What Do Oysters Eat?

Key Takeaways:

• Location Is Everything for Flavor: Oysters eat whatever drifts in on the tide, and we can’t change the menu.
• Tidal Buffet: With farmed shellfish, there’s no feed to source, no protein conversion to manage, and no bait fishery being overharvested to keep the operation running. The tide comes in, the oysters eat, the tide goes out, and we harvest.
• Water Quality Is an Environmental and Business Concern: Anything that improves stormwater runoff or water treatment also improves the local oyster industry. The economics and the ecology are aligned, which makes this industry unusual.

The usual answer is that oysters eat plankton.

Diatoms in the spring, and dinoflagellates in the fall! Here we have some magnified phytoplankton and zooplankton, aka oyster food [1]. 

Some tasty looking diatoms...

...and a few microscopic monsters. (Although the green dude on right is clearly a plant).

The word 'plankton' derives from a Greek word meaning “to wander” and encompasses a big group of stuff that drifts around in the ocean. Microalgae is a common type, and if there’s enough algae in the water you can see it, like with this noctiluca bloom a few years back [2]:

The more algae in the water, the more food for the oysters (usually). If the water is super clear, it’s generally Christmas Dinner at the Cratchits.

To rephrase our question and ask, "what do you feed oysters?" illustrates why shellfish farming is sustainable: We don't feed them anything [2.5]. The tide comes in and the oysters eat. A few hours later, the tide goes out and we harvest. This repeats twice a day, every day, in a cycle older than dirt. There's no getting up at 5 am to milk the cows. No arranging someone to toss hay to the goats when we leave the farm. No overharvest of a bait fishery, or worries about protein conversion rates. Shellfish farming requires "nothing more than seawater and sunshine."

This is not to say that we don't think about what they eat. We do, pretty much constantly. Diet determines their flavor, growth rate, seasonality, shell strength, safety, and even color. Some kinds of algae are harmful to humans [3]. An oyster can starve in an estuary full of the wrong kind of food. We wonder if the coccolithophore we've had the past three years imparts a chalky flavor, and suspect the blooms are interfering with oyster reproduction [4]. It's just that we can't control what they eat... we can only change our farming and harvest practices in reaction.

Left, hungry, wee-bit skinny. Right, well-fed oyster, fat and happy. These photos were taken on the same day, but the oysters are from different estuaries.

But if you only say "oysters eat plankton," you're leaving out a lot of the story. An oyster food diary would show that they also consume dissolved organic material like leaf litter from upstream forests or small pieces of sea grasses [5]. This is one reason why location is so important to oyster flavor: they are what they eat, and they can't move to greener pastures. It's also why economics and environment are closely linked on an oyster farm. Anything that cleans up storm water runoff or improves water treatment facilities protects both the shellfish industry and the overall health of the ecosystem.

Final answer: oysters are sedentary filter feeders that dine like well-behaved children: they'll eat what they get, and they're [usually] happy about it. [6]. 

Q: What’s for dinner? A: Food.[7]

 

Notes

1. These photos come from a microscope on loan from Sound Toxins, a Sea Grant program that works as an early warning system for harmful algal blooms. They were taken by our good pals and former colleagues Tiffany & Brady.

2. It’s red! It’s a tide! IT’S NOT RED TIDE. Noctiluca is harmless and super cool… it’s bioluminescent, baby… and the real bad boys on the street (alexandrium, psuedo nietsche, etc) aren’t red. Who named it red tide?

2.5. Jumping in here to say that oysters are fed as juveniles in hatcheries. Much of the work in a hatchery centers around growing algae in tanks, and getting the food mixture just right for happy mommy, daddy, and baby oysters. Also, about the tides: know that not every place on earth has two tides a day. It's pretty complicated. Read this beautiful book to learn more!

3. We're fortunate to farm in an incredibly clean estuary in Washington State, which has an extensive monitoring program to warn shellfish communities about toxic algae blooms far in advance.

