World governments have made little progress in the past 20 years when it comes to their pledges to protect marine life and reduce overfishing, experts said on Thursday.

With ocean health among the top 10 issues at the Rio 20 summit on sustainable development June 20-22, international experts called for concrete action to avoid "empty ocean commitments."

Targets set at UN summits in 1992 and 2002 have largely gone unmet, and implementation "has been difficult, ineffective or practically nonexistent," the authors wrote in the US journal Science.

Contributors came from the Zoological Society of London, Simon Fraser University in Canada, the Pew Environment Group in the United States, the University of British Columbia and the University of Oxford.

"Our analysis shows that almost every commitment made by governments to protect the oceans has not been achieved," said Jonathan Baillie, director of conservation at ZSL.

"If these international processes are to be taken seriously, governments must be held accountable and any future commitments must come with clear plans for implementation and a process to evaluate success or failure."

An international action plan to end illegal, unreported and unregulated fishing remains voluntary and has not put a stop to the $23 billion per year industry, the article said.

While local level protections of marine biodiversity have improved in some places, the global picture is "bleak" for many forms of sea life, it said.

"Even the most closely watched species -- such as turtles, sharks, and coral reef fisheries -- are not safe."

Problems and politics have complicated efforts to meet the goals that world leaders have set in the past, and so future efforts should be "more nuanced and context-specific to be realistic and achievable," the authors said.

The team made three recommendations: to bring global fishing in line with resources in domestic and international waters, redirect harmful subsidies and instead use the money to fund efforts to halt illegal fishing, and implement "even a minimal" ecosystems approach to protect vulnerable species.

"Rio+20 is a unique opportunity for governments to collectively show courage and leadership to reverse the worsening state of the world's ocean, and to take action to protect the largest reservoir of biodiversity left on our planet," said Susan Lieberman, deputy director of Pew Environment Group's International Policy team.