Guppies, an invasive fish used in India for mosquito control.
Rchampagne via Wikimedia Commons
Across India’s rivers, lakes, reservoirs, and wetlands, a silent ecological shift is underway. Fish species introduced for aquaculture, ornamental trade, mosquito control, and commercial fisheries are spreading rapidly through freshwater ecosystems, often at the cost of native biodiversity.
Many of these introduced species are highly adaptable. They tolerate pollution, survive extreme temperatures, breed aggressively, and outcompete indigenous fish for food and habitat. As their populations expand, they are altering aquatic ecosystems, reducing native fish populations, and affecting the communities that depend on freshwater fisheries for livelihoods and food security.
India is home to nearly 10% of the global fish diversity, with around 2,500 native species inhabiting marine, brackish, and freshwater ecosystems. Yet studies show that at least 291 exotic fish species have already entered Indian freshwater systems, with several now categorised as invasive alien species (IAS). Their spread raises urgent questions about water governance, biodiversity conservation, and the future of freshwater ecosystems in a warming climate.
Invasive Species (IAS) are plants or animals introduced into environments where they do not naturally occur. Once established, some species spread rapidly because they lack natural predators and adapt easily to changing environmental conditions. In freshwater ecosystems, invasive fish often survive better than native species in polluted or disturbed waters. Many reproduce quickly, consume a wide range of food, and tolerate fluctuations in temperature, salinity, and dissolved oxygen. These traits allow them to dominate rivers, reservoirs, lakes, and wetlands. The consequences extend beyond biodiversity loss. Invasive fish can degrade aquatic habitats, alter food chains, reduce water quality, and threaten fisheries that sustain local communities.
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.
A recent review by Anuragini Kadwalia published in the International Journal of Fisheries and Aquatic Studies informs that the National Biodiversity Authority in India has declared 14 fish species as invasive alien species. These include:
Parana sailfin catfish
Sucker-mouth armored catfish
Nile tilapia
Black carp
Guppy
Western mosquito fish
African catfish
Bighead carp
Common carp
Eastern mosquito fish
Mozambique tilapia
Vermiculated sailfin catfish
Amazon sailfin catfish
Red-bellied piranha
Among these, Mozambique tilapia, Nile tilapia, Common carp, Sailfin catfish, Bighead carp are considered to be more aggressive, rapidly replacing native fish species.
It is widely recognised as a food, sports and ornamental fish and was introduced into Indian waters in 1957 for aquaculture. The fish searches for food at the bottom of the freshwater body and often uproots sediment that leads to habitat disturbance due to uprooting of aquatic plants that serve as food for native fish, causing a decline in water quality and habitat degradation. Due to similar feeding habits, the fish competes for food resources with indigenous species such as Mrigal carp, White carp and Reba carp.
The Common carp
It is being found in large numbers in River Girna in Maharashtra and Krishna Raja Sagar reservoir in Karnataka and has also been found to have invaded Dal Lake in Kashmir and Gobind Sagar reservoir in Himachal Pradesh, outcompeting local snow trout populations and leading to a significant decline in their numbers. Common carps are now found in high numbers in the River Ganga and its associated tributaries, adversely affecting the Indian major carps.
Nile tilapia is a tropical fish that thrives in shallow freshwater habitats. It feeds on periphyton that includes a slimy layer of algae, microbes, and organic matter attached to submerged surfaces, along with aquatic plants, small invertebrates, and detritus found at the bottom of water bodies.
The species reproduces rapidly. Female Nile tilapia breed every three months and protect their eggs and young by carrying them in their mouths. Their ability to tolerate wide variations in salinity, dissolved oxygen, and temperature allows them to survive in diverse environments where many native fish struggle. Combined with aggressive breeding and strong parental care, these traits have enabled the fish to spread quickly across Indian rivers and reservoirs, often competing directly with indigenous species for food and habitat.
The Nile Tilapia.
Today, Nile tilapia are found in large numbers in the Godavari, Krishna, Cauvery, and Yamuna rivers. Their rapid expansion in the Yamuna highlights the scale of the problem. In 2005, the species was present only in negligible numbers, but within just two years, its population increased by nearly 3.5 percent, indicating how quickly the fish can establish itself in a new ecosystem.
In the lower stretch of the Yamuna at Prayagraj, Nile tilapia now contributes nearly 23 percent of the total fish catch, while the catch of Indian major carps has sharply declined to between 2 and 6 percent. The species is also increasing across stretches of the Ganga and other connected freshwater bodies in the basin, raising concerns over the long-term survival of native fish populations.
Mozambique tilapia is a mouth-breeding fish that can survive across a wide range of temperatures and salinity levels. Introduced into India through aquaculture in 1952, the species spread rapidly through ponds, lakes, and reservoirs across the country and is now listed among the world’s 100 worst invasive species.
Mozambique tilapia
The fish has affected fisheries in several major water bodies, including the Vaigai, Krishnagiri, Amaravati, Bhavanisagar, Tirumoorthy, Uppar, and Pambar reservoirs in Tamil Nadu; the Walayar, Malampuzha, Pothundy, Meenkara, Chulliar, and Peechi reservoirs in Kerala; the Kabini reservoir in Karnataka; and Jaisamand Lake in Rajasthan. Concern over its rapid spread led the Fisheries Research Committee of India to ban its propagation in 1959.