4. Total guess. But we’re sticking with it until someone informs us otherwise. And also: these cocco blooms, and the fact that our grandpa never saw one, illustrate one small part of the human struggle: It’s hard to tell if things are scary changes or normal [long term] trends. (This is why we need science.)

5. We could list some unappetizing stuff here, alongside this poetic flora, but we’re not gonna.

6. Of course, the obvious drawback to this is that sometimes it pays to be a picky eater. For more info read about the history of the New York oyster industry. Moral of that story: trust your gut.

7. Direct quote from Hama Hama Oyster Mama the 2nd, circa 1991.

What a Well-Fed Oyster Actually Tastes Like

Diet is where things get personal. A fat, well-fed oyster, one that’s been sitting in a productive estuary with strong current and plenty of microalgae, will have a plump, firm belly, good liquor in the shell, and real flavor complexity: sweet up front, briny in the middle, with a clean finish. The belly and the liquor are two of the first things we look at when assessing a harvest, and Oyster Anatomy breaks down what each part tells us and why they look different depending on where the oyster ate.

A hungry oyster tells a different story. The meat shrinks away from the shell, liquor runs sparse, and whatever flavor the growing location should deliver gets muted. That’s one reason harvest timing matters so much. Pulling an oyster before the algae blooms have had a chance to fatten it up means sending out something technically alive but underwhelming.

What This Means for the Oysters You're Eating

An oyster's flavor is a direct record of its environment. The algae, the organic matter, the water temperature, the current, all of it moves through the oyster and ends up, in some form, in what you taste. A Hama Hama grown at the mouth of the Hamma Hamma River tastes the way it does in part because of the cold, spring-fed water and the forest upstream of it. A Blue Pool grown in a different part of our beach, with slightly different current exposure and a different algae mix, tastes noticeably different. We're not entirely sure why, and we're not sure anyone is, which is one of the more humbling things about farming something you can't fully control. From that perspective, anything we do to protect water quality isn’t just environmental management, it's flavor management, one tide at a time.

Frequently Asked Questions About An Oyster’s Eating Habits

How often do you feed the oysters?

Ha ha - trick question! We don’t feed the oysters. Oysters feed continuously in live seawater when they're submerged. On a beach farm like ours, oysters are typically underwater for 12 to 18 hours a day. 

Oysters grown subtidally, below the low tide line, have access to food more or less around the clock, and that’s a big reason they tend to grow faster. The tradeoff is that they need more tumbling and management to develop a good shell shape, without the natural toughening that comes from air exposure at low tide. More time submerged means faster growth, but the shell needs more hands-on shaping along the way. 

Can oysters eat too much? Is overfeeding a thing?

It’s possible. Oyster seed is fairly hard to keep alive, and sometimes we suspect that too much algae in the water can stress the seed, causing it to die. But in general, overfeeding isn’t something we have time to worry about much. 

Does what oysters eat change by season?

It does, and it's one of the more interesting variables in oyster flavor. Algae populations shift seasonally with water temperature, daylight, and nutrient levels, which means the food available to oysters in March is different from what's on the menu in August. Warmer water typically brings more algae, which means faster growth, but it also brings greater variability in algal types, including blooms we'd rather not see. Cooler months tend to produce slower-growing oysters with denser, more flavorful meat, which is part of why fall and winter are generally considered peak eating season for raw oysters on the West Coast. We pay attention to these shifts not just for safety reasons, but because they tell us what the oysters taste like.

Does what oysters eat affect whether they're safe to eat raw?

Yes, and shellfish monitoring programs exist for a reason. Certain types of phytoplankton produce toxins that accumulate in the oyster’s tissue. The oyster isn’t harmed, but the human who eats it can be. 

Washington State’s monitoring program tests the water and shellfish regularly, closing growing areas when toxin levels run too high. Oysters can filter water at a remarkable rate, and that sensitivity to the water column is precisely why the monitoring matters. 

Why do oysters from different places taste so different if they're the same species?

The answer comes down to diet, and diet comes down to location. A Pacific oyster grown in the clear waters of Hood Canal will have less to eat than one grown in the algae-rich waters of South Puget Sound.