THE deaths of giant freshwater stingrays in the Mae Klong River could have been caused by a combination of pollution created by human activities and natural phenomena.
However, the exact reason behind the death of more than 50 critically endangered giant stingrays is still the subject of dispute.
A team from Kasetsart University has said pollution – namely, wastewater – was the major factor behind the deaths. But vets from Chulalongkorn University have said traces of a cyanide compound were found in tissue samples from the dead rays and toxic chemical contamination could have caused their deaths.
Multiple sources have said that many factors could have caused the quality of water in the river to decline, leading to the mass die-off.
There has been a lot of suspicion that wastewater from an industrial facility upstream caused the deaths, with an environmental group based in Ratchaburi having shown a video clip of an ethanol factory directly releasing wastewater into the river. The video clip by the environmental group was recorded on September 29.
People along the Mae Klong River in Samut Songkhram said they started to spot abnormal activity by the rare stingrays about the same time late last month, when many of them floated to the surface. This was unusual, as they are normally found on the riverbed.
By early October, stingray carcasses were being found along the river and its tributary canals. The number of dead rays was estimated to be more than 50 – around one third of the total stingray population in the river.
However, wastewater discharges into the river are not unusual. Ban Pong Environmental Protection and Anti Corruption Network secretary-general Ratthawut Wallathanaroj said factories in Ban Pong district have emitted wastewater for years, which might rule out wastewater being the immediate cause of the deaths.
“Our organisation has been trying very hard to reveal this bad practice. There are seven big factories in the district that regularly discharge wastewater into the river and we have informed several local official agencies but nothing happened,” Ratthawut said.
“I don’t know exactly that the pollution from here caused the mass stingray deaths, but I’m sure that it is a big factor.”
Weerakit Joerakate, head of Samut Songkhram Fisheries Research Station, said he was sure the stingrays died from a sudden decrease of oxygen in the water based on evidence his team had compiled.
He said there had been heavy rain upstream in September, so the Mae Klong Dam had discharged a large amount of water. Sediment in the river, contaminated by pollution from human activities along the river, was carried downstream with the run-off and deposited at the river delta.
“The pollution in the sediment caused a rapid drop in the dissolved oxygen level in the water, especially along the riverbed, resulting in the rapid death of large stingrays that could not escape the pollution in time,” he said.
“I don’t think we should only blame industry because we are all responsible for dumping wastewater from our homes into the river as well. And the reason that it was a mass death this year is because the river could no longer take more pollution.”
Pratchaya Suphap, a local fisherman in Samut Songkhram, said not only stingrays had been killed as shellfish that also inhabit the riverbed had also been affected.
“I’ve never seen a mass die-off like this before. During the time of this incident I saw a cloud of polluted water slowly drift down the river and it slowly flowed out to the sea,” Pratchaya said.
“This time of the year, we also experience the seasonal algae bloom, which heavily degrades the water quality and kills several aquatic animals every year too. A combination of the polluted water from the river and algae bloom in the sea caused the mass deaths to occur.”
Meanwhile, a different hypothesis came from Dr Nantarika Chansue of Chulalongkorn’s Faculty of Veterinary Science. She said test results of the dead rays showed they had been exposed to a substance with a very high level of cyanide.
“From our study, we found that the stingrays’ kidneys and livers were damaged and there were malfunctions in their nerves and blood systems. Such bioindicators point to the cyanide toxin,” Nantarika said.
However, her conclusion was contested by Weerakit, who said if cyanide or another toxic substance was really the cause of the stingrays’ deaths, other smaller aquatic animals would have died as well.
“The toxin is more lethal to smaller fish, even though stingrays are very sensitive to toxin. But there would have been other fish that died too,” he said.