Andhra Pradesh releases 10 million mosquitofish in waterbodies to control malaria, dengue
The Andhra Pradesh government has released approximately 10 million Gambusia fish into the state’s water bodies to combat mosquito-borne diseases like malaria and dengue. The fish, also known as mosquitofish, is widely used as a biological agent for controlling mosquito larvae.
However, the release of these invasive alien fish species has raised concerns about the potential harm that will be sustained by native species that abound in the state’s freshwater bodies.
Andhra Pradesh reported approximately 6,391 dengue cases and 2,022 malaria cases in 2022, according to the Union Ministry of Health and Family Welfare, Government of India (MoHFW). However, the cases have risen significantly, according to local news reports: There have been 2,339 dengue and 1,630 malaria cases registered in the state in the last six months.
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Gambusia affinis (G affinis) is native to the waters of the southeastern United States and a single full grown fish eats about 100 to 300 mosquito larvae per day, according to MoHFW.
Gambusia have been a part of mosquito-control strategies for over a century in various parts of the world, including India. G affinis has a sister species, Gambusia holbrooki (G holbrooki), also known as the eastern mosquito fish.
Mosquitofish has been part of various malaria control strategies in India since 1928, including the Urban Malaria Scheme.
The fish are generally understood to be a good biological control method against mosquito breeding, but can only be a part of an integrated approach that involves various other methods like chemical spraying and, most importantly, source reduction, said Desham PR, a public health practitioner working in Telangana.
“Reducing the number of mosquito breeding sources like puddles and open drainages, and clearing vegetation that mosquitoes feed on must be prioritised over releasing fish into waterbodies,” she said.
In fact, studies haven’t been conclusive about the effectiveness of Gambusia as a viable mosquito control.
While there are studies that showed reduction in malaria cases where the fish were introduced, there are others that reported increased mosquito larvae population where they were introduced, as the latter preyed on other predators that ate mosquito larvae.
There are also studies that reported that Gambusia’s predatory efficacy reduced when they were introduced in running water streams, water bodies with high insecticide levels and waterbodies with thick vegetation.
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Specifically in India’s case, the discussion on the effectiveness of Gambusia includes another question — whether the species said to be introduced originally in the country in 1928 is G affinis or G holbrooki. “It is not clear which of the two species was introduced, as there has been no systematic molecular taxonomy work,” read an article published in 2020.
This is a major concern since G holbrooki has been suggested to be ineffective for mosquito control, as mosquito larvae comprise a negligible part of its diet. Therefore, it could well be possible that the authorities have been introducing an invasive alien species into freshwater bodies across the country with no real benefit.
There are other concerns too: The fish has a high breeding capacity. A single female may produce between 900 and 1,200 offsprings during its lifespan, according to MoHFW.
The young females have two gestations per season, while the older females may have up to six generations per season. A season lasts about 30 days and young ones are released in broods of 25-30 at a time. The fish can also survive in diverse environments, pointed out the 2020 paper.
It is this adaptability that has allowed the fish to survive on six out of seven continents and has made the International Union for Conservation of Nature declare Gambusia one of the 100 worst invasive alien species in the world.
Multiple countries, including India have listed Gambusia as invasive species. However, the fish continues to be a prominent part of the country’s malaria control programmes and the fish continue to be released into freshwater bodies across the country, like Andhra Pradesh, Chandigarh and Uttar Pradesh.
Known to outcompete and prey upon native species, Gambusia also get aggressive in environments where they need to compete with other species for resources. They are known to eat the eggs of competing fishes and frog tadpoles. Studies have also observed them chasing other fishes and resorting to fin-nipping.
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In India, mosquitofish affected the ecosystem health of the lake after its introduction into the Nainital Lake in the 1990s to control malaria, found a study. This, however, is one of the very few studies in India that investigated the effects of this alien species, despite it being introduced in freshwater bodies across the country in large numbers every year.
A World Health Organization publication supported the effectiveness of breeding and use of G affinis as a larval control method in man-made breeding habitats, like swimming pools and garden ponds, with no access to the natural environment.
The report, however, warned against the release of the exotic fish species into natural environment, as they “may cause unwanted side-effects by replacing local species or affecting other aquatic animals.”
In household ponds or tanks where there might not be many other dietary options, Gambusia could be effective in controlling larval populations, the author of the 2020 report Nobin Raja M told this reporter. Even then, there is a possibility of these fish spreading to natural environments.
“Gambusia is yet to be recognised as a problem in India, said Raja. “Operations like Gambusia breeding, distribution and introductions are rarely regulated and data about these operations is very sparse,” he added.
Instead of relying on mosquitofish, the researcher suggested encouraging “biologists and fish taxonomists to come up with river basin-based lists of native fish species that can control mosquito larvae and releasing them into the natural environment.”
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Gambusia fish have a high breeding capacity. A single female may produce between 900 and 1200 offsprings during its lifespan. Photo: iStock.png)
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