The five-day meeting is the first bid by the United Nations to address the toughest problems facing our oceans, from coral bleaching and plastic pollution to overfishing and rising seas due to climate change.

"We must put aside short-term national gain, to prevent long-term global catastrophe," Guterres told the gathering at the UN General Assembly.

"Conserving our oceans and using them sustainably is preserving life itself."

Guterres raised alarm over the state of the world's oceans, saying pollution, overfishing and climate change are causing severe damage and that "the situation is getting worse."

A recent study showed that plastic could outweigh fish in the seas by 2050 if nothing is done, he said.

Rising sea levels threaten entire countries, fisheries are collapsing and coastal ecosystems are under unsustainable levels of stress from industry, fishing, mining, shipping and tourism, Guterres said.

The first step to turn the tide is "ending the artificial dichotomy between economic demands and the health of our seas," he asserted.

"The conservation and sustainable use of marine resources are two sides of the same coin."

Guterres called for concrete steps, from expanding marine protected areas to the management of fisheries, reducing pollution and cleaning up plastic waste.

UN member-states are working on a "call to action" from governments, civil society and businesses that commit to taking steps to clean up the oceans and work preserve what is arguably the Earth's most important resource.

Targets include protecting at least 10 percent of coastal and marine environments by 2020, reducing ocean pollution and strengthening ways to fight illegal, unreported and unregulated fishing.

- US pullout from climate accord -

The conference opened just days after President Donald Trump announced his decision to withdraw the United States from the Paris climate accord. 

Bolivian President Evo Morales took a swipe at the United States from the UN podium, saying the withdrawal was "denying science, turning your back on multilateralism and attempting to deny a future to upcoming generations."

These actions are "the main threat to Mother Earth and to life itself," he said.

Swedish Deputy Prime Minister Isabella Lovin, whose country is co-hosting the conference with Fiji, expressed hope that the conference will be "the game-changer our ocean so badly needs."

"The ocean is now 30 percent more acidic than in pre-industrial times," she said. "Big predatory fish stocks have declined by some 70-90 percent. The surface waters are getting warmer. In some areas, there are more microplastics than plankton."

Gabon's President Ali Bongo Ondimba announced the creation of Africa's largest marine protected area, a network of nine marine parks and 11 aquatic reserves.

The measure will extend protecting of the existing Mayumba National Park to the limit of Gabon's exclusive economic zone.

A new report released Monday showed that more marine protected areas could help fight climate change.

Marine reserves can lessen the impact of ocean acidification -- which kills coral reefs -- and provide refuge for species that are in decline, according to the report by international researchers led by University of York professor Callum Roberts.

They can also "promote uptake and long-term storage of carbon from greenhouse gas emissions, especially in coastal wetlands, which helps reduce the rate of climate change," the study said.

Just 3.5 percent of the world's oceans are set aside for protection, and only 1.6 percent are fully protected from fishing and other exploitation. (**)