Tide of acid-ocean fear rolls over oyster industry

Growers rely on wild oysters, which typically grow in clusters like this. Third-generation shellfish farmer Brian Sheldon now must turn to oysters started in hatcheries. Photo by MCT.

The collapse began rather unspectacularly.

In 2005, when most of the millions of Pacific oysters in this tree-lined estuary failed to reproduce, the shellfish growers of Willapa Bay, Washington state largely shrugged it off.

In a region that provides one-sixth of the nation's oysters - the epicentre of the West Coast's $US111 million ($NZ160 million) oyster industry - everyone knows nature can be fickle.

But then the failure was repeated in 2006, 2007 and 2008.

It spread to an Oregon hatchery that supplies baby oysters to shellfish nurseries from Puget Sound to Los Angeles.

Eighty percent of that hatchery's oyster larvae died, too.

Now, as the US oyster industry heads into the fifth summer of its most unnerving crisis in decades, scientists are pondering a disturbing theory.

They suspect water that rises from deep in the Pacific Ocean - icy seawater that surges into Willapa Bay and is pumped into seaside hatcheries - may be corrosive enough to kill baby oysters.

If true, that could mean shifts in ocean chemistry associated with carbon-dioxide emissions from fossil fuels may be impairing sea life faster and more dramatically than expected.

And it would vault a key Washington industry to the centre of international debate over how to respond to marine changes expected to ripple through and undermine ocean food webs.

Scientists seeking to explain what's plaguing these coastal oysters say the link to more corrosive water is strong but anecdotal.

It could be just one of several factors.

But the possibility leaves some shellfish farmers uneasy about more than just their future business.

Indications that ocean acidification may already play a role in the decline of oysters are a "sign of things being out of balance, and that scares the living daylights out of me," said third-generation oysterman Brian Sheldon.

Ruffling his 8-year-old son Jebediah's head, he added, "For this guy."

Pacific oysters aren't native to Willapa Bay, but shellfish growers have farmed them here since the 1920s.

It's about the only place left on the West Coast where growers look to the wild to get their oysters.

Normally, oysters spawn in the water, producing larvae that swim and eventually attach to a hard surface - typically other oyster shells.

This creates oyster seed, called a "set".

These succulent molluscs are then moved by hand throughout the bay and take two to five years to fatten up.

But somewhere between the larval stage and settling on a shell, these embryonic oysters are dying.