A massive invasion of an alien fish in Bangladesh’s inland waters, including all major rivers, is threatening the aquatic ecology and freshwater fisheries.
Known as suckermouth catfish, native to South America, the fish’s invasive characteristic is globally recognised, especially after it invaded North America, the Caribbean and South Asia, causing aquatic degradation and replacing some of their native fishes.
‘We haven’t seen anything like this before,’ fisheries department’s aquaculture wing chief Azizul Haque told New Age.
The species poses a threat to the aquatic ecosystem as it is competing with native fishes for space and nutrient sources, according to experts.
Fisheries department officials are aware about the fish’s abundance in heavily polluted rivers flowing around and through the capital, including the Buriganga, where fishes cannot generally survive because of low oxygen and toxicity.
Researchers believe that the armoured catfish which belong to the Loricariidae family was imported in the 1980s as ornamental fish in Bangladesh and then it somehow got released or escaped into the wild in the 1990s.
At least three types of catfish known by their scientific names have been reported in Bangladesh. They are Pterygoplichthys anisitsi, Pterygoplichthys pardalis and Hypostomus plecostomus.
‘Pterygoplichthys anisitsi, also known as paraná sailfin catfish, is found in freshwater countrywide from Kaptai lake to Chalan Beel,’ said Dhaka University zoology professor Md Niamul Naser.
Hypostomus plecostomus is spreading locally in wetlands such as haors and beels, he said.
A 2018 research by scientists from Bangladesh, USA and UK revealed the invasion of the Ganges and the Brahmaputra rivers by the third species Pterygoplichthys pardalis, also known as the Amazon sailfin catfish.
On Sunday morning, Alamgir, a 10-year-old boy, standing in ankle-deep water in the Buriganga, was seen using a tiny patch of torn fishing net attached to a slender stick to catch a catfish swimming to the surface of the water under the Basila bridge.
‘Catching tiger fishes is real fun,’ said Alamgir, calling the fish by its locally given name.
Alamgir released most of the fishes he caught in the morning but kept one tied by its bony-plated tail as a trophy of his hunting expedition.
New Age took a five-minute ride in a boat over the Buriganga with an amateur fisherman who netted 13 of the nocturnal catfish in one go on Sunday noon.
‘This river is full of tiger fishes and one can spot so many at night,’ said fisherman Nur Islam.
DU teacher Niamul said that the first sighting of the catfish species outside an aquarium was reported in Gulshan and Banani lakes in the mid-1990s. Fisheries department officials said that media reports show that the catfish got spread to the river Shitalakkhya in Narayanganj in the mid-1990s.
Recently, the same catfish species has also been captured in the river Surma.
This catfish species is eaten in its native land as well as Mexico. In Bangladesh, there are scattered reports of it being sold in the market for up to Tk 200 a kilogram.
Fisheries department recently launched an investigation into the matter with initial reports confirming its widespread presence in Noakhali, Cumilla, Bogura and Mymensingh.
‘This is the first-ever exotic fish that appeared to have established itself in Bangladesh’s ecosystem,’ said Sarker Mohammed Rafiqul Alam, the fisheries department officer investigating the fish.
He said aggressively invasive fish such as African magur, scientifically called Clarias gariepinus, and red piranha, scientifically called Pigocentrus nattereri, failed to survive in nature.
Bangladesh banned African magur in 2013, about three decades after its introduction in the country, and red piranha in 2007, about a decade after its introduction.
The suckermouth catfish was captured in neighbouring India’s river Ganga during 2008-2009. Indian media reported after the 2018 flood that the catfish got spread over a vast area.
Floods served as an important route of dispersal of the catfish after its introduction to at least 17 countries largely because of its algivore nature to keep the growth of algae under control.
Aquarists call this catfish species tank cleaner as it sticks to the aquarium glass or cobble or floor through suction, scraping their surfaces to clean any vegetative growth off them.
The catfish can also get attached to larger fishes through suction to eat algae gathered on their bodies and injure the host in the process. Their ability to stick to surfaces allows them to remain stationed in the flowing water creating scope for colonising nearby areas.
The catfish can also live on detritus, which is abundant in supply in Bangladesh’s floodplain. They scrape the riverbed in search of detritus turning the water turbid.
‘The fish is a major threat to aquatic lives and biodiversity,’ said Rajshahi University fisheries department associate professor Mohammad Yeamin Hossain.
Also on the catfish’s menu is phytoplankton, the base of the aquatic food web, the disappearance of which could mean the collapse of the aquatic food chain.
Bangladesh Agricultural University fisheries management professor Zoarder Faruque Ahmed said that rapid propagation of the catfish can trigger an immediate food crisis for native algivores.
Faruque pointed out that ‘eventually the impact would be felt by all native fishes, including ruhi, catla, mrigel and hilsa.’
The US fish and wildlife service in 2018 categorised the catfish species of Hypostomus plecostomus as highly risky after assessing the ecological impact of its ability to rapidly monopolise nutrient resources and increase population.
The US assessment revealed that the catfish can survive under hypoxic conditions, when there is not enough oxygen required for survival, and thrive in a wide range of temperatures from 16C to 32C. It can stay out of water for 20 hours and live up to eight years in the wild.
The catfish grows up to 50cm and matures so fast that it starts breeding before reaching its half size.
The breeding season is protracted, expanding over a minimum of five months with a catfish laying 3,000 eggs with a hatching time of three to five days, said the US assessment.
The catfish inhabits sloppy river banks creating burrows with multiple chambers which the male catfishes guard when there are eggs inside, said the US assessment.
The burrows could extend up to 1.5 meters which caused river banks to collapse and siltation problems in Mexico, Florida and Texas, said the US report.
The US assessment report revealed that the proliferation of Hypostomus and Pterygoplichthys reduced the tilapia production in six Florida lakes two thirds between 1993 and 2006.
In Mexico the annual loss from disrupted freshwater fishing stood at $16.4 million after the proliferation of the catfish.
The US report associated the spread of the catfish with the declines of a native snail in California and two native freshwater fishes in Texas.
One of the declining freshwater fish, Etheostoma fonticola, better known as fountain darter, is considered endangered by the International Union for Conservation of Nature. The fountain darters inhabit bottom vegetation of clean spring-fed waters and eggs on algae.
Fountain darters eggs were found in the stomach of the catfish, said the US report.
The UK-based Centre for Agriculture and Bioscience International said that attempts to remove the armoured catfish once widely established in nature is likely to be unsuccessful.
The head and upper part of the body of the fish is covered in bony plates that keep aquatic predators away from them, said the CABI. When threatened, the catfish erects its dorsal and pectoral fins, making matters difficult for predators and may end up killing them.
CABI said that the catfish is not generally aggressive but their robust morphology makes them a threat to native and endemic fishes, especially when their adult members compete for spaces and resources.
Bangladesh Fisheries Research Institute chief scientific officer AHM Kohinoor said that they are aware about the catfish’s spread to nature but never investigated it.
‘The fish is not harmful as far as the ecology is concerned,’ he said.
The amateur fisherman Nur Islam however sniffs danger in the spread of the fish. He recalled capturing about 700 of them during a recent nightlong fishing spree.
‘It is not that the catfish only looks scary. It keeps making a consistent, ominous sound when out of water,’ said Nur.