How we took decisive action when we found evidence of disease among oysters at two upwellers
Last fall, as we prepared to plant our oysters from the Gloucester upweller, we had some unexpected and unpleasant news. Before oysters can be transferred from the nursery to Massachusetts waters, they require routine laboratory testing for common diseases, and this year, some of our oysters tested positive for a little-known disease called Neoplasia. In order to protect our Mill River restoration site, we destroyed the Gloucester and Marblehead crops of oysters. The rest of our oysters (Nantucket and Hyannis) were disease-free and were planted as normal.
The oyster planting process and our rigorous testing regime
Before anyone can plant shellfish from a nursery like our upweller in the wild, they need to get the shellfish tested for disease. This protects wild and farmed shellfish populations. In 2020, testing for Mass Oyster was conducted by the Haskin Shellfish Research Laboratory at Rutgers University. Typically about 100 individual oysters are sent for testing, and these oysters are dissected and analyzed for a number of diseases. If the samples pass the tests, the state’s Division of Marine Fisheries, as well as the town shellfish constable or warden, must permit the transplant before it can go forward. Our previous two successful crops from Gloucester (over 120,000 total) went through this testing and received a clean bill of health. This year, neoplasia was detected in some of the samples sent to Rutgers.
Neoplasia and shellfish disease
Disease is a serious threat to oyster populations and the oyster industry. Neoplasia can be a transmissible disease also described as shellfish leukemia. It’s usually associated with soft-shelled clams, not oysters. In New England, the diseases of most concern to oysters are Juvenile oyster disease, Dermo, Multinucleated Sphere Unknown (MSX) and Seaside Organism (SSO). These diseases - Dermo and MSX in particular - have long loomed over the oyster farming industry. The strict testing regimes required for transplanting shellfish, as well as routine and ad hoc monitoring of all designated shellfish growing areas, are in place to protect the industry, human health, and ensure the health and safety of oysters.
We did the only thing that could be done: destroy the oysters raised at its Gloucester and Marblehead hatcheries. This can’t be sugar coated - after months of labor to keep the upweller tanks clean and functional, and financial investments in purchasing the seed and maintaining the upwellers, this was incredibly disappointing for us and all the volunteers involved with the project.
From the hatchery to the upweller
Growing oysters is more akin to ranching than farming, given that growing oysters doesn’t require inputs. But before oysters are growing on the tidelands or in open water culture, they start off in hatcheries unless growers are collecting what’s known as natural set.
Hatcheries use select oysters to spawn millions of larvae. Each of those larvae attach to a piece of hard substrate, like the shells of other oysters, and start developing a shell themselves and growing into what we all recognize as an oyster. These new sets oysters can only stay at the hatchery for a short period. They are voracious feeders and they start to grow very quickly. Hatcheries have techniques that force oyster larvae to grow as single oysters rather than in clumps. Just weeks after attaching, oyster larvae have grown to the point where they are ready to be shipped off to eager customers who order this “seed” by the hundreds of thousands.
Those customers, whether commercial oyster growers, nonprofits or municipalities, have a choice to make. If they plant the oyster larvae immediately, it will be susceptible to predators. Most will continue to grow the seed to a larger size in a more protected and regulated nursery (like our upwellers). When they reach the grower’s target size, oysters are then planted in grow bags, cages, or even directly on-bottom. The larger those outplanted oysters are, the greater their chances of survival are. After that, growers take a largely hands-off approach to growing them to adulthood, which might take 1 to 4 years depending on the area and oyster species.
Because of the close conditions oysters grow in, disease is a major concern for hatcheries, commercial growers, and organizations dedicated to restoration. We reported our Neoplasia to the hatchery, and their subsequent tests didn’t yield any evidence of neoplasia among their stock.
So where did our oysters get a disease from?
Several months later, we still don’t know the source of the disease from our oyster crops. Our hatchery has tested other oysters from the same spat and hasn’t found anything.
It’s possible that there were neoplastic oysters near the water intake for the upwellers, which could have transmitted the neoplastic cells to our spat.
It’s also possible that this disease is linked to the environment the oysters developed in. As waters warm due to climate change and there are extended periods of warm water conditions, oxygen levels can decrease. Although more research is needed, it is believed that there is a connection between low oxygen levels and neoplasia.
Plans for 2021
Without knowing definitively the source of the neoplasia, we plan to conduct a dive survey of the areas surrounding the upwellers to look for contaminated shellfish. We’re also taking extra precautions. This year, we’re purchasing seed from a different hatchery. We’re also making additional efforts to reduce stress in the upweller tanks and closely monitoring water quality parameters. We plan to test a double set of oysters prior to release next year.
There’s always a possibility that testing will show a repeat problem, but we are optimistic that 2021 will be successful and our North Shore upwellers will help introduce over 100,000 native oysters into Massachusetts waters.