Major commercial fish stocks around the world could collapse within decades as global warming compounds damage from pollution and overfishing, UN officials said.

"You overlay all of this and you are potentially putting a death nail in the coffin of the world fisheries," the head of the UN Environment Program, Achim Steiner, told a phone-in news conference in Monaco.

His agency released a report Friday detailing new research on how rising ocean surface temperature and other climate changes are affecting the fishing industry. It says more than 2.6 billion people get most of their protein from fish.

The research sheds new light on an undersea flushing mechanism that helps renew fish stocks in three-quarters of the world's primary commercial fishing grounds. Report author Christian Nellemann said global warming is disrupting this circulation.

"If this mechanism stops, we may risk a collapse in major fishing grounds" in the coming few decades, he said.

While threats to fish supplies from pollution and overfishing have been well-documented, Nellemann said this was the first time the combined effect of this and changing temperatures has been closely studied.

"We are seeing shifts in marine life that we have never observed before," he said.

He said deep-sea shrimp in the western Mediterranean were among species under threat. The report says that some shellfish that had thrived in once warmer water in the Atlantic have moved as much as 1,000 kilometers (600 miles) north in recent decades.

Still, the report says fisheries could recover from all these problems - if countries reduce global carbon emissions and shipping pollution, and stop overfishing and damaging fishing practices such as bottom trawling.

The Worldfish Center, an international research organization, has also warned that global warming could undermine fisheries, but has been more cautious in its estimates. It said in a recent report that global warming could also create opportunities, such as allowing farmers to use flooded areas no longer suitable for crops to cultivate fish.

Nellemann also said more than 50 percent of the world's coral reefs could die by 2050 because of bleaching caused by higher ocean surface temperatures, based on climate projections by international scientists.

(China Daily February 23, 2008)