In 2021, several districts in the state of Uttar Pradesh battled a deadly outbreak of dengue and malaria. To check the spread of disease and control the mosquito menace, district administrations of Kannauj, Moradabad, Firozabad, Lucknow and Amroha, among others, started releasing mosquitofish, a species of freshwater fish of the genus Gambusia, into local water bodies. These fish feed on mosquito larvae.

Firozabad’s Chief Medical Officer Dinesh Kumar Premi had said to the media that this strategy was previously successful in the Bareilly and Baduan districts of the state. As per his statements to the media, 50 packets of fish seeds had been acquired from Badaun to start with and would be released into ponds and rural areas of the district.

Employing biological control measures, in the form of a natural predator to control the spread of a pest species, is not uncommon. For controlling mosquito numbers alone, to prevent the spread of diseases such as dengue and malaria that claim lives, local administrations across the world have turned to mosquitofish, dragonflies and aquatic turtles. Another species of fish, known as copepods, when introduced in large water-storage tanks, were successful in limiting the spread of dengue in Vietnam.

The Gambusia, however, remains among the most widely used biological control agent against mosquito larvae. According to the National Center for Vector Borne Disease Control – an umbrella programme for prevention and control of vector-borne diseases – this particular species has been in use in India since 1928. In National Center for Vector Borne Disease Control’s guidelines for larvivorous fish for vector control, the Gambusia affinis (western mosquitofish) is described as a “very hardy fish and can adapt to wide variations in temperature as well as to chemical and organic content of the water but does not tolerate very high organic pollution.”

It further states that a “single full-grown fish eats about 100 to 300 mosquito larvae per day”. It has advised states in the past to upscale the use of this fish as a biological control method in rural areas.

The Gambusia is a hardy fish and is adaptable to wide variations in temperature as well as to the chemical and organic content of the water but does not tolerate very high organic pollution. Photo credit: NOZO/ Wikimedia Commons

Invasive species

Researchers, however, are of the opinion that the introduction of Gambusia fish into local waterbodies may actually be harmful in the long run. The IUCN lists the Gambusia affinis among 100 of the world’s worst invasive alien species, calling it a pest in many waterways around the world following initial introductions in the early last century as a biological control of mosquitoes. It further states that it is no more effective than native predators of mosquitoes and that one of the main avenues of its spread is continued, intentional release by mosquito-control agencies.

Mosquitofish are difficult to eliminate once established. “It is really difficult to manage the species because female Gambusia can get impregnated and lay fries whenever the situation is favourable,” said Nobin Raja, a researcher at the Ashoka Trust for Research in Ecology and the Environment, who co-published a paper in 2020 on the invasion and management of Gambusia in peninsular India. “It has also been reported that Gambusia can delay or prepone their sexual maturation and alter their body size based on the environment.”

Gambusia fish are often referred to as plague minnows due to their explosive spreading, higher reproduction rate and their negative impact on local ecosystems. They compete with other fish for resources and evidence suggests that they predate on other fishes’ fries and frog larvae. “After the introduction, the Gambusia fish hijacks the food web and breaks the existing cycle, causing an imbalance in the ecosystem,” said Raja. “The fish also negatively affect the aquaculture industry as they prey on the eggs of and compete for resources with economically-important fish.”

The Gambusia remains the most widely used biological control agent against mosquito larvae. However, it is also among some of the worst invasive species. Photo credit: Clinton & Charles Robertson/ Wikimedia Commons

The invasiveness of the fish has led some countries to introduce laws to control them. Gambusia were first introduced into Australia from North America as a biological control for mosquitoes. However, this was unsuccessful. Instead, the species had a detrimental effect on native fish through competition for resources and their aggressive behaviour.

In 2019, the Department of Agriculture and Fisheries in Queensland, in its guidelines stated that “Gambusia are a category 3, 5, 6 and 7 restricted invasive fish under the Biosecurity Act 2014. They must not be kept, fed, given away, sold, or released into the environment without a permit. If caught, Gambusia must be humanely destroyed immediately and disposed of as soon as practicable by burying a suitable distance from the waterway where it was caught or placing it in a rubbish bin.”

Way ahead

Invasive alien species are the second biggest threat to the environment following habitat degradation. Researchers and nature enthusiasts have been building awareness about invasive alien species.

“In recent years, the National Biodiversity Authority has declared Gambusia as an invasive alien species in India, but the communication gap between governmental agencies and a general lack of awareness on the negative impacts of invasive alien species has led to the continued introduction of Gambusia in local water bodies,” said Raja.

There are several tools such as chemical/pheromone-based traps and physical removal of individuals from the infested area that can help limit the invasive species. But more needs to be done, explain experts.

First, the current distribution of the species must be mapped. Then, one needs to measure the socio-economic and environmental impacts with scientifically-backed studies. Third, there is a need to devise effective control methods based on the landscape and the impact the species is having on local ecology.

“But before everything else, we need to stop introducing Gambusia into our waterbodies,” said Raja. “They will become more virulent [gain genetic diversity] and pose a larger threat to our biodiversity and local livelihoods. They may become more adaptable and spread more widely into protected areas where they may pose a threat to endemic and endangered aquatic fauna.”

This article first appeared on Mongabay.