Last month, we kicked off our Tilghman Islanders Grow Oysters program for the 2020-2021 season. The oysters that were placed with volunteer growers in September are now growing rapidly in their new homes. How do we know?
Well, earlier this month, we trained some of our growers to measure their oysters so that we can keep track of their growth over time. We also have oysters hanging from the dock at Phillips Wharf and Black Walnut Point Inn that we measure each month. As part of this program, we will do a Facebook Live video each month where we walk through measuring our oysters, talk about our oyster restoration program, and discuss some of the critters we find in with our oysters. Eventually, when we reopen our campus to the public, we will host a session once a month where anyone can join us in person and be a part of measuring our oysters. We already streamed our first measuring session, which you can check out on Facebook.
In October, we found a number of different critters in our oysters, including a blue crab, lots of mud crabs, grass shrimp, oyster toadfish, striped blennys, barnacles, mussels, and what may be oyster flatworms (as identified by some of our followers on social media). The oyster spat, across all areas sampled, grew an average of 9.1 millimeters (about ⅓ of an inch) between September and October. This month, the average size of our oyster spat was 27 mm, which is just over an inch in size. As the water temperature drops, we expect the monthly growth of these oyster spat to slow down, so we probably won’t see this much growth in a single month until spring.
These oyster spat were provided by the Marylanders Grow Oysters program and produced by the Horn Point Hatchery in Cambridge, MD. Because of this, we don’t know exactly when the oysters settled, but it was likely sometime in the summer, based on their rapid growth. The general rule of thumb for wild oysters is that they grow about an inch per year (probably slightly faster when young and then slightly slower as they get larger). However, these oysters are nowhere near a year old and have already reached an average of one inch in size, suggesting that oyster cages provide enhanced growth of oysters. We hypothesize that this is due to a couple reasons. One, the relatively few number of oysters in nearby proximity may mean more food for these oysters (and therefore less competition for that food). Two, because the cages hang in the water, the oysters may receive better water flow (which means more food) than wild oysters growing on a stationary item such as an oyster reef or riprap. Three, because our growers occasionally shake the cages to remove mud and other particulate matter, the oysters are cleaner and again, more likely to have better water flow and access to food than wild oysters.
In addition, the cages provide some protection from predators, which means more of them should survive to adulthood than wild populations. From September to October, the number of oysters that died was relatively high. This mortality rate was 0.14, which means 14% of the oysters we put out to measure died in one month. However, we think this will be one of the highest mortality rates that we see this season. This is because of the stress put on the oysters when we prepare them to go to our growers, which includes being tossed around a bit and being out of water for several hours. In addition, we expect to see some mortality when oysters are moved from one part of the Bay to another. Even though the Horn Point Oyster Hatchery is not very far away from the Bay Hundred area, we still expect to see differences in salinity, temperature, and the phytoplankton (oyster food) populations in the creeks and coves where our oyster cages are placed. Those differences can be stressful for the oyster spat, causing a lot of stress after that transition.
Ultimately, the data we collect is shared with our growers, so they have an idea of what is going on with their oysters. We also look at this data to determine best practices for our oyster restoration program. Are there areas that allow for better growth and survival of the oyster spat we place with volunteers? If so, then we can focus on recruiting volunteer growers in these regions as we add new participants and grow our program.
If you are interested in learning more about our program or signing up to grow oysters for the 2021-2020 season, please visit our website.