Image copyright SPL

The amount of plastic in the ocean is set to triple in a decade unless litter is curbed, a major report has warned.

Plastics is just one issue facing the world's seas, along with rising sea levels, warming oceans, and pollution, it says.

But the Foresight Future of the Sea Report for the UK government said there are also opportunities to cash in on the "ocean economy".

They say this is predicted to double to $3 trillion (£2 trillion) by 2030.

The report says much more knowledge is needed about the ocean. The authors say the world needs a Mission to "Planet Ocean" to mirror the excitement of voyaging to the moon and Mars.

The Foresight reports are written by experts to brief ministers on medium and long-term issues of significance. This one has been signed off by ministers from four different departments as the authors emphasise the need for a joined-up oceans policy.

One of the authors, Prof Edward Hill from the UK National Oceanography Centre told BBC News: "The ocean is critical to our economic future. Nine billion people will be looking to the ocean for more food. Yet we know so little of what's down there.

"We invest a lot of money and enthusiasm for missions to space - but there's nothing living out there. The sea bed is teeming with life. We really need a mission to planet ocean - it's the last frontier."

Image copyright Getty Images Image caption The report projects an increase in the industrial capture of fish

Another of the authors, the chief scientist for the UK government's environment department Ian Boyd, agreed: "The ocean is out of sight, out of mind," he said.

He told BBC News: "There's a continuous process of exploring for new things to exploit in the oceans, and that's happening faster than we scientists can keep up with. My suspicion is legislation is also struggling to keep up - and obviously there are risks in that."

He said offshore wind farms, oil industries and mining firms were spreading into unexplored areas. "Scientists need to get in there faster than the commercial people or at least at the same time - to put proper regulation in place to govern those industries."

The report highlights many concerns, including the current worry about ocean plastic litter, which it forecasts will treble between 2015 and 2025.

Media playback is unsupported on your device Media caption A giant build-up of plastic was captured by underwater photographer Caroline Power

But it stresses that the ocean is being assailed from many different types of pollution - including run-off pesticides and fertilisers from farms, industrial toxins like PCBs, and pharmaceuticals.

The authors say if governments can identify ways of protecting biodiversity in the seas, there are riches to be harvested - including nodules of metals and possibly even cures for cancer.

They predict that the biggest industrial growth in the seas will come from offshore wind, followed by marine aquaculture and fish processing. The report also projects an increase in industrial capture of wild fish.

This latter suggestion alarmed Rachel Jones, a marine expert from London Zoo, ZSL. She told BBC News: "Given that 90% of global fisheries are either at or in excess of sustainable catch levels, I can't really see how they are going to expand capture fisheries."

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