different river basins.

Conservation and Management Measures

Efforts to protect the Nilgiri Mystus should focus on habitat conservation and sustainable freshwater resource management. Key measures include:

• Protection of river catchments through afforestation and prevention of soil erosion.
• Control of pollution sources in agricultural and urban areas.
• Regulation of fishing practices, especially during the breeding season.
• Impact assessment of hydropower and dam projects in the Western Ghats.
• Public awareness and local community involvement in conserving native fish diversity.

Government initiatives such as the National Biodiversity Authority (NBA) and the Western Ghats Biodiversity Hotspot Programme support the documentation and protection of endemic species like the Nilgiri Mystus.

Related Species

The genus Hemibagrus comprises several South and Southeast Asian species, many of which inhabit hill streams and river systems. Related Indian species include:

• Hemibagrus menoda – found in the Ganges and Brahmaputra basins.
• Hemibagrus punctatus – occurring in the Mahanadi and Godavari river systems.
• Hemibagrus microphthalmus – distributed in the southern Western Ghats.