Monaco - A deadly combination of climate change, over-fishing and pollution could cause the collapse of commercial fish stocks worldwide within decades, said Achim Steiner, head of the United Nations Environment Program.

"You overlap all of this and you see you're potentially putting a death nail in the coffin of world fisheries," Steiner told reporters on Friday on the fringes of a climate conference involving more than 150 nations and 100 environment ministers.

Some 2.6 billion people worldwide depend on fish for protein, said a UNEP report "In Dead Water" published on Friday.

Climate change has compounded previous problems such as over-fishing, as rising temperatures kill coral reefs, threaten tuna spawning grounds, and shift ocean currents and with them the plankton and small fish which underpin ocean food chains.

"The question is not whether we should stop fishing but to address climate change, which is creating a degree of impact we've not seen before," said lead author of the UNEP report, Christian Nellemann.

"We are getting more and more alarming signals of dramatic changes in the oceans. The recovery from the changes we're making will probably take a million years."

The report found the most affected areas included those responsible for half the world's fish catch.

A slowing of ocean currents as a result of climate change may over the next 100 years interrupt the transport of nutrients to the most valuable coastal fishing zones, and the flushing away of pollution.

In other impacts, Nellemann said he expected more than 50 percent of coral reefs to die by 2050 as a result of rising temperatures, with resulting impacts on tourism.

Carbon emissions from burning fossil fuels create an acid when dissolved in water, and could over the coming decades make the sea more acidic than at any time in the past 65 million years, and by 2100 could prevent mollusks in some seas from forming shells.