[MARRAKESH, Morocco] When nearly all the young oyster crop died two years in a row at shellfish farms in the US Pacific Northwest, workers at first suspected a virus.

But the real culprit was a new worry: a change in the acidity of the sea water feeding the oyster tanks.

As the world's oceans absorb carbon dioxide that is building up in the atmosphere, seas have become 30 per cent more acidic than they were before the industrial era, said Carol Turley, a senior scientist at Britain's Plymouth Marine Laboratory.

The increasingly corrosive water threatens a wide range of sea life, particularly shellfish such as oysters and scallops, making it hard for them to form and maintain shells.

Warming of the world's oceans, as they absorb rising heat associated with climate change, also is killing coral reefs and driving more fish species toward cooler seas and away from the regions where they have traditionally lived and been caught, Ms Turley said on the sidelines of the recent UN climate talks in Marrakesh.

Another effect of warming is a reduction in the amount of oxygen in the sea, threatening fish, said Ulf Riebesell, a German ocean researcher who works on acidification, among other problems.

"The ocean is under a major challenge. It's not only heating up, it's also acidifying and losing oxygen. The three stressors come simultaneously and they play out worldwide," he said in Morocco.

That is fuelling new challenges for both rich and poor communities around the world, from small-scale fishermen who can no longer bring in a catch, to conservationists watching fish move out of hard-won reserves, and coastal and island states fearful their tourist industries will collapse with their ailing reefs.

"It's happening too fast for organisms and ecosystems to develop strategies to cope," said Hans Pörtner, a scientist with the German-based Alfred Wegener Institute and a contributor to the work of the Intergovernmental Panel on Climate Change.

"There's a high risk of losing up to 90 per cent of coral reefs in a 1.5-degree Celsius warmer world by the end of the century. This is a system that has already gone beyond its tolerance limits," he said.

Rapidly cutting planet-warming emissions is the surest way to deal with the problem and limit potential damage, scientists say. But with a global shift to clean energy happening more slowly than is needed so far, fishing communities around the world will have to find ways to adapt to the changes - and some are already trying out ideas, scientists say.

At oyster farms in the US Pacific Northwest, for instance, monitors now check the acidity level of ocean water coming into the tanks of young oysters. If levels begin to rise, as a result of upwellings of acidic ocean water, intakes are shut off or can be adjusted to draw water from different levels, Ms Turley said.

At least one big oyster operation, after major losses due to ocean acidity in 2007 and 2008, shifted production to Hawaii, where upwellings of acid water are less of a problem, she said.

Poorer fishing communities may be able to adopt early warning systems too, said Ana Queirós, a marine ecologist at the Plymouth laboratory.

For instance, they could quickly harvest a bigger number of fish if satellite monitoring of currents showed warm, low-oxygen water that could kill fish moving toward fishing grounds, she said. "These are real-life adaptation measures - though they are also temporary solutions," Ms Queirós said, particularly if more of the oceans become regularly inhospitable for fish and other sea life.

Ms Turley believes poor communities that risk losing their reefs could also turn to "carbon farming" by growing and harvesting seaweed, which takes up ocean carbon.

Turning seaweed into anything from food to drugs, fertiliser and roof thatching could help bring in an income to supplement or replace fishing, she said, and communities may be able to earn money from carbon credits as well.

"We've been looking at impacts (of climate change), but now we also need to look at solutions, for both the short term and the long term," she said.

Mayrah Shaltout of Morocco's National Institute of Oceanography and Fisheries said coastal communities in Africa are already seeing declines in fish catches - and not just because of competition from industrial trawlers.

Two thirds of the countries most vulnerable to fishing declines are in Africa, she said.

In other parts of the world, Atlantic cod are moving north toward the Arctic, seeking cooler waters, while young barnacles off the British coast are gradually moving north as well, Ms Queirós said. "The species will go if they can," she warned. "Science tells us the changes are going to happen, and they're going to be much more prevalent than what we're seeing now."

REUTERS