By Satoko Mills
About two hundred years ago, Olympia oysters (Ostrea lurida), native oysters in the SF estuary, were abundant. The average brood of larvae is between 250,000-300,000 (which by the way, we need a microscope to see larvae by our eyeballs) and they successfully grow up about 3-4 cm (equal to 1-1.5 inches. Some Olympia oysters are bigger, but on our reef balls, 3-4 cm. Yes, it’s a small oyster!) bivalve shells to thrive. However, during the Gold Rush era (19th century), pollution, over-harvesting, and habitat loss led to the collapse of Olympia oyster populations. (It is not endangered, yet)
Some organizations started Oyster Restoration but why the Olympia oyster is featured? Olympia oyster is the only species that is native to the west coast of North America and is famous as a “beneficial shell”. Which benefits they can bring? They can create shelters for other tiny creatures by making cluster-like reefs, generate cleaner water by filtering up to 12 gallons of water per day with their 3-4 cm body (which is amazing!), and reduce wave energy before it reaches land (you can imagen that the wave crush with oyster’s hard shells and the wave energy will be reduced). This beneficial shell attracts some researchers to target Olympia oysters about their populations, habitat, and abilities for their study, and some organizations to start oyster restoration projects in SF Bay.
In 2013, The Watershed Project (TWP), also one of those organizations who believe in Olympia oysters’ potential, deployed 100 oyster reef balls in the mudflat at Point Pinole since Olympia oysters need hard substrate to attach to grow up. Our volunteers made these reef balls with concrete, bay sand, and crushed oyster shells to install into the mudflat with the expertise of Dixon Marine Services. TWP has collected data and estimated the Olympia oyster population on the reef balls and salinity data for about 10 years.
We monitored the Olympia oyster twice a year, which is before and after the breeding season to see how their breeding was successful. Let’s explore the scientific world with our findings! First, have you seen Olympia oysters? Can you identify their life stage? It’s very easy to identify. You can assume closed oysters as “live oysters”, opened oysters as “recently died”, and one-shell oysters (missing the other shell, which is called “scar”) as dead oysters.
Oyster larvae are free-floating until they are attached to a hard substrate (like reef balls or rocks). When they find the hard substrate to settle down, they look light white-ish in color and they are called “spats” (It’s weird, but they look like spats)! Then they change yellow to orange in their young stage! Then, they change their color to gray in their adult stage.
Now, let’s look at the Olympia oyster population data that we have collected. You can see the population went up and down, which looks random (Figure 1). The oyster population is close to zero in some years, and current data (2023) shows a low population. Why has it changed so much? What is the factor impacting their population? Can you guess?
The answer is oysters don’t like low salinity, they are so intolerant of lower salinity! TWP has collected salinity data with the support of a long-term volunteer’s effort and the salinity usually goes down after heavy rain (about 2-3 days later). Our data showed the Olympia oysters on our reef balls tend to die when the salinity drops (Figure 2). Rain caused huge freshwater input from the sky and creeks to the SF Bay, and we expect that lower salinity might cause the oysters to die off as some researchers mentioned. If the salinity can recover quickly, it may be the better habitat for Olympia oysters, however, SF Bay may take some time to recover the salinity because it is a bay where the water is surrounded by terrestrial land and isolated.
Guides to see the graphs: You can find the oyster population was about 100 per reef ball this year while the peak population was 500 oysters in 2021 (Figure 1). What do you expect the salinity in 2021 and 2023? Yes, in 2021, the salinity was very stable and kept high (orange dots in the graphs) by summer. However, in 2023, the salinity dropped multiple times because of an exceptionally wet winter as you may remember (Figures 2 for 2021 and 3 for 2023)!
We need rain, and so do many creatures, but some creatures who live in high-salinity areas are very vulnerable to freshwater input. Oysters, rainfall, and salinity affect each other, and when the oyster population decreases, other tiny creatures who live under the oyster shelters might die due to the loss of their habitats. While the oyster population dropped in 2023, we found chunks of spats (you already know “spats”, these are young oysters), so we hope they will grow on the reef balls to build clusters and bring their benefits! We will keep monitoring the Olympia oyster population at Point Pinole to see how it will change.
Today, we are exposed to storms and/or devastating impacts of climate-related events. I am hoping that Olympia oysters may be nature-based great helpers to reduce wave energy to protect our place. We are always looking for adventurous volunteers of all ages to monitor the Olympia oysters on our reef balls. Please sign up here by choosing “citizen science” if you are interested. We will host an “Oysters and Birds Watching at Point Pinole” event in February. Please check our event calendar, event information coming soon.
Last, we are so lucky to have excellent community science volunteers every time, and with their support, we have collected long-term data. Thank you very much to those who attended in the past or were interested in this monitoring event. We all very much appreciate your work.
All graphs and photos credits: The Watershed Project
San Francisco Bay Subtidal Habitat Goals Project, Sea Grant California, Presidio, UC Davis Native Olympia Oysster Collaborative, A Guide to Olympia Oyster Restoration and Conservation