by Erin Flattley (Intern at BH)
The Firth of Forth was once home to a thriving European flat oyster (Ostrea edulis) population, which was one of the largest oyster beds in Europe. However, overharvesting, combined with industrial pollution, led to the complete disappearance of the species by the early 20th century.
The Restoration Forth project has been successfully returning the species to the Firth of Forth sine 2022, with the first 3,000 oysters being deployed in September 2023, introducing a baseline population, which will hopefully become self-sustaining over time. [1]
So, what actually goes into the successful deployment and restoration of this keystone species, and how does the Forth Marine Hatchery play a part in this restoration?
Step 1 – Finding a suitable restoration site
The first step in successful oyster restoration is identifying suitable translocation sites, which requires consideration of a range of environmental factors. The Firth of Forth is a has a history as a site for successful oyster populations, however due to changes in water quality, pollution, and human activity- its not as simple as simply reintroducing oysters to the site and requires habitat suitability modelling and surveys to confirm suitability.
These habitat suitability surveys consider many factors and are important to site shortlisting alongside theoretical modelling. These surveys are completed through a range of techniques including physical surveys, video camera surveys, and intensive, more detailed SCUBA surveys.
Firstly, if an area is already designated as a “protected area” – meaning that it is managed for long-term conservation of a species, habitat, or ecosystem- it may not be possible to get permissions and pass legislation for new restorative activity. The site also needs to be suitable for our accessibility needs. In order for restoration work to be completed, staff and volunteers need to be able to access sites by boat or on foot in a safe manner for initial work and continued monitoring.
From a species-specific perspective, the site needs to be assessed for appropriate seabed material/ substrate. Oysters are more likely to grow successfully in a substrate with old oyster shell material. The Firth of Forth is successful in meeting this criterion, as this material can be found along this coastline, representing historic oyster beds, and indicating the chance for successful oyster growth.
The site also has presence of other bivalves such as horse mussels (Modiolus modiolus), which is another indicator of site suitability, as they hold a similar ecological niche. However, presence of other species such as those predatory towards oysters may be indicators of the lack of suitability of the site and are also considered during habitat suitability surveys and modelling.
Step 2 – Sourcing the oysters
Once the site has been selected, the oysters themselves need to be sourced. European Flat oysters are less commonly farmed than species like the invasive Pacific oysters (Magallana gigas), as they have slower growth rates and are therefore less profitable. Due to Restoration Forth not needing them to be of a size suitable for commercial demand, they can buy smaller oysters (from 2 years old), creating a supply model that offers oyster farms a way to make a profit while contributing to European Flat oyster restoration.
This supply model allows for a wider network of suppliers, thus diversifying the restored populations, and preventing inbreeding depression which can negatively impact the sustainability of future populations- which has been seen in Pacific oyster populations [2].
The oysters used in the Restoration Forth project largely come from the west of Scotland, either from farms, or on occasion from wild populations, such as those from Loch Ryan. Oysters released this past week are from a farm on the Isle of Barra, which is a trusted site which, as all sites of oyster collection, have strict biosecurity protocols to aid in restoration success.
These protocols are key to prevention of the spread of parasites and pathogens such as Marteilia refringens, a parasite that affects bivalve digestive systems [3]; Ostreid herpesvirus-1 (Oyster herpesvirus), a viral disease which causes mass mortality in oysters [4]; and Bonamia ostreae, a parasite that is particularly dangerous to European flat oysters [5]. However, these laboratory-based disease screenings aren’t the only important safeguards to safe translocation of the species.
Step 3 – Oyster Cleaning
With transporting oysters from the west coast of Scotland to the east coast, comes risk of movement of diseases and Invasive Non-Native Species (INNS). While fisheries are inspected and tested for diseases, cleaning of all individual oysters is incredibly important for maintaining that European Flat oysters are the only species being moved to restoration sites.
The cleaning of oysters takes a village, with over 30,000 oysters having so far been successfully deployed in the Firth of Forth. Volunteers are key to working through the vast number of oysters, working with Restoration Forth and Heriot Watt staff, including members of the Balanced Horizon team such as myself, Kerrie Flockhart (Scottish coastal cleanup project lead), and Jake Norton (Forth Marine Hatchery project lead), who has been particularly hands on in cleaning and deployment stages of the restoration process.
