A river out of balance: How the invasive fish, Nile Tilapia, is transforming the Godavari river ecosystem

The story of the Godavari has long been one of abundance. Flowing across six states and sustaining more than 70 million people, India’s second largest and most complex river system has supported livelihoods, nourished fisheries, and sustained rich freshwater ecosystems for centuries. Yet beneath its waters, a quieter transformation is underway. An invasive fish introduced to enhance food production and aquaculture incomes is rapidly altering the river’s ecological balance.

The rise of Nile tilapia in the Godavari is more than a fisheries concern. It highlights how human interventions, changing habitats, and economic priorities can reshape ecosystems in unforeseen ways. As native fish become increasingly scarce in both nets and markets, an uncomfortable question emerges: when a species introduced in the name of development comes to dominate an ecosystem, who bears the cost: biodiversity, local communities, or both?

Known as the Dakshin Ganga or Ganges of the South, the Godavari originates in Maharashtra’s Western Ghats and travels over 1,450 kilometres before meeting the Bay of Bengal. Along its course, it supports a remarkable range of habitats, from upland forests and freshwater ecosystems to the country's second largest mangrove forest at its delta. The basin is a  critical biodiversity hotspot. It is home to endemic fish species, migratory birds, and numerous aquatic organisms, even as it faces mounting pressures from pollution, infrastructure development, and habitat degradation.

Scientists have recorded 152 freshwater fish species in the basin, including 44 species found nowhere else and 20 species classified as threatened or near threatened. Freshwater fish that are commonly found in the river waters sustain a rich assemblage of carps, catfish, snakeheads, featherbacks, gobies, eels, and pearl spots. Key species include catla, rohu, mrigal, and wallago attu. Among the most celebrated species is the prized monsoon delicacy pulasa or hilsa, renowned for its distinctive flavour. However, several native species are already under stress. The white catfish is now listed as endangered, while the mrigal carp is classified as vulnerable, underscoring the growing fragility of the river’s aquatic biodiversity.

A recent study titled ‘Exotic fish and aquatic ecosystem disservices in River Godavari’ published in Current Science however reveals that Tilapia, an exotic and highly invasive fish, has rapidly proliferated and outnumbered the native fish found in the river, threatening biodiversity, the survival of other fish species in the river and the livelihoods of several fisherfolk who depend on the river for their fish catch.

What are invasive species?

Invasive species proliferate very fast, have the capacity to reproduce even in adverse environmental conditions such as extreme heat and pollution, and are able to survive on a wide range of food items. They often get introduced accidentally or intentionally, and human migration and demand for food often lead to the introduction of commercially important exotic agriculture and aquaculture species into the environment. Ornamental trade, unscientific aquaculture practices, sport fishing, mosquito control, natural disasters and ballast water can also lead to the introduction of invasive alien species into water bodies. Some of these species can accidentally escape into the wild and then evolve as invasives.

The exotic and  invasive Nile Tilapia fish

The exotic Nile tilapia was introduced to India in the late 1970s due to its capacity to grow and breed fast, and government approval was granted for its culture in 2009. Several fish farmers thus started breeding Tilapia fish in their ponds to ensure food security and generate economic opportunities. However, biologists soon started raising concerns about its competitiveness with native fish as it escaped into water bodies such as rivers. Following this, the Government of India issued guidelines for responsible tilapia farming through the National Fisheries Development Board.

Tilapia are swarming the Godavari, displacing native fish and threatening livelihoods of fisherfolk

However, tilapia fish are now being found in large numbers in the upper stretch of the Godavari, including the reservoirs, and this has led to a rapid displacement of indigenous fish species from the river, posing a risk to the river ecosystem. Tilapia contributed between 20% and 60% of the total catch in the upstream section of the Godavari during pre-monsoon 2025 and post-monsoon 2025 surveys. 

The spread and establishment of exotic fish has also to do with habitat disturbances in ecosystems caused due to natural and anthropogenic factors. The main stretch of the Godavari has approximately 67 dams/barrages, with an average of one every ≈ 22 km. The upper stretch, extending from the Brahmagiri hills (Nashik) to the Manjira confluence (Basar) (710 km), is dotted with 34 barrages and dams. This has altered the riverine flow and habitat quality, resulting in shifts in fish community structure. The altered habitats are also known to favour the establishment of exotic fish. Surveys in the Godavari show that tilapia of different stages of growth and varied sizes reside in its waters, confirming that the populations are well established, with active breeding and successful propagation occurring in such altered habitats.

The spread of tilapia and altered river flows have increased pressure on local indigenous species of fish in the area, and fishermen in the region report a marked decline in species such as freshwater eel; Jerdon's catfish, or koori; and white catfish in the upstream stretches, which may be correlated with the dominance of tilapia. Tilapia are also known to prey on local indigenous fish species. For example, studies show that the native ‘common spiny loach', commonly found in the river, is unable to recognise the chemical cues released by invasive tilapia in the water, which are normally essential for fish to detect predators, and is thus becoming vulnerable to predation and competition.

Fisherfolk are at loss as indigenous species fetch more incomes as compared to tilapia, but are disappearing fast

Fishermen in the region are facing the problem of loss in incomes as the qualitative value of exotic fish such as tilapia is considered to be much lower as compared to that of native fish. For example, carps fetch 120–150 INR/kg, murrels 300–400 INR/kg, and catfishes 125–350 INR/kg, whereas tilapia is sold at only 60–100 INR/kg. Many full-time fishers are thus shifting to part-time fishing due to lower income from fishing in this river system.

Many of the fisherfolk also resort to overfishing to make ends meet, and this also disturbs the ecological balance of the river ecosystem. They often resort to using small mesh-sized fishing gear, which removes the young native fish species but leaves out the young tilapia who live in shallow littoral areas or at the river bottom or in the mouths of the female tilapia fish. This ensures high survivability of tilapia and a rapid rise in their numbers when conditions are favourable. 

However, fisherfolk also do not want tilapia out of the water, as this will affect their livelihoods until another viable alternative fishery resource becomes available. The local/regional markets along the main stretch are flooded with tilapia. This restricts the customer choices and results in reduced food basket diversity. This has direct implications for nutritional security, as many fish are locally consumed/preferred for their dietary, medicinal and cultural properties. 

The article argues that there is an urgent need to explore the causes and dynamics of the rapid increase in Nile tilapia fish in the Godavari river, particularly in the context of the changing environment. The Nile tilapia’s unchecked spread in the upper Godavari is eroding native fish diversity and destabilising livelihoods. Urgent management—anchored in international conservation guidelines—must combine community-led removal, collaborative research, and adaptive policy to safeguard aquatic ecosystem services and protect native germplasm for future generations.