- Recent observations of tilapia in the coastal waters near Devipattinam have prompted an investigation to understand and assess their colonisation patterns.
- Observations have revealed fish of various sizes, even in areas where they are not typically found and that the fish is adapting its diet and breeding behaviour to the inshore waters of the Palk Bay.
- With a low disease rate and high breeding capacity, Tilapia fish have been artificially farmed in India since the 1950s and hold economic significance.
The coastal waters of Devipattinam in Tamil Nadu have been experiencing an unusual increase in the population of the invasive tilapia fish (Tilapia sp.). Fish of various sizes have been spotted, including young and mature individuals, even in areas where they are not typically found.
“This prompted an investigation into various population characteristics,” says Muthusamy Anand from the Department of Marine and Coastal Studies of Madurai Kamaraj University and lead author of a July 2023 study in Tamil Nadu’s Palk Bay region which unravels how the invasive tilapia fish rapidly adapt to the habitat, by exploring their capacity to alter their surroundings and the genetic changes induced by the habitat. The study looked at abundance, gut content analysis (GaSI), gender distribution, size range and maturity levels to shed light on the natural invasion and colonisation of marine waters by tilapia in the Palk Bay region, says Anand.
Tilapia, typically found in aquaculture facilities, now occurring in marine environments such as Palk Bay, could have potential long-term impacts on the native ecosystem and aquaculture industry, emphasising the need for further study and management strategies.
Adaptable, invasive with economic significance
Tilapia include a host of tropical freshwater fish species belonging to the Cichlidae family that are native to Africa and the southwestern Middle East. Some species among them are high in demand in the aquaculture industry due to their low disease rate and prolific breeding. The Mozambique tilapia, for instance, was among the first species introduced for aquaculture in India during the 1950s, primarily for their protein. Since then, various tilapia species have been farmed at the industrial level. The total tilapia production in 2022 was estimated to be about 70,000 tonnes, with 30,000 from aquaculture alone, according to a joint study by WorldFish and the Confederation of Indian Industry. India has also set an ambitious goal to produce 0.766 million metric tons of tilapia by 2027.
“Tilapia are hardy and highly adaptable, which is reflected in significant differences in tilapia gut microbiota across diverse habitats. They can tolerate pollution, temperature fluctuations, and low oxygen levels in water,” says Anand. This adaptability makes tilapia fish ideal for farming across India, from brackish waters in Tamil Nadu to freshwater regions in the Himalayas.
These traits also mean that they can spread easily in new habitats. Unintended releases and escapes from aquaculture pens have resulted in their widespread presence across freshwater habitats in India, including streams, lakes and backwaters. Tilapia dominate waterways like the Ganga and Yamuna rivers and can even be found in the biodiversity-rich Andaman Islands and Western Ghats.
“Most invasive species exhibit plasticity and are able to adapt to changing climatic conditions,” notes A.K. Singh, Emeritus Scientist at the ICAR-National Bureau of Fish Genetic Resources, Lucknow. Plasticity refers to their ability to adapt anatomically to new environments. Once established, however, tilapia become a threat to native species and ecosystems. This has prompted its inclusion in the National Biodiversity Authority’s list of invasive alien species of India, despite its economic significance.
However, there was previously little information available on the diet and habitat ecology of tilapia during new invasions.
Adapting to coastal waters in Devipattinam
The 2023 study was conducted near Devipattinam, an ancient port town in Tamil Nadu, close to an estuary of the Vaigai river. This area is rich in marine species and supports an active fishing community, with both mechanised and non-mechanised boats operating in the vicinity. The study concluded that tilapia are now thriving in fully marine environments within Palk Bay.
An analysis of specimens collected from the Devipattinam coast offered insights into tilapia’s growth patterns, diet and reproductive biology. In a recent survey conducted in January 2023, adult specimens with fertile gonads (reproductive organs) were documented. However, the majority of the specimens discovered were juveniles and sexually immature, suggesting that the inshore waters of the Palk Bay region serve as a breeding and nursery ground for young tilapia.
Surveys indicated that tilapia are more prevalent in coastal waters with an average depth of two to five metres, primarily dominated by seagrass. An analysis of their gut contents revealed their preference for crustaceans, molluscs and plankton, underscoring their adaptability as opportunistic feeders. On examining the gonads of samples from various sites confirmed the presence of all life stages, including immature, maturing, and mature fish, with a high number of mature female specimens. Additionally, a close look at length-weight relationships to understand population dynamics suggested isometric growth in both male and female tilapia, meaning that the weight of an individual fish increased proportionally to three times the length.
“The consistent male-to-female ratio also indicates breeding activity within the marine environment, which was unexpected. These observations suggest that the population is successfully establishing itself in the area,” says Anand.
Cost and effect
It is also concerning that local fishermen are aware of the presence of these invasive species and regularly harvest them from the inshore waters. “The older generation of fishermen did not encounter this fish during their time, but now they are increasingly sought after, with the new generation exclusively targeting tilapia. Particularly on Saturdays, there is a noticeable increase in its collection. A thriving market has emerged, particularly evident on Sundays, indicating a growing demand for this fish,” says Karthik Arumugan, a local fisherman.
Tilapia pose a significant threat to indigenous fish and fisheries in the Palk Bay. “Their presence can lead to competition with native species for food, depriving them of essential resources. Tilapia may alter or degrade habitats, affecting the overall biodiversity of ecosystems where they are introduced. Over time, tilapia populations may become dominant, potentially displacing native fish species and affecting the revenue of the fishing industry,” says Anand.
In 2020, the Department of Fisheries had issued guidelines for the responsible farming of tilapia in India. They specified a list of approved species and required that farms maintain bio-security by preventing any escape of the invasive fish into water sources, even in situations like flooding, where they would pose a risk to the native biodiversity. “It, however, needs stricter enforcement,” says Anand.
At the same time, further understanding of the ecological impacts and population dynamics of tilapia is crucial for developing effective conservation strategies. To achieve this, the study authors recommend further research to comprehensively assess tilapia’s invasion across the entire Palk Bay region. “This information can inform targeted management strategies aimed at reducing the spread and minimising the impacts of tilapia on the ecosystem,” says Anand.
Read more: [Commentary] Tilapia: How an invasive fish came to dominate our ecology and food
Banner image: Tilapia fish in a zoo in Indonesia. When released in the wild, over time, tilapia populations become dominant, potentially displacing native fish species. Photo by Nafisathallah/Wikimedia Commons.