US president Barack Obama has vowed to use his executive powers to protect US waters in the central Pacific Ocean and draw up a national strategy to combat illegal fishing.
Leaders from more than 80 countries have gathered at the State Department for a two-day conference uniting scientists and industry experts aiming to draw up an action plan to clean-up the world's oceans.
"We all know how fragile our planet can be," Mr Obama said.
"Rising levels of carbon dioxide are causing our oceans to acidify. Pollution endangers marine life. Overfishing threatens whole species as well as the people who depend on them for food and their livelihoods," he said.
In a video address, Mr Obama said he had directed the US government to "build a national strategy to combat black-market fishing".
This initiative is a practical solution to an ugly problem and will forever change the way we think about our seafood.
Beth Lowell, Oceana group campaign director
The US president is also set to announce plans to declare a vast area of the central Pacific Ocean off-limits to commercial fishing or energy exploitation, the Washington Post has reported.
The proposal would expand the Pacific Remote Islands Marine national monument from 87,000 square miles to around 782,000 square miles and create the world's largest marine sanctuary.
But it could ignite a new battle with Republicans in congress, angered by Mr Obama again using his executive powers to bypass the legislative.
"If we ignore these problems, if we drain our oceans of the resources, we won't just be squandering one of the humanity's greatest treasures, we'll be cutting off one of the world's major sources of food and economic growth," Mr Obama said.
Hollywood lends support
Mr Obama won support from Hollywood actor Leonardo DiCaprio, an avid diver, who pledged his foundation would pump another $7 million over the next two years into projects to help the oceans.
"I've witnessed environmental devastation first-hand," DiCaprio told the conference, recalling two dives he made some 18 years apart on the Coral Reef in Australia.
"What once had looked like an endless underwater utopia is now riddled with bleached coral reefs and massive dead zones."
Environmentalists also welcomed Mr Obama's plans as "a historic step forward in the fight against seafood fraud and illegal fishing worldwide".
"This initiative is a practical solution to an ugly problem and will forever change the way we think about our seafood," said Beth Lowell, campaign director with the international advocacy group Oceana.
A recent report found that between 20 and 32 per cent of wild-caught seafood imported into the US in 2011 came from illegal or "pirate" fishing.
"Because our seafood travels through an increasingly long, complex and non-transparent supply chain, there are numerous opportunities for seafood fraud to occur and illegally caught fish to enter the US market," Ms Lowell said.
Marine Conservation Institute president Lance Morgan, who put together the case to support extending the protection zone in the central Pacific, welcomed the announcement.
"Part of the reason this area is important to protect right now is it's about as close to a pristine ocean as the United States still has," he told the ABC.
"I think it's very important for setting that high bar for everyone.
"The economic impact to US fisheries is very minimal in this region and I think it's showing there are important areas of the ocean."
Call for action about 'survival of people'
The announcement comes a day after US secretary of state John Kerry called for a global regime to protect the oceans, which he said were under threat from too much fishing, acidification from climate change and marine pollution.
On Monday, Mr Kerry opened the conference with a call for all nations to move beyond talks and studies to taking specific steps toward a global agreement to protect the oceans.
"We are not going to meet this challenge unless the community of nations comes together around a single comprehensive global ocean strategy," he said.
He was joined by Kiribati president Anote Tong, who said the small Pacific nation would ban commercial fishing from its Phoenix Islands Protected Area by January 1.
The low-lying state, consisting of 33 islands, is highly vulnerable to rising sea levels, one of the most severe impacts of climate change.
Mr Tong called climate change the greatest moral challenge of modern times, adding the main hope of addressing climate change lies within the oceans.
"It is about the survival of people," he said.
"It is not about economics, not anymore. It is not a political football. It is not about the course or who is responsible anymore.
"It is now about what we must do together as responsible global citizens."