The world's oceans are under threat from both illegal and mass industrial fishing, in which millions of fish are caught and discarded, experts warned Tuesday.

"The overfishing of our oceans is largely the result of illegal fishing plus discarded fish," said Juan Vilata, a conservationist at the environmental group WWF Chile, on the sidelines of a major conference on saving the world's oceans.

"And let's not forget that (the fishing industry) also ignores scientific criteria and fishes beyond the recommended limits," he told AFP at the second annual Our Ocean conference in the picturesque Chilean port of Valparaiso.

Chile and the United States both declared large new marine reserves Monday at the opening of the conference, which has brought together scientists, politicians and business leaders from around the world.

Chilean President Michelle Bachelet declared a sanctuary larger than France around iconic Easter Island.

US President Barack Obama, addressing the gathering by video link, announced two new National Marine Sanctuaries, on Lake Michigan and the Potomac River.

The US also unveiled a major global initiative to use satellite technology to better police over-fishing and track illegal catches.

The conference is a pet project of US Secretary of State John Kerry, who piloted the first edition last year in Washington and was in Chile Monday to urge a concentrated international effort to combat the main threats to the world's oceans: overfishing, pollution and acidification caused by carbon emissions.

Since the 1970s, those three problems have wiped out half the population of marine animals -- mammals, birds, reptiles and fish.

The conference's goals include developing policies to make the fishing industry sustainable and tackling the problem of pollution from land, such as agricultural pesticide and fertilizer run-off and waste plastics.

Last year's conference in Washington saw $800 million pledged to support various environmental initiatives.