Environment experts say plants would not grow back on river banks and shoals in a nearly 80 square kilometres stretch that have come into contact with the furnace oil.
The plants near the river and canals inside the world's largest mangrove forest that came into contact with oil will also die slowly, says Dr Dilip Kumar Datta, a professor at Khulna University's environmental science department.
Datta, who is also a member of the environment and forest ministry-formed investigation committee, told bdnews24.com the amount of oil on the Shela River decreased on Friday compared to last two days.
Most of the oil has spread further and got stuck on the river branches and canals.
"I think we won't see any oil floating on the water in the next couple days. It will mix with the water and go underwater," Datta said.
But the affects of the oil spill would become visible after several days.
He said there were no dead fish on the river yet. "We can say for now that the fish habitats haven't reached critical stage yet."
Bangladesh Wildlife Conservation Society's Rubaiyat Mansur, who has been staying in the Sundarbans for the last several days, also made similar observations.
Locals said no crocodiles were seen on the shoals inside the forest while migratory birds have moved away from the Pashur River. The oil floating on water and stuck on river banks have also turned the breeze in the forest greasy.
However, forest officials said no dead fish had been spotted on the Shela River since Tuesday's accident, but a number of dead crabs were spotted at several places.
'Southern Star 7', carrying 350,000 litres of furnace oil, sank on the Shela River Tuesday after being hit by another cargo vessel, 'MV Total'.
The sunken tanker was towed to the shore on Thursday, but, by then, the oil had spilled across a vast swathe of the Sundarbans, said forest officials.
The environmental experts fear the spill would hamper the ecology and diversity of life there, including the aquatic life and plants, specially the Irrawaddy dolphins.
The forest ministry's nine-strong investigation team visited around 50 kilometres area in the forest on Friday.
They will stay there until Dec 18 and make recommendations to tackle the disaster and losses.
However, only progresses the government made until Friday are closing the Shela River route to all modes of vessels and sealing the entrances to canals with nets to stop the oil from spreading further.
Apart from these, local fishermen and villagers have begun mopping up oil with pots, pans and sacks on the river.
State-owned Padma Oil Company Limited contractors are buying it from them at a rate of Tk 30 per litre.
Padma Oil's contractor Abdullah Traders owner Rafiqul Islam Babul said they had acquired altogether 5,200 litres of oil, approximately 26 barrels, on the first day.
This exercise would continue for two more days and 10 more vessels from Mongla port would join on Saturday, said shipping ministry's Additional Secretary Rafiqul Islam who is overseeing the operations.
Authorities, meanwhile, have backpedalled on a decision to use oil chemicals to disperse oil due to fears of further endangering the ecology of the mangrove forest.
Tugboat Kandari-10 after arriving from Chittagong is waiting at the scene with 10,000 litres of chemicals. The initial decision to release the chemicals was suspended due to objections by the forest department.
Forest department officials said even though the chemicals worked in containing oil spill on the sea, they were not sure whether it would harm the forest's freshwater animals and plants more.
The use of oil spill dispersant to bring the pollution under control would begin only after getting clearance from the Ministry of Environment and Forest.
Though the forest department, BIWTA and Mongla Port Authority have claimed to have taken all necessary steps, there was no visible progress as there was clear plan of action yet.
Deputy Minister for Environment and Forest Abdullah Al Islam Jakob did acknowledge that the oil spill would cause irreparable damage to the Sundarbans, but actions of the government agencies still lack direction.
The mopping up may help to reduce the amount of oil on the canals, but officials overseeing the operations could not confirm whether it posed any health hazards.
A 31-km stretch of the Shela and Pashur rivers are dolphin sanctuary. Dolphins are mostly seen near a breeding centre for the wildlife along the Pashur River.
However, Forest Ranger Abdur Rob said no dolphin had been seen there since Wednesday.
Sore mouth was spotted in seven young crocodiles at the Karamjal Wildlife Reproduction Centre in Chandpai Range on Thursday after the contaminated water entered the facility, he said.
No migratory birds could either be viewed in the Pashur River since Wednesday, Rob added.
[Additional reporting by Shahidul Islam]