Oyster cleaning involves six steps, four of which volunteers assist with. First, the oyster’s species is checked, to ensure no pacific oysters have accidentally been mixed in with the target species. Following this, a scrubbing brush is used to remove any mud, sand on the surface of the shell and in the hinge of the oyster, where they are also assessed for any damage, such as broken shells, which may be a sign of death of the oyster or will prevent further growth of the individual.
Once the shell has its initial clean, any barnacles and keel worms uncovered are scraped off. While barnacles and keel worms aren’t invasive species themselves, they may harbour invasive species within them- so it’s important that any growth on the oyster shells are removed. During this stage, it’s important to keep an eye out for any INNS that may be present, such as sea squirt species (Didemnum vexillim, or Styela clava), or Slipper Limpets (Crepidula fornicate), as they can have immense ecological harm.
After a final scrub to remove any left-over debris, the oysters are taken by staff for chemical surface sterilisation. This step involves submersion with either 4% Formaldehyde, or 1% bleach to further prevent transmission of unwanted organisms. The oysters are then counted by staff, and placed in a five-day quarantine, in a UV sterilised tank. These final steps further reduce biosecurity risks, by cleaning any water retained within the oyster during initial cleaning. Then they are ready for deployment in their new home in the Firth of Forth [1].
Step 4 – Deployment
For their deployment, oysters are released in one of two ways. Either they are hand dropped off a boat for subtidal release sites, or they are released from the shore in intertidal sites. In both cases, they can be released either individually or glued to strings. Jake has been spending time with Restoration Forth dropping the cleaned oysters off a boat near Granton, Edinburgh, following his time spent as part of the team cleaning thousands of oysters at Heriot Watt University.
Restoration Forth in collaboration with Dornoch Environmental Enhancement Project (DEEP), the Marine Conservation Society and Heriot Watt University sometimes advertise opportunities for volunteers to get involved in this step of the restoration process, as well as oyster biosecurity and cleaning days- so keep an eye on the Marine Conservation Society Eventbrite for future opportunities.
Jake Norton – Releasing oysters into the Forth
Step 5 – Monitoring
Following their release, continuous monitoring of the deployment sites is vital to maintaining survival of new oyster beds. Growth of the placed oysters, and any subsequent juveniles, is monitored to ensure that they are not only surviving but thriving in their new locations. As well as monitoring the oysters themselves, their surrounding ecosystem is monitored to assess their impacts on ecological changes, and other marine species near the oyster site. This monitoring is done in similar ways to the initial habitat suitability surveys. Drop-down video surveys can help assess the seabed, while SCUBA/snorkelling allows for more detailed investigation of the oysters and nearby biodiversity.
Similarly to the cleaning stage of this process, volunteers and citizen science are vital to the continuous monitoring and success of this project. The Marine Conservation Society offers guides on how to get involved through assisting with habitat assessments, biodiversity monitoring near release sites, and direct monitoring of oysters, and washed-up shells along the Firth of Forth [6]. If you are interested in getting involved in monitoring for this project, and other related projects such as the seagrass project please go to:
Restoration Forth | Marine Conservation Society
While the Restoration Forth project is a highly successful project, the process is very expensive, requires great manpower and intensive labour, and is fairly small scale in terms of establishing a self-sustaining, native oyster reef.
This is why the establishment of a native oyster hatchery on the east of Scotland is so important. Through the development of the Forth Marine Hatchery, led by Jake Norton, we plan to massively upscale European Flat oyster production, to help alleviate the pressure of cost, risk of disease transmission and INNS spread, and supplement the work done by Restoration Forth.
“At the Forth Marine Hatchery, we aim not just for quantity, but for excellence. Our foremost goal is to produce healthy, native oysters for restoration. Because true success in marine restoration depends on quality, not just numbers.” – Jake Norton, Project Leader of the Forth Marine Hatchery.
[1] Restoration Forth: Our Story | WWF
[3] Marteilia refringens – GOV.UK
[4] Oyster herpes virus (OsHV-1) | Disease guide | The Fish Site