Dissolved oxygen falls sharply, ammonia exceedingly higher
Hundreds of dead fish of different species, and other aquatic animals have been found floating in the river Halda for the last couple of days.
The dead fish are posing a serious threat to the aquatic biodiversity of Bangladesh’s largest natural breeding ground for carp.
According to the Department of Fisheries lab, the level of dissolved oxygen (DO) went below 1mg per litre on an average against the required level of at least 5mg per litre.
According to the District Fisheries Office, the level of dissolved oxygen was found low in the two feeder canals of the river. In some points, the level of ammonia was found to be 0.5-1.5ppm against the tolerable limit of 0.01ppm.
The high concentration of ammonia in water makes it difficult for aquatic organisms to sufficiently excrete the toxicant, leading to toxic build-up in internal tissues and blood, which leads to their death.
Humans need air to breathe, and the aquatic organisms need dissolved oxygen to respire. It is necessary for the survival of fish, invertebrates, bacteria, and underwater plants. Dissolved oxygen is also needed for the decomposition of organic matter.
Locals said the water of Halda River turned pitch-black on June 20, spreading the rotten stench of dead fish.
Following the matter, the locals formed a human chain at Madunaghat area on June 23, demanding that the river be saved from industrial pollution.
According to Halda River Research Laboratory of Chittagong University, as many as 20 species of fish, including Mola carplet (Amblypharyngodon mola), five species of prawn, Mozambique Tilapia (Oreochromis mossambicus), Aer (Sperata aor), Rohu (Labeo rohita) Catla (Catla catla), Mrigel carp (Cirrhinus mrigala), Kalbaus (Labeo calbasu) Kuchiya (an eel-type fish), were found floating in the river.
A Mrigal fish, weighing over 15kg, was also found floating in the river.
Officials of Bangladesh Fisheries Research Institute (BFRI), Halda River Research Laboratory of Chittagong University, Department of Fisheries (DoF) and Department of Environment (DoE) have already visited different spots of the river and its feeder canals.
The BFRI and the DoE also collected water samples for lab tests from different points of the river to find out the causes behind the death of such a large amount of fish.
While talking to the Dhaka Tribune, noted Halda river researcher Prof Dr Manzoorul Kibria said the levels of dissolved oxygen fell sharply and the ammonia went up hundred times the tolerance level due to pollution.
“Untreated effluents discharged from different industrial units from oxygen to Kulgaon areas, domestic waste and poultry bird excreta are responsible for the death of a huge number of fish species,” Kibria said.
“Bamonshahi canal was blocked by Chittagong Development Authority (CDA) a few years ago. It should be opened without delay.”
“The DoE should a keep a vigil so that the industrial units keep the ETPs operative round the clock. A commission for conserving the Halda should also be formed” Kibria, also president of Halda River Protection Committee, demanded.
Stating that Tk800 crore is earned from the fish of the river every year, he said: “The river supplies water to around 6,500,000 people every day.
“It is the living gene bank of pure Indian major carp. A huge number of people, including fry collectors and hatchers live on the spawns produced in the river. They will lose their livelihood if we cannot stop the rampant pollution.”
Chittagong DoE Azadur Rahman Mollick told the Dhaka Tribune that the level of dissolved oxygen required for aquatic animals was not up to the mark.
“We have collected water samples from different points for tests. Primarily, it seems that domestic wastes, which got mixed with other wastes after the recent flood in Chittagong, are to blame for the falling level of dissolved oxygen in the river.”
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“We have collected water samples from nine points of the river and tested them in the lab in different parameters. In most cases, the level of dissolved oxygen was below 1mg per litre,” Md Kamrul Hassan, senior chemist of DoE, Chittagong (Laboratory), said.
“Two teams of the DoE have visited different spots of the Halda. Based on the field visit, we have already compiled a report which has been sent to the higher authorities of the department,” Sangjucta Das Gupta, DoE assistant director (Technical) of Chittagong Metro, said.
The DoE official added: “While developing a housing project located in Kulgaon area, Chittagong Development Authority (CDA) blocked Bamonshahi canal a few years ago. The household wastes used to find its way into the Karnaphuli through the canal. But both industrial and domestic waste are now being channeled to the Halda through Khandakia canal due to the diversion.”
“The production at the industrial units adjoining the river remained suspended during the Eid-ul-Fitr vacation. So, there is no chance of industrial pollution behind the recent deaths of fish in the river,” the DoE official said.
Chittagong District Fisheries Officer Mominul Haque said they had inspected different points of the river for three consecutive days and witnessed different species of fish like major carps, prawns, eels, Tilapia, Rohu, and Aer floating in the river.
“To avoid such death of fish, the industrial units adjoining the river must install Effluent Treatment Plants and keep them operative round the clock,” suggested the fisheries officer.
Originating in Khagrachari’s Ramgarh upazila, the 107km-long river passes through Hathazari and Raozan upazilas before merging into the Karnaphuli River.
During April and May every year, different species of carp fish start migrating to the spawning ground of the Halda from rivers like Karnaphuli, Matamuhuri, and Sangu.