The indigenous fish species, particularly the small ones, are facing extinction on the basin of the Old Brahmaputra River for a host of reasons, especially habitat loss, according to experts.
They laid emphasis on the conservation of rivers and wetlands like khal, beel, haor and baor to protect the species.
Other reasons identified by the experts include unchecked use of agro chemicals, scarcity of water, overfishing, and the impacts of climate change.
“Agro chemicals are destroying the breeding grounds of the fishes endangering their existence,” said Dr Mahmud Hasan, a professor of Dhaka University's fisheries department.
Framers indiscriminately use agro chemicals and pesticides to boost their crop production and prevent pest attacks, but after rains, the agricultural runoff containing pesticides and other chemicals come down to the water bodies, destroying the natural habitats of fish, he said.
The freshwater fishes in Bangladesh are comprised of 265 species and about 140 of them are the small indigenous ones, according to the Wildfish Centre in Dhaka.
Some of the threatened species include mola, dhela, koi, napit koi, kajoli, baspata, chital, chanda, sarputi, jat puti, tit puti, meni, bele, bain, taki, foli, boal, rani, chela, darkina, pathorchata, khailsha, lal khailsha, tara biam, bata, gutum and tengra.
Pesticides destroy zooplanktons, said Prof Mahmud. “As the native fish species, particularly the small ones, can't take food for lack of zooplanktons, they can't properly grow and breed.”
A retired professor of the Marine Science and Fisheries Institute at Chittagong University, Abdul Quader, said farmers were using pesticides whimsically as there was no law to regulate their use.
According to the experts, the breeding grounds of the fish have been drastically reduced due to inadequate rainfall apart from siltation in rivers, water-bodies and floodplains.
Dr Manzarul Karim, a scientist at the Worldfish Centre, blamed climate change for the erratic rainfall, and said many rivers and wetlands were dying due to insufficient rainfall.
As a lower-riparian country, he said, Bangladesh is also facing scarcity of water because of withdrawal of that from the common rivers by the upstream countries [like India].
Dr Karim said climate change would pose a serious threat to aquaculture in the country's coastal region, particularly in the Sundarbans, as the sea level would keep on rising due to global warming in the future affecting the coastal freshwater ecosystem.
Speaking about overfishing, Prof Quader said people were aggressively catching fishes from rivers and waterbodies to meet their food demand without thinking about the future.
“During the dry season, local people catch all varieties of fish from waterbodies withdrawing water from them. They indiscriminately catch all mother fishes,” he said.
Quader said the indigenous fish varieties were also disappearing fast since the water bodies, including rivers, haors, canals and wetlands were being grabbed by influential people.
Prof Mahmud said the entire Brahmaputra basin had already filled up with silts. “A decline of water volume in the river is significantly contributing to the extinction of fish species.”
He warned that if the remaining waterbodies and rivers could not be protected from grabbing, the native fish species would disappear from the country in the coming days.
“We used to catch a plenty of fishes round the year from the river (Brahmaputra) only a decade back. But, fishes are rarely found now in the river even in the month of Ashar, the prime time of fishing,” said Khalilur Rahman, who fishes in the Brahmaputra River in Kishoreganj.
“Indigenous varieties like puti, sarputi, khailsha, kajuli, tengra, koi and boal used to be found in plenty during monsoon in the past, but now those are being disappeared. You will find only sada pani (clean water), not fish during rain,” said Moti Miah, who lives on the bank of the Jhira Nodi, a tributary of the Brahmaputra, in Kuliarchar upazila.