Before Mozambique tilapia appeared in the Lake Jaisamand in Rajasthan during 1990–91, major and minor carps accounted for nearly 91 percent of the total fish catch. Within two decades, their share fell sharply to just 14 percent, while Mozambique tilapia made up almost 80 percent of the catch. The species was also found in large numbers in the lower stretch of the Yamuna, where it once contributed nearly 25 percent of the total fish catch. However, recent reports suggest that Nile tilapia has now become the dominant invasive fish in the river. Researchers have additionally raised concerns about possible hybridisation between Nile tilapia and Mozambique tilapia in Indian waters.
Sailfin catfish is commonly sold as an aquarium fish, but many are released into ponds, lakes, and rivers once they outgrow home tanks and become difficult to manage. Released by traders and aquarium hobbyists, the species has gradually entered natural freshwater ecosystems across India.
The fish has now spread to Kerala, Tamil Nadu, Maharashtra, Bihar, Andhra Pradesh, Uttar Pradesh, West Bengal, and several northeastern states. Scientists warn that it threatens native fish populations by feeding on fish eggs, damaging breeding grounds, and competing with local species for food and habitat.
Sailfin catfish
Although its current distribution is still more limited than some other invasive fish species in India, researchers fear the species could spread rapidly in the future. Its ability to survive changing environmental conditions and adapt to rising temperatures may allow it to expand into new water bodies as climate conditions shift.
Bighead carp is a fast-growing invasive fish that feeds mainly on zooplankton and other plankton found in freshwater ecosystems. The species can survive extreme environmental conditions and reproduces rapidly, allowing it to spread quickly once introduced into rivers and reservoirs.
The fish is believed to have entered India unofficially, possibly from Bangladesh, for aquaculture purposes. Today, it is found in large numbers in rivers such as the Yamuna, Chambal, Sharada, Dewa, Gomti, and Wainganga.
Bighead carp, one of the invasive species in Indian waters.
Scientists warn that the species competes directly with native South Asian carp and Rohu because all these fish depend heavily on zooplankton as a food source. This competition can reduce food availability for indigenous fish and affect their growth and survival. To protect local fish diversity, states such as Assam and Chhattisgarh have banned the seed production and farming of bighead carp.
Scientists warn that climate change may accelerate the spread of invasive fish across India’s freshwater ecosystems. Rising temperatures, shifting rainfall patterns, declining water quality, and increasing ecological stress can weaken native fish populations and disrupt their breeding cycles. In contrast, many invasive species are more tolerant of environmental changes and can survive in harsh conditions, allowing them to spread rapidly and outcompete indigenous fish.
Extreme weather events and flooding also increase the risk of invasive fish escaping from aquaculture ponds into rivers, lakes, and wetlands. India, ranked among the countries most affected by extreme weather events, is likely to face growing ecological pressures on its freshwater systems in the coming decades.
Large infrastructure projects are adding to these concerns. There is a fear that proposed river linking projects could connect previously isolated river systems, allowing invasive fish to spread more easily into new habitats and threaten endemic species. Such unrestricted movement of invasive fish could further damage aquatic biodiversity and alter fragile freshwater ecosystems.
Freshwater ecosystems support fisheries, agriculture, groundwater recharge, biodiversity, and food security. Rivers, lakes, and wetlands also provide livelihoods, nutrition, and cultural identity to millions of people across India.
The decline of native fish is therefore not only an ecological issue but also a social and economic concern. As invasive species continue to reshape freshwater ecosystems, traditional fisheries and communities dependent on local water bodies may face increasing uncertainty and livelihood losses.
Prevention and early detection might be the most effective ways to control invasive fish species before they become established in rivers, lakes, reservoirs, and wetlands.
Stricter regulations are needed to prevent the introduction of exotic fish for aquaculture, commercial fisheries, ornamental trade, and sport fishing. Scientists also stress the importance of conducting detailed ecological assessments before introducing any non-native species into Indian water bodies.
Public awareness is equally important. Researchers warn that many invasive fish spread after aquarium owners or traders release them into natural water bodies once the fish become difficult to manage in tanks.
Where invasive species have already spread, a combination of management measures are recommended. These include physical removal of invasive fish, increasing fishing pressure on target species through larger gill net mesh sizes, habitat restoration, and the use of barriers to restrict movement. Other methods such as bubble curtains, electric fields, light deterrents, biocides, and carbon dioxide barriers are also being explored to reduce their spread.
Scientists further emphasise the need for regular restocking of native fish species to strengthen local populations and maintain ecological balance in rivers and reservoirs.
Long-term monitoring of freshwater biodiversity is also essential. Techniques such as electrofishing, net surveys, and environmental DNA or eDNA metabarcoding are increasingly being used for early detection and tracking of invasive fish populations.
India’s freshwater ecosystems are already under severe pressure from pollution, dam construction, groundwater depletion, sand mining, and climate change. The spread of invasive fish adds another layer of stress to rivers, lakes, reservoirs, and wetlands that are already struggling to survive.
The crisis is not only about fish. It also threatens freshwater systems that support biodiversity, fisheries, food security, livelihoods, and cultural traditions across the country.
Protecting native aquatic ecosystems will require stronger regulation, scientific monitoring, and long-term water governance that recognises rivers and wetlands as living ecological systems rather than simply sources of extraction.
The growing spread of invasive fish is a warning that once ecological balance is lost, restoring freshwater ecosystems becomes far more difficult than protecting them in the first place.