An FOS sponsored Martha’s Vineyard Shellfish Group Project
Oyster Bar Experiment
Sengekontacket Pond, like most other ponds on the island, is struggling with high nitrogen levels and dirty water. Sengekontacket unfortunately happens to be one of the most impaired ponds on the Vineyard. To combat this issue, we came up with a proposal to use a natural solution to help filter and clean the pond.
In recent years, the benefits of oysters have become more well known and recognized. The towns shellfish departments have already invested into growing the oyster population in Sengekontacket, and we proposed an experiment somewhat different from that of the towns. Read below to learn about our proposal, the results of the project, and the benefits of oyster propagation.
Proposal: to investigate the potential to develop small oyster bars in several areas in the upper reaches of Major's Cove.
Similar to projects already underway by the town shellfish departments to culture single oysters in the pond, this project would increase the numbers of oysters in Sengekontacket.
It differs somewhat from the existing efforts in that rather than planting single oysters to be harvested, the goal would be to eventually establish larger groupings of clustered oysters that would remain largely unharvested with the hopes that they would reproduce and become a self-sustaining population.
Clustered oysters growing in a bar offer additional ways to reduce nitrogen through bacterial denitrification and burial in their underlying sediments. Clustered oysters also have the added benefits of providing marine habitat for numerous other species within their nooks and crannies. We also believe an established oyster population in the rich waters of the upper reaches of Majors Cove may help to mitigate nutrient inputs and reduce algal blooms before they work their way downstream to the larger pond. In addition, this proposal will examine the effectiveness of planting younger, smaller oysters cemented on shell (spat-on-shell) as a less costly and labor intensive alternative to planting larger, older, single oysters.
Shellfish, especially oysters, are being extensively deployed in eutrophied estuaries in efforts to reduce nitrogen levels. Shellfish reduce nitrogen levels through several major pathways:
Sequestration in tissues: Nitrogen (a building block of protein) is incorporated in the shells and soft tissues of shellfish as they grow and removed from the system when they are harvested. The amount of nitrogen that can be removed through harvest is relatively easy to measure and quantify and does not vary too much in time and space. Both wild and aquacultured shellfish sequester nitrogen and the harvest of either can result in nitrogen reductions. http://www.sciencedirect.com/science/article/pii/S0025326X16310761
Bacterial denitrification: Under anaerobic conditions, certain bacteria are capable of converting the nitrate in seawater to nitrogen gas, returning it to the atmosphere and thereby reducing the available nitrogen in the water. While generally recognized as a significant pathway for nitrogen reduction, it has been difficult to measure and the amount of nitrogen reduction varies greatly. It is generally believed to occur more frequently in natural oyster reefs rather than with cultured oysters. https://www.sciencedirect.com/science/article/pii/S0272771414002789
Deep burial of biodeposits in sediments: Nitrogen available for eutrophication is also lost through burial in sediments. When they filter feed, shellfish release fecal pellets and mucoid pseudo feces that fall to the bottom sediments where under ideal conditions they are buried deeply and no longer pose a threat to water quality. As with bacterial denitrification, this means of nitrogen reduction is more likely to occur in a natural oyster bar than in aqua cultured oysters.
It has also been reported that organisms that co-inhabit natural beds of oysters play a significant role in nitrogen reduction. In many cases these species also filter feed and by their sheer numbers replicate the water quality enhancing effects of the oysters. Again, this added nutrient reduction is less likely to occur in a culture of single oysters. https://www.oysterrecovery.org/wp-content/uploads/2013/04/Oyster-Reef-Denitrification-Report.pdf
Further, oyster bars provide habitat and are attractive to an array of mobile organisms (shrimp, crabs, fish, etc.) that support a healthy marine ecosystem and likely assist in the export of nitrogen offshore into the larger food web.
Finally, new research suggests that potentially denitrifying bacteria inhabit the shell and gut of oysters that may further enhance their nitrogen bioremediation eco-services. http://www.vims.edu/newsandevents/topstories/2017/oyster_microbiome.php
The current status
In the previous project in 2018 we planted oyster spat-on-shell in four locations in the upper reaches of Majors Cove. The oyster spat was produced in the hatchery and transported to Majors Cove. Some of the shell was planted immediately on the experimental sites. The rest were grown in nursery cages to grow larger before being planted. We found that the spat-on-shell from the nursery cages had
better survival and grew faster than the oysters that were planted out directly from the hatchery. When the oysters were sampled in the fall of 2019, many of the oysters had spread from the original planted locations due to wind and wave action. Some of the smaller oysters had chipping around the edge of their shells indicative of crab predation. The growth and survival appeared to be better on hard bottom sites than those planted on soft muddy bottoms. The harder bottoms tend to have more water flow which provides more phytoplankton for the oysters. The soft muddy bottoms have less flow, less food,
more buildup of silt, and tend to be more acidic. The smaller oysters are more likely to sink into the mud of soft bottoms and suffocate.
Building on the results of the previous project, we propose to modify our methods to increase survival and growth of oyster seed planted in Majors Cove. We intend to increase the nursery period in both floating and bottom cages from one season to two seasons so that oysters are larger when planted. In our previous project, we found much of the oyster seed was scattered off the sites we planted it on due to wind and wave action. Lighter scallop shell was used for some of the oyster cultch in our previous investigation, and we intend to use heavier oyster shell cultch exclusively for this proposal. Growing the oysters in the nurseries for a longer period of time will result in faster growth and we expect that the larger oysters will have better survival from predation when planted out. Using heavier oyster shell cultch and the larger seed will also help keep the oysters from being moved by the wind and waves.
In 2020, the project focused on growing oysters to a larger size than in previous years, before planting them on the proposed sites. The larger oysters will a better chance of survival from predation. The oysters were spawned in summer of 2020, will be over-wintered in cages (2020-2021), and planted in fall of 2021.
Native oysters, from Edgartown and Tisbury Great Ponds, were spawned in early July and the larvae were kept in tanks at both the Vineyard Haven Solar Hatchery and Oak Bluffs Hughes Hatchery. Every other day the tanks were drained, the larvae were resized, counted, and put back into the cleaned tanks. Each day they were fed a steady diet of algae that was cultured in the hatcheries. After two weeks of swimming, the oyster larvae started their process of metamorphosis and were put into large setting tanks which contained bags of shell. The shell had been recycled from restaurants and raw bars across the island, by our Shell Recovery program. An estimated 1.5 million oyster larvae were added to the setting tank used for this project. The swimming oysters stick to the shell in the bags (thus spat-on-shell) and make for easy transport to the grow cages. The oysters were grown in the setting tank for three weeks before being deployed into Sengekontacket.
On August 6, 2020, with the help of the Oak Bluffs Shellfish Department, the 17 bags of spat on shell were successfully transported to Majors Cove and put into floating and bottom cages. They will grow in these cages until early September before being split into new bags and additional cages. Every few weeks the cages will be checked and air dried to reduce fowling. All bags of oysters will be held on the bottom of Major’s Cove for the winter, and brought back to the productive surface waters in the spring, to continue growing until September of 2021. At that time, they will be planted onto the oyster bars.