The pristine, sheltered sounds off Vancouver Island, British Columbia, offer the cleanliness and protection ideal for farming oysters, clam, scallops, and other shellfish. Since the 1970s, the industry has grown so rapidly that the area once supplied nearly two-fifths of Canada’s farmed shellfish and is the coastal community’s economic backbone.

But something is killing them off. In the last two years, nine-tenths of baby oysters have died in Desolation Sound farms (the normal mortality rate is about 50%). Scallop farmers off Vancouver Island have reported mass die-offs of their hatchlings since 2010. British Columbia’s share of Canada’s aquaculture industry is in a tailspin.

Further down the coast, the US’s $270-million Pacific Northwest shellfish industry is teetering (paywall) following the mysterious 2008 oyster die-off.

Scientists aren’t sure what the culprit is. Environmental stressors are rising, creating a complex interplay of factors. For instance, BC’s typically chilly coastal waters are warming—and that’s shifting the timing of zooplankton blooms, which in turn feed the shellfish. Scientists say seemingly slight ecosystem changes have likely compound the destruction of a deadly oyster herpes virus that has been wiping out oysters in France and Australia.

Though not all scientists agree, many suspect that one lethal factor is the the area’s falling pH levels, which have slipped from an average of 8.1 between 1954 and 1974 to 7.2 by 2001. (A pH of below 7 is considered acidic, with 6 about the acidity of urine and 5 that of black coffee.)

In other words, the sea is bathing shellfish in water that increasingly resembles acid, which deprives the water of the carbonate ions that baby mollusks need to grow tough shells. That likely leaves them more vulnerable to parasites and disease, says Curtis Suttle, a marine biologist and virologist at the University of British Columbia. A recent study on sea butterflies—tiny snail-like creatures that are important in the food chain—found them struggling to survive as a result of their shells being eaten away.

At the root of these changes are rising CO2 levels. Broadly, the ocean absorbs nearly a third of the carbon dioxide released into the atmosphere each year, say scientists. As CO2 levels rise, so do the amounts sucked up by the ocean, changing the seawater’s chemistry in the process. By 2,100, scientists predict the ocean will be 170% more acidic than it was before the Industrial Revolution. This will put commercial fishing stocks all over the planet at risk.

Earth System Research Laboratory, National Oceanic & Atmospheric Administration

But the Pacific Northwest is unusually vulnerable. Winds that gust south in the summer whip deeper water to the surface. Since CO2 tends to be trapped in colder depths for many decades, these upwellings are naturally more acidic.

The oysters grown off the Pacific Northwest aren’t native to the area; the lack of similar upwellings in their native Japanese waters leaves them poorly adapted even to these natural extremes in upwelling acidity. The additional acidity that comes from manmade CO2 now exceeds their ability to tolerate it (see Seattle Times reporter Craig Welch’s wrap-up of the science for more).

Many Pacific Northwest shellfish farmers have switched to raise baby shellfish in buffered hatcheries until their shells are strong enough to withstand the ocean. It’s probably not a long-term fix, though. The upwelling acidity likely comes from CO2 absorbed back in the 1960s, when atmospheric CO2 was less than 80% of today’s levels. Now, there’s a heck of a lot more where that came from.