If you follow food issues, one of the topics that surfaces again and again is how exactly we’re going to feed a planet expected to exceed 9 billion hungry mouths by 2050.

Experts talk about crop yield and study ways to increase animal production. Others predict future food shortages will eventually require us to adapt to a vegetarian lifestyle. Mmmm … kale.

But an oft-overlooked part of the sustainable protein conversation is aquaculture. This has less to do with farmed bluefin tuna and farmed Atlantic salmon — which both have serious sustainability issues — and more to do with oysters, clams and mussels.

Bivalves are the darlings of the sustainable seafood world. They’re filter feeders that clean the water as they grow. They don’t require supplemental feed. No pesticides or antibiotics are used in their cultivation. There are virtually no issues with escapement. They’re a delicious, affordable source of protein. Every way you look at it, they could become an important part of the solution of how to feed the planet. There’s just one problem with all of this: It’s called ocean acidification.

The world’s oceans are changing. Not only are sea levels rising and ocean water getting warmer, but the sea has been acting as a huge sponge, soaking up our excess carbon dioxide.

Unfortunately, it’s being dissolved into the ocean, making our waters acidic, and it’s happening at an alarming rate. For oysters, and other creatures that make calcium carbonate shells, the ocean’s changing pH levels mean tiny larvae have a hard time creating the shells they require to survive.

Right now, this change is primarily visible in the Pacific Northwest (although seafood-watchers on the East Coast are worried too — the Washington Post just published this story on how acidification is affecting Maryland’s treasured blue crabs). Hardest hit are oyster companies like Taylor Shellfish Farms and Whiskey Creek Shellfish Hatchery that produce the seed aqua-farmers up and down the West Coast use to grow oysters.

Scientists are beginning to take a closer look off our own San Diego shoreline.

“We’re not immune to ocean acidification. The pH of our waters offshore are changing here as well,” said Dennis Peterson, biologist and director of science at Carlsbad Aquafarm, which produces the Carlsbad oysters that appear on raw bar menus and at dollar oysters nights across San Diego. (They’re also the only oyster farm in Southern California.)

While Peterson says ocean acidification hasn’t caused massive oyster die-offs at the Carlsbad farm yet, he is worried about the company’s mussel production.

“The byssus thread is what I’m really worried about. I lose a lot of product in the summer already, and if it gets worse, I could see that as a real problem,” he said.

That byssus thread is the fiber mussels produce to cling to rocks. As mussels mature, they get heavier. When the byssus thread is weak, it makes it hard for the bivalve to stay attached to its growing surface. Whether pH changes in the water are impacting the development of the byssus thread is still uncertain, but Peterson said there’s cause for concern.

“It’s important that we tease apart what’s ocean acidification, what’s global warming and what’s La Nina,” he said.

To help solve the mystery, Peterson connected with Gretchen Hoffman, a marine scientist from the University of California, Santa Barbara. Hoffman and her team have been placing monitoring equipment up and down the coast of California to track ocean acidification levels.

“We want to learn what’s going on in the water at economically important sites,” she said. “We want to understand which species are more resilient, and to identify those that are sensitive. Without a doubt, San Diego is an important region for us.”

While ocean acidification may make it difficult for oysters to form shells, mussels to form healthy byssus threads or may eventually impact species like rock fish or other economically important fisheries; some species like sea urchin, seem to be more resilient. At least so far.

The California Ocean Protection Council has formed a working group to monitor the effects of ocean acidification and to develop recommendations, similar to the way Washington state created a Blue Ribbon Panel on the issue, Hoffman said.

“We’re trying to do for California what Washington did, and to understand the science,” she said.

Clare Leschin-Hoar is a contributor to Voice of San Diego. Follow her on Twitter @c_leschin or email clare@leschin-hoar.com.

Clare Leschin-Hoar is a contributor to Voice of San Diego. Follow her on Twitter @c_leschin or email her clare@leschin-hoar.com.

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