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In Sri Lanka, the hunt is on for alien fish in native lakes

  • A young fish enthusiast from suburban Sri Lanka is on a personal mission to remove invasive species from the island’s lakes and other waterways, starting with clown knifefish and alligator gars introduced as part of the aquarium fish trade.
  • Sri Lanka’s freshwater habitats are plagued by at least 30 exotic fish species, according to the most recent assessment — either released intentionally for aquaculture or mosquito control, or accidentally through the aquarium trade — with a number of them having turned invasive.
  • Some of the carnivorous species are listed among the world’s worst invasive fish, including the knifefish, which has become established in a number of key habitats and poses a threat to native freshwater species, many of which are endemic.
  • Experts have called for stronger regulation to prevent the continued introduction of alien invasive fish species into Sri Lanka’s freshwater habitats.

COLOMBO — Located in the suburbs of Sri Lanka’s biggest city, Colombo, the picturesque Thalangama Lake is plagued by at least two invasive predatory fish species that are contributing to the decline of the native aquatic life.

“I have heard villagers living close to the lake claiming that they had seen a fish with a long snout like an alligator in the Thalangama Lake waters,” said Pathum Madhusanka, a young fish enthusiast who has made it his mission to rid the lake of the invasive species.

The particular fish he’s referring to is the alligator gar (Atractosteus spatula), native to North America and one of the largest fish from that region, which can grow to a length of 3 meters (10 feet).

Madhusanka has for the past few years been painstakingly fishing out another species from the lake: the clown knifefish (Chitala ornata), also a voracious feeder, and native to Southeast Asia. He had to put his mission on hold when the Sri Lankan government imposed a lockdown in response to the COVID-19 pandemic. But within a day of the lockdown being lifted in May, Madhusanka was back at Thalangama with his rod and reel. And the first fish he hooked was an alligator gar, about a meter (3 feet) in length.

“It seemed quite impossible to find an alligator gar in this water body,” he told Mongabay.

Invasive fish species are released into natural bodies of water by owners who get tired of them when they outgrow their tanks or after breeding, as in the case of this alligator gar (Atractosteus spatula), found in an urban lake. Image courtesy of Pathum Madhusanka.

There’s growing interest among fish breeders in Sri Lanka to cultivate “monster fish” — exotic, large, often carnivorous species — and alligator gars have become a popular aquarium fish in many Sri Lankan commercial aquariums.

“The owner of the alligator gar would have purchased it from a pet aquarium while it was small, but the fish would have soon outgrown the tank, making the owner release it into Thalangama Lake,” Madhusanka said.

Reports suggest there are at least three alligator gars in the lake; Madhusanka has to date caught two. He’s also caught more than 70 knifefish in the five years he’s been going after that species, widely recognized as one of the world’s most invasive freshwater fish. Madhusanka discovered juvenile knifefish in Thalangama in 2017, indicating that they were breeding in the lake.

Of the 97 species assessed for Sri Lanka’s red list of threatened freshwater fishes, 61 are endemic to this Indian Ocean island. Of these, 12 are considered critically endangered, 29 endangered, and 10 vulnerable.

The aggressive Mayan cichlid (Mayaheros urophthalmus) is the latest addition to the list of alien invasive fish found in Sri Lanka. Image courtesy of Shantha Jayaweera.

The list, compiled this year, recognizes alien invasive species as a big threat to the island’s freshwater fish. It identifies 30 such introduced freshwater fish species in Sri Lanka’s natural freshwater habitats.

“However, not all the exotic fish released to the natural waterways can be listed as invasive,” says Sampath Goonetillake, IUCN Sri Lanka’s senior program officer for biodiversity.

For a fish to be considered invasive, it must directly impact native species either by competing with them for resources or directly feeding on them.

“We also have observed some other species such as gouramis, zebras, platy fish in natural waterways that are still not considered invasive, though it is extremely important to keep a watchful eye over them,” Goonetillake said.

