Giant stingrays under threat in Mae Klong River

The Nation 09 October 2016 | Pratch Rujivanarom

GIANT freshwater stingrays are facing extinction in the Mae Klong River, a leading marine biologist warned yesterday as mass deaths attributed to water pollution killed a large segment of the population.

As many as 20 giant freshwater stingrays were found dead this week along the Mae Klong River in Samut Songkhram. The cause of death was still unknown yesterday, but researchers said poor water quality was a major cause of ecological damage.

Thon Thamrongnawasawat, deputy dean of the Fisheries Department at Kasetsart University, said he was concerned about the die-off affecting the rare species of stingray Himantura polylepis in the Mae Klong River.

“Giant freshwater stingrays are rare fish due to the loss of their habitat and their low fertility rate. The sudden death of a large proportion of the population is very serious. It will take decades to recover the population to levels before the incident and if the situation get worse, we may lose this species of stingray from the river forever,” Thon said.

Giant freshwater stingrays are viviparous fish that produce only one to four offspring at a time and it takes more than 10 years for the fish to mature and become ready to mate.

Given the recent loss of mature rays, it will be very difficult for the population to recover, Thon said.

The species, which is the largest of freshwater stingrays, is listed as endangered by the International Union for Conservation of Nature and Natural Resources (IUCN).

The stingrays are found in the Chao Phraya River basin, the Mekong River and various rivers in the Malaysian state of Sabah.

The population has decreased worldwide, with the IUCN reporting that the stingray population in the Mekong River and Tonle Sap Lake had fallen by 50 to 95 per cent while in Thailand the population had declined by 30 to 50 per cent.

Meanwhile, local fisheries researchers are working to save the stingrays with the head of Kasetsart University’s Samut Songkhram Fisheries Research Station, Weerakit Joerakate, reporting that two rays had been brought to the station for treatment.

“We received a report from local residents on Friday night that they had found one living female stingray washed ashore. She was two metres wide and four metres long and it was estimated that she was more than 40 years old. Luckily, we could save this precious animal and bring her to the station. Right now her condition is better, as she can swim and move by herself, but we have to let her rest here until the river conditions are better,” Weerakit said.

“The count of dead stingrays now is around 20, but it seems to be more because of a duplicate count. This is a destructive event for the stingray population and we are doing our best to save as many surviving stingrays as we can,” he said.

Thon said an analysis of water quality in the river had found that levels of dissolved oxygen (DO) in the river were very low. In some areas, DO levels were as low as one milligram per litre, while the level that is required to sustain marine life is six milligrams per litre, Thon said.

“From the observation of the mass fish death, big fish like stingrays die first, while many smaller fish are still alive. This indicates that the low DO levels are responsible for the fishes’ deaths. However, there are many factors that can lower the water quality, such as water runoff during flood periods, wastewater from residential areas and pollution from industry,” he said. “We will need more tests and evidence to pinpoint what is the real cause of this incident and who is behind it.”