Chilika Lake's Hidden Biodiversity: The Aquatic Species Living in India's Largest Coastal Lagoon

Chilika is a brackish water lagoon, not a freshwater lake, and that distinction is the biological engine behind everything that lives in it. Spread across roughly 1,100 square kilometres along the Odisha coast, it sits at the meeting point of the Mahanadi river system and the Bay of Bengal. Salinity levels inside Chilika shift with the seasons: fresher during the monsoon as river inflow dominates, saltier in summer as the sea pushes back through the outer channel. That fluctuating salinity gradient creates multiple micro-habitats within a single water body, which is why the species count here is so disproportionately high for its size. The Chilika Development Authority, which has monitored the lagoon's ecology since 1992, records over 800 species of fauna across its three ecological zones: the outer channel, the central sector, and the northern sector near the Mahanadi delta.

The lagoon supports around 225 species of fish. Commercially significant ones include the Indian white prawn (Penaeus indicus), the mud crab (Scylla serrata), and mullet species that move between fresh and salt water as salinity changes. The Zoological Survey of India has recorded the short-snouted seahorse (Hippocampus hypselurus) in Chilika's seagrass beds, a species listed as vulnerable on the IUCN Red List. Seahorses are biologically unusual: the male carries the fertilised eggs in a brood pouch and gives birth to live young, a trait found in no other vertebrate group. Chilika also shelters the horseshoe crab (Tachypleus gigas), an arthropod whose lineage predates the dinosaurs by over 200 million years. Its blue, copper-based blood is used in biomedical testing to detect bacterial contamination in injectable drugs, a fact that gives this ancient creature a direct role in modern medicine.

Between October and March, Chilika receives approximately one million migratory birds from Central Asia, Siberia, and the Himalayan foothills. The species list runs to 160 migratory varieties, including the bar-headed goose (Anser indicus), which crosses the Himalayas at altitudes above 7,000 metres, the highest recorded migration route of any bird. Flamingos arrive in smaller numbers, congregating in the shallow northern sector where algal growth is dense. The lagoon also hosts the endangered Irrawaddy dolphin (Orcaella brevirostris), a population of roughly 150 individuals, which is the primary reason most visitors come at all. What the dolphin tourism brochures leave out: the Irrawaddy dolphin's presence depends directly on fish stocks, which depend on seagrass health, which depends on water salinity staying within a workable range. The dolphin is the visible tip of a food web that runs several layers deep.

Chilika's seagrass meadows cover significant portions of the lagoon floor, particularly in the outer channel and central sector. The dominant species include Halophila ovalis and Halodule uninervis. Seagrasses are flowering plants, not algae, they produce oxygen, trap sediment, and provide feeding and nursery habitat for fish, crustaceans, and the seahorses mentioned above. When the outer channel silted up in the 1990s and seawater exchange dropped, seagrass cover declined sharply and fish populations followed. The channel was mechanically reopened in 2000, salinity recovered, and seagrass beds expanded again. The Chilika Development Authority documented a 37 percent increase in fish catch within three years of the channel reopening, a direct measurement of how aquatic vegetation underpins the entire wetland economy. Phytoplankton in the water column adds another layer: these microscopic organisms form the base of the aquatic food chain and their seasonal blooms regulate oxygen levels throughout the lagoon.

Chilika was designated a Ramsar Wetland of International Importance in 1981, the first site in India to receive that status. The designation brought monitoring, but the pressures are structural. Aquaculture enclosures have historically blocked water circulation in parts of the northern sector. Agricultural runoff from the Mahanadi catchment carries fertilisers that cause algal blooms, which deplete oxygen and suffocate fish. Urban expansion along the Odisha coast adds sedimentation. Each of these stressors acts on a different part of the lagoon's biology, but they converge on the same outcome: reduced species diversity and declining fish catch for the fishing communities, roughly 200,000 people, who depend on the lagoon for their livelihood. The lagoon's ecology and the local economy are not separate problems. Every species that disappears from the water column removes something from the net.

The dolphin draws the crowds, but it is the seagrass, the phytoplankton, the horseshoe crab, and the fluctuating salinity that make the dolphin possible. Chilika's biodiversity is not a collection of interesting animals living near each other, it is a set of interlocking biological dependencies, each one calibrated to a water body that is neither river nor sea but something the continent has spent millennia building at the exact point where both meet.