Thousands of people protested in the Mauritian capital of Port Louis on Saturday, calling for an investigation into an oil spill from a Japanese ship and the death of at least 40 dolphins found near the site.

Environmentalists have also called for an investigation into whether the dolphins died as a result of the spill, caused when the bulk carrier Wakashio struck a coral reef last month.

One protester held a banner with a dolphin covered in oil reading “our lives matter” and another held one calling for the government to resign. Mauritian flags were waved across the packed square of St Louis Cathedral.

Fabiola Monty, an environmental scientist, said: “We do not trust the government and the diluted information they’ve been feeding us regarding the management and responses to the oil spill.”

The government has said it will carry out autopsies on all of the dolphins and has set up a commission to look into the spill. Two investigations are being carried out: one by the police on the crew’s responsibilities and one by a senior shipping ministry official on what happened to the ship.

Vets have so far only examined two of the dolphin carcasses, which bore signs of injury but no trace of oil in their bodies, according to preliminary autopsy results. The autopsy on the first two was conducted by the government-run Albion Fisheries Research Centre.

Autopsy results on 25 dolphins that were washed ashore on Wednesday and Thursday are expected in the coming days, according to Jasvin Sok Appadu from the fisheries ministry.

A local environmental group, Eco-Sud, which took part in Saturday’s protest, said in a statement on Friday that civil society representatives should be present during the autopsies and called for a second opinion from independent specialists.