About 3,50,000 litres of furnace oil spilled in the river.
The first evidence of harm caused by an oil spill, from a capsized tanker in Bangladesh's Shela river, on the local flora and fauna surfaced on Sunday, with reports of the first dead dolphin.
The body of the dolphin was spotted by Syed Zakir Hossain, a photojournalist with the Dhaka Tribune on Saturday, in the Harintana-Tambulbunia channel of the Shela river, according to a Xinhua report.
Oil tanker Southern Star 7, carrying 357,664 litres of furnace oil, sank in the river, in the Sundarbans mangrove forests, after being hit by a cargo vessel on Tuesday. The body of the tanker's captain was recovered earlier in the day.
According to the Xinhua report, news of the discovery of the dead dolphin was published by the Dhaka Tribune, on Sunday.
Reports of sightings of corpses of other animals were already coming in since Friday, but this was the first confirmed sighting of dolphin remains, Xinhua reported, citing the Dhaka Tribune.
Since the Shela river is a sanctuary for two types of dolphins, a major concern after the tanker capsized was regarding the well-being of these sensitive and rare marine creatures.
Environmentalists have expressed grave concerns over the accident, saying that the plants and aquatic resources of the world's largest mangrove forests -- the Sundarbans, would suffer a big blow due to the oil spill in a vast area.
The oil spill poses a serious threat to all the aquatic creatures including the dolphins in that part of the river as the amount of oxygen in the water is likely to decrease, they added.
A 100 km oil slick was seen floating on the Shela river on Sunday, though officials assured that the country's government had strengthened efforts to remove the layer of oil that had seeped into the Sundarbans mangrove forest.