Tilghman Islanders Grow Oysters is the name of the volunteer oyster restoration program that we manage here at the Phillips Wharf Environmental Center. This program is under the larger Marylanders Grow Oyster program, managed by the Oyster Recovery Partnership. As part of this program, we receive bags of spat on shell from the Horn Point Oyster hatchery. The bags are split up and put into oyster cages which are then delivered to our volunteer oyster growers. The oyster cages hang from docks around Tilghman Island and throughout Harris and Broad Creeks. After about 9 months or so, the oyster cages are picked up by our staff and volunteers and planted in the Harris Creek Oyster Sanctuary, per the request of the Department of Natural Resources.
We typically receive the spat on shell in the fall of the year; this year we received it at the end of October. This year was unusual because we paid for bags of spat on shell for the first time. This is because the Oyster Recovery Partnership lost a significant chunk of their funding, which caused a reduction in the amount of spat bags they were able to purchase and provide to sub-programs like ours at no cost. This year, ORP determined that they were only able to provide 40% of the bags that were provided last year.
This affected programs throughout the state, causing many programs to pay for spat or reduce the size of their program. While Phillips Wharf did not reduce the number of volunteers participating in our program, we did choose to cap the number of cages volunteers could host and we also did not push to expand the program as we have been attempting to do, post-COVID shutdown. We also purchased a number of bags of spat on shell so that we were able to provide all the cages requested by our volunteer growers.
So, while receiving fewer bags of spat on shell was not the start we had hoped for the 14th season of the Tilghman Islanders Grow Oysters program, the program is on track. Over the last week of October and the first day of November, all oyster cages were delivered to our volunteer growers. In addition to delivering cages, we collect size data on the oysters so that we have a rough idea of what size the spat are, on average, when they arrive into our care. To do this, we take a sub-sample of about 10% of the spat bags and measure all the baby oysters on the inside of the shells in those bags. Of that sub-sample, we also put together measuring bags with 100 spat each to measure throughout the growing season and see how the oysters are doing.
Based on our initial sub-sample, the oysters we received were an average of 8.94 mm in size to start. This number varies from year to year and depends on how long between when the baby oysters settled and became spat to when we received them, in addition to how warm it is. Baby oysters grow faster in warmer months (up to a certain point) and the ideal temperature for oysters in our region is 77 to 79°F. We have observed through this measuring program, that the growth of our oysters begins to slow down below 65°F and stops completely around 50°F.
Since we received our oyster spat so late in the year, we didn’t see a huge amount of growth in the first month after the cages were placed with our volunteer growers. This also makes sense based on the water temperatures we observed at our 1 month measuring point, which were between 44 and 53°F. We saw a general increase of about 3.6 mm on average across all our measuring bags and we expect most of this growth occurred in early November before water temperatures dropped.
At our 1 month measuring, we also begin recording biodiversity data, which is where we track the variety and abundance of other species that are in with our oysters. Some of these are animals that grow on the oyster shells like barnacles, mussels, and lacy bryozoan. It also includes animals like bristle worms, fish, and shrimp. We also observe oyster flatworms, which prey on the oyster spat. While these are normal members of the oyster reef community in the Bay, our diamondback terrapins also think they are delicious so they are usually eaten, rather than left alone to wreak havoc on our baby oysters.
As our oyster growing season continues, we’ll likely take a hiatus from measuring during the coldest months of the year as we know the oysters aren’t growing during this time. However, as the water begins to warm up in the spring, we’ll begin measuring the oysters again to track their growth over time. If you enjoy hearing about how the oyster spat are doing, let us know and we’ll be sure to include an update on them in one of our Spring 2025 Heron Herald newsletters!
Yes. Please keep us updated. This is fascinating to learn about the development of oysters. I appreciate your writing & caring for the Fishmobile.