Antonio Guterres says one recent study warns discarded plastic rubbish could outweigh fish by 2050 if nothing is done.

Secretary-General Antonio Guterres opened the first-ever UN conference on oceans with a warning that the seas are “under threat as never before” with one recent study warning discarded plastic rubbish could outweigh fish by 2050 if nothing is done.

The UN chief told presidents, ministers, diplomats and environmental activists from nearly 200 countries on Monday that oceans – “the lifeblood of our planet” – are being severely damaged by pollution, overfishing, and the effects of climate change as well as refuse.

The five-day conference, which began on World Environment Day, is the first major event to focus on climate since President Donald Trump announced last Thursday that the United States will withdraw from the landmark 2015 Paris Climate Agreement – a decision criticised by Bolivia’s President Evo Morales and other speakers.

Guterres said the aim of the conference is “to turn the tide” and solve the problems that “we created”.

He said competing interests over territory and natural resources have blocked progress for far too long in cleaning up and restoring to health the world’s oceans, which cover two-thirds of the planet.

“We must put aside short-term national gain to prevent long-term global catastrophe,” Guterres said. “Conserving our oceans and using them sustainably is preserving life itself.”

General Assembly President Peter Thomson, a Fijian diplomat, said: “the time has come for us to correct our wrongful ways”.

“We have unleashed a plague of plastic upon the ocean that is defiling nature in so many tragic ways,” he said. “It is inexcusable that humanity tips the equivalent of a large garbage truck of plastic into the ocean every minute of every day.”

Thomson also warned that illegal and destructive fishing practices and harmful subsidies for fisheries “are driving our fish stocks to tipping points of collapse”. And he said increasing human-caused carbon emissions are not only driving climate change but causing rising sea levels by warming the oceans and making them more acidic with less oxygen, which harms marine life.

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Thomson said the conference probably represents the best opportunity ever “to reverse the cycle of decline that human activity has brought upon the ocean”, and to spur action to meet the UN goal for 2030 to conserve and manage the ocean’s resources.

The conference asked governments, UN bodies, and civil society groups to make voluntary commitments to take action to improve the health of the oceans. So far, more than 730 commitments have been received, most on managing protected areas, according to conference spokesman Damian Cardona.

At the end of the conference on Friday, nearly 200 countries will issue a Call for Action addressing marine issues that Cardona said have already been agreed.

It will urge nations to implement long-term and robust measures to reduce the use of plastics, including plastic bags, and counteract the sea-level rise that threatens many island nations as well as rising ocean temperatures and increasing ocean acidity.

Micronesia’s President Peter Christian said Pacific islanders are concerned the ocean has been “left to heal itself” after being used as “a dumping ground for industrial waste”, a weapons’ testing ground, and being polluted by humans on shores and ships at sea.

Stressing the importance of all countries being part of the Paris agreement, Christian said in an apparent reference to Trump’s decision: “While some may continue to deny man’s culpability for the damaging effects of climate change on islands and islanders … no man, no island, no village and no nation can deny that trash in our oceans is of man’s own doing.”

“And for this, man must clean up his mess,” he said.

Bolivia’s Morales was more forthright, telling the conference that the government of the United States, one of the world’s “main polluters”, decided to leave the Paris agreement, “denying science, turning your backs on multilateralism, and attempting to deny a future to upcoming generations”.

This “has made it the main threat to Mother Earth and life itself”, Morales said.