Fishy invaders: Think before you dump them!

A fish once released in a seemingly harmless act has now become a formidable ecological and economic problem, writes Nobinraja M.

​On Kolkata’s eastern edge, amidst the tightly clustered ponds, I showed a photograph of an exotic fish to a local fisherman. His eyes widened: “I’ve got this bad fish in my pond!” He pointed to the small, prolific fish teeming in his pond.

Promptly, he led us along narrow ridges between the ponds to his plot, angrily scooping fish from the water with a handheld net and flinging them onto the ground. “These fish eat everything. I haven’t been able to cultivate my own fish for the last two years. If this continues, I’ll have to leave this job altogether.”

Unfortunately, the fisherman’s story is common across India. The fish he scooped that day was the Gambusia, also known as the mosquitofish. Once introduced for mosquito control, Gambusia species have ravaged aquatic ecosystems, driving small-scale fishers to the brink. And Kolkata is not alone.

This story underscores a larger ecological crisis that is silently taking place across our neighbourhoods. Invasive alien fishes(IAF) are introduced due to human activity—intentionally or accidentally—outside their natural range, where they establish, spread, and cause ecological or economic harm.

What makes them “invasive” is not just their exotic origin but their ability to outcompete native species, reproduce aggressively, and thrive in disturbed environments. In India, 12 IAF are currently recognised as a major threat, but this list will likely grow without stronger policies and more scientific monitoring in place.

From pets to pests

One of the most troublesome IAF is Pterygoplichthys spp., commonly known as the “suckerfish” or “tank cleaner fish.” These catfish were introduced globally through the ornamental aquarium trade. For many aquarium hobbyists, they are the perfect low-maintenance companion. When these fish outgrow their tanks or when owners tire of maintaining them, they are often released into nearby lakes and rivers.

Even these small, seemingly harmless releases can quickly turn into a full-scale invasion. Once released, these fish thrive in new waters, reproducing rapidly, outcompeting native species, and even destroying the eggs of local fish. In India, Pterygoplichthys spp. have already spread across several states, wreaking havoc. Fishers complain that their nets are torn apart when the species is accidentally caught as bycatch. Attempts to remove this spiny fish cause cuts and injuries, and unlike many natives, they can survive drought by breathing air or burrowing into banks, which also destabilises lake edges.

In short, a fish once released in a seemingly harmless act has now become a formidable ecological and economic menace. This problem is not limited to large fish like suckerfish; even small aquarium fish such as guppies and goldfish, often dumped when owners can no longer care for them, can establish wild populations. Instead of releasing them, hobbyists should return unwanted fish to pet shops, rehome them with other aquarists, or seek guidance from local aquariums and fisheries departments.

Economic and ecological burden

Globally, the scale of economic loss is staggering. The Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) estimates that invasive alien species cost the world over $423 billion annually, a figure that has quadrupled every decade since 1970. Asia bears a heavy share of this burden, with agriculture and aquaculture losses running into billions. India is no exception, having lost around ₹127.3 billion over the past 60 years.

The ecological consequences are even more sobering. Native species are especially vulnerable because freshwater systems are already under pressure from pollution, dams, and over-extraction. An alien fish that breeds prolifically or alters food webs can tip the balance, pushing endemic species into decline. The result is not just the loss of biodiversity, but also the erosion of cultural and economic practices that depend on these waters. Yet, with sparse national-level data, researchers, policymakers, and stakeholders are left grappling with the crisis without a clear picture of its true scale.

Legalities and loopholes

India’s legal framework does acknowledge the threat. For example, the Environment (Protection) Act (1986) prohibits the introduction of alien species that may threaten native biodiversity. The Biological Diversity Act (2002) also restricts the import and release of non-native species without approval from the National Biodiversity Authority. Yet, in practice, loopholes abound. Enforcement is patchy, ornamental fish imports continue with minimal oversight, and an active illegal trade ensures a steady inflow of potentially invasive species.

The gap between legislation and ground reality is precisely where invasions thrive. Prevention is key, and monitoring new arrivals is essential to stay ahead of potential invaders. Which species might become the next major invader in India? That is a question science cannot yet answer with certainty. The risk depends on ecological compatibility, life history traits, and pathways of introduction. What is clear, however, is that constant surveillance is essential.

Advanced tools such as environmental DNA (eDNA) are now enabling such monitoring globally and have recently been applied in India. Alongside this, quarantine measures, routine monitoring of the aquarium trade, and community-level awareness campaigns are the first lines of defence. In the short term, India must focus on managing the 12 invasive alien fishes already recognised in its waters. In the longer term, attention must turn to preventing the entry of globally notorious invader species that have disrupted ecosystems elsewhere.

(The writer is a researcher in the field of ecology and management of invasive alien fishes in India)