Bangladeshi fishermen using sponges and sacks began cleaning up a huge oil spill in a protected area that is home to rare dolphins after some environmentalists had warned of an ecological "catastrophe". Thousands of litres of oil have spilled into the protected Sundarbans mangrove area, home to rare Irrawaddy and Ganges dolphins, since a tanker collided with another vessel on Tuesday. The government has sent a ship carrying oil dispersants to the area, which is inside one of three sanctuaries set up for the dolphins. But environmentalists say the chemicals could harm the delicate ecology of the Sundarbans, a Unesco world heritage site. The head of the local port authority told reporters that fishermen would use "sponges and sacks" to collect the spilled oil, which has already spread over an 80 km area. But Amir Hosain, chief forest official of the Sundarbans, admitted authorities were in the dark about what to do. "This catastrophe is unprecedented in the Sundarbans and we don't know how to tackle this," he said. "We're worried about its long-term impact, because it happened in a fragile and sensitive mangrove ecosystem." Rescue vessels have now salvaged the tanker, which was carrying an estimated 357,000 litres of oil when it sank. But officials said the damage had already been done as the slick had spread to a second river and a network of canals in the Sundarbans, the world's largest mangrove forest, which straddles India and Bangladesh. Rubayat Mansur, Bangladesh head of the New York-based Wildlife Conservation Society, said most of the oil appeared to have already leaked out. "I visited the sunken trawler this morning. Only few hundred litres of oil remain inside, so almost all the oil has spilled into the Sundarbans," he said. "Irrawaddy Dolphins can be found in Southeast Asia. But their population … is very small compared to Bangladesh," said Mansur. The Sundarbans is also home to hundreds of Bengal tigers.