When Yves Perreault looks out over the pristine waters of Desolation Sound, where his family annually harvests half a million oysters, he fears for the future of the ocean – and the industry that supplies Canada with half its shellfish.

Something is killing oysters and scallops in dramatic numbers, causing suppliers to warn of shortages and producers to worry about the future of their businesses. The cause is unknown, but ocean acidification is the main suspect.

"It's a remote area, the water is clean … we haven't had any environmental concerns, so I'm not sure what's going on," said Mr. Perreault, who owns Little Wing Oysters and is president of the BC Shellfish Grower's Association.

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Over the past two years, Mr. Perreault's oyster farm on B.C.'s south coast has experienced 80 to 90 per cent mortality of young shellfish – the normal attrition rate is 50 per cent – and last year, nearby Pendrell Sound had a massive die-off of wild oysters.

"It was in the billions," he said of the Pacific oysters that died only a few months after they hatched.

"It's hard to say without having somebody there monitoring what's going on. It could be food related. Maybe there were too many oysters and there was not enough food and they just starved – or something else [is happening] in the water like the acidity level," he said. "To be frank, we don't know a lot about it and that's what's scary."

Mr. Perreault routinely monitors the ocean for food abundance, temperature and salinity – but thinks he should test the pH level too, to keep track of how acidic the water is.

The Vancouver Aquarium has been doing just that – and its records show the pH level in Vancouver's harbour steadily declining, from 8.1 (1954-74) to a low of 7.3 by 2001.

A pH unit measures acidity with a range of 0-14. The lower the value, the more acidic the environment.

The U.S. National Oceanic and Atmospheric Administration has noted a direct correlation between rising levels of C02 in the atmosphere and levels in the ocean. As more C02 accumulates in the Pacific, the pH decreases and the acidic level rises.

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Sophia Johannessen, a research scientist with the federal Institute of Ocean Sciences in Sidney, said it is clear oceans are becoming more acidic.

Asked if ocean acidification is to blame for oyster die-offs – and the recent collapse of scallop stocks in a Vancouver Island operation – she said: "I'm not sure yet. … We need to know if there is some local problem."

Dr. Johannessen said waters off the coast of B.C. are getting warmer and there has been a change in the timing of zooplankton blooms, which shellfish eat. She said a shortage of food, or increased temperatures, could have put shellfish under stress, and then a slight change in pH could knock them out. Chris Harley, a zoology professor at the University of B.C., feels the same way.

"It's an interesting puzzle. … I'm not sure what's killed all those scallops out in the Strait of Georgia. … It might have been low pH, but I'm not sure we can say that with much confidence," he said.

But Rob Saunders, CEO of Island Scallops, says he has tracked pH levels closely and sees a link between increased acidity and shellfish die-offs.

"I'm convinced the ocean is getting much more acidic, and much more acidic than anyone anywhere believed it could happen that fast," he said.

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Mr. Saunders' operation has lost 10 million scallops over the past two years, and smaller companies have had similar problems. Mr. Saunders is pushing for a research project to find out what's happening. "Is it a disease? Is it just strictly C02 stress or acid stress? If we don't figure it out, then we don't have an industry," he said.

Guy Dean, vice-president and chief sustainability officer at Albion Fisheries Ltd., one of BC's biggest suppliers of fresh seafood, said the scallop die-off has rung alarm bells. There is still a supply of B.C. oysters, but local scallops are a rarity now.

"It's definitely a sign. It's like the canary in the coal mine," he said. "That is the early indicator of climate change and how it is going to affect the availability of various products."

Canada produces more than 12,000 tonnes of oysters annually, worth more than $18-million, with B.C. responsible for 60 per cent of the crop. Nationally, the yearly production of scallops tops 58 tonnes, with B.C. accounting for 76 per cent.

Editors' note: Sophia Johannessen is a scientist at the Institute of Ocean Sciences in Sidney, B.C., not Nanaimo, as incorrectly stated in an article published online on Feb. 27 and in the print edition on Feb. 28.