Accidental and deliberate escapees

The first recorded instance of exotic fish being introduced into natural bodies of water in Sri Lanka dates back to 1889. That’s when the rainbow trout (Oncorhynchus mykiss), native to the northern Pacific region, was deliberately introduced to natural streams in Sri Lanka’s central highlands for sports fishing. The next deliberate introduction was tilapia from Africa (Oreochromis mossambicus and Oreochromis niloticus), released in 1952 to boost aquaculture and inland fisheries. At first they were placed in man-made reservoirs, but soon spread far beyond, and are now found in many rivers and other natural water bodies.

Suckermouth catfish (Hypostomus plecostomus), native to South America, and walking catfish (Clarias batrachus), from Southeast Asia, are also recognized as alien invasive freshwater fish in Sri Lanka, along with the clown knifefish, accidently introduced to natural systems by the  ornamental fish trade in the 1990s.

Common invasive species

The suckermouth catfish, commonly known as “tank cleaners,” are the most widely distributed and harmful invasive fish species found in Sri Lanka’s freshwater habitats, according to Ramani Shirantha, a researcher on aquatic species with the National Aquatic Resources Research and Development Agency (NARA).

“Suckermouth catfish lays more eggs than most other species,” she told Mongabay. “And as the mother protects her eggs, their survival rate is about 80%, whereas the survival rate of native fishes is around 30%.”

This catfish is a hardy species able to survive several hours outside water and with fewer threats compared to native fish. This has allowed for its rapid expansion and ability to outcompete native species for food. Shirantha rated the knifefish as the second-worst aquatic invasive species, given that it directly feeds on the larvae of other species.

The presence of invasive fish has become a serious ecological issue in Sri Lanka’s natural water bodies, especially in the suburbs and island’s dry zone, according to Shantha Jayaweera, a senior instructor with the aquatic group of the Young Zoologists’ Association.

Jayaweera has observed how Mayan cichlids (Mayaheros urophthalmus), native to Central America, have spread from another water body in Colombo to several other wetlands. “Mayan cichlids are known for their aggressive nature attacking other fishes. It will be a serious concern if not controlled immediately,” Jayaweera told Mongabay.

There’s also the case of the guppy (Poecilia reticulata), native to the Caribbean region, and mosquitofish (Gambusia affinis), native to the Mississippi, which were introduced to feed on mosquito larvae. Both species are now established in many natural bodies of water, and research indicates that guppies have become more carnivorous, feeding also on amphibian eggs.

Introduced in the 1930s to keep mosquito populations in check, guppies have spread to some of the biodiversity-rich natural streams of the island’s wet zone, threatening not only to other fish species, but also rare amphibians and dragonflies, as they can attack both eggs and larvae. Image by Hiranya Sudasinghe.

Guppies have even been found inside biodiversity-rich habitats in Sri Lanka’s wet zone and montane central highlands, home to already threatened amphibian species found nowhere else on Earth.

“During our freshwater fish survey, we found a guppy fish inside the Sinharaja Rainforest freshwater pool,” said Madura de Silva, the president of the Wildlife Conservation Society Galle. “It is a mystery as to how guppies found their way into the forest regions, but this indicates that alien invasive fishes would pose a significant threat in the future.”

In light of their invasive nature, Sri Lanka banned imports of knifefish in 2003 and piranhas in 1998. But these fish continue to be locally bred, and accidental releases remain a risk.

As a containment measure, the ban should be extended to cover the domestic aquarium fish trade, essentially prohibiting the cultivation of invasive species, said environmental lawyer and environmentalist Jagath Gunawardena.

Gunawardena, who played a lead role in helping draft a bill to regulate alien species, said the continued introduction of such species, both flora and fauna, posed a serious threat to natural  habitats.



Banner image of a fully grown clown knifefish caught alive from a lake in urban Colombo, courtesy of Pathum Madhusanka.


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