NatureScot Research Report 1316 - Guidance and recommendations for native oyster enhancement projects in Scotland

Year of publication: 2024

Authors: David Donnan (NatureScot), Cass Bromley (NatureScot) and Flora Kent (Marine Directorate)

Cite as: Donnan, D., Bromley, C. and Kent, F. 2024. Guidance and recommendations for native oyster enhancement projects in Scotland. NatureScot Research Report 1316.

Keywords

native oysters, Ostrea edulis, restoration, enhancement, best practice guidance

Summary

We have produced this native oyster guidance to assist anyone considering a European native oyster enhancement project as there are legislative, policy and procedure considerations which are specific to Scotland. Please note that to avoid any confusion, “native oyster” throughout this document refers only to the European native flat oyster, Ostrea edulis. In publications from beyond Scottish waters, “native oyster” may refer to other oyster species.

This guidance should be used alongside the Scottish Marine and Coastal Enhancement Framework resources, which provide generic guidance and can also be accessed via Marine Enhancement | NatureScot. The Native Oyster Restoration Alliance (NORA) and the Native Oyster Network (UK & Ireland) also provide detailed technical and practical guidance for native oyster enhancement projects. In addition, The Scottish Code for Conservation Translocations provides an underpinning framework for the development of an oyster enhancement project in Scottish waters.

The guidance is presented in three sections:

1. Background to native oyster conservation and status in Scottish waters, including threats and risks to the species;
2. Native oyster restoration guidance, including site selection, licensing, biosecurity and monitoring;
3. Key recommendations for native oyster restoration.

When developing a native oyster enhancement project, it is important to contact the Marine Directorate (formerly Marine Scotland) and NatureScot at the earliest possible stage to seek advice. Contact details are listed below, together with contacts for other authorities who also have responsibilities related to this type of activity. Early communication can help address queries around project feasibility, the inherent risks associated with any shellfish translocation, and identify the consents that may be required.

It is the responsibility of restoration practitioners to ensure that they have sought advice, acquired the appropriate licenses and permissions before starting a project and throughout the project, and have developed an effective biosecurity management plan.

Background

NatureScot’s role in relation to native oyster enhancement

NatureScot has the following key roles to play in relation to native oyster enhancement in Scottish waters. There is a balance to be struck between developing suitable enhancement projects whilst ensuring that activities do not adversely affect existing populations and other protected features.

• We have an obligation to conserve Ostrea edulis as a Priority Marine Feature (PMF), both as a species and as a habitat (native oyster beds)
• NatureScot is also the competent authority for the licensing of species translocations, per the Scottish Code for Conservation Translocations (Scottish Code for Conservation Translocations)
• We also have a role in supporting and advising the Scottish Government, other competent authorities, and all stakeholders involved in the marine environment. In the context of enhancement/ restoration, this is focused on supporting responsible and appropriate activities aimed at meeting biodiversity and climate objectives.

We have therefore developed this guidance to reflect these considerations, with the aim of assisting prospective restoration projects to plan their work accordingly. It has also been developed on the basis of NatureScot fulfilling our responsibility to protect and conserve Scotland’s existing native oyster populations.

The status of native oysters in Scottish Waters

Scotland once had a widely distributed, and in places abundant, native oyster population living in suitable habitats around the coast. Hand gathering was the main form of collection for thousands of years, with oyster shells being found in middens dating back to the Palaeolithic to Neolithic periods (from ca. 10,000 years BCE to ca. 2,500 BCE).

With the dawn of the industrial revolution from ca. 1750 CE came increasing demand for cheap food for the growing urban populations. A combination of technological advances in vessel and fishing gear technology, together with steam trains providing quicker routes to market, led to increased rates of exploitation that rapidly became unsustainable. As well as harvesting for consumption, another driver of overexploitation was collecting oysters, often juveniles or smaller adults, for re-stocking dwindling populations elsewhere. Within a relatively short period, many fisheries became economically unviable and in some areas, native oysters became locally extinct.

Scotland’s Marine Assessment 2020 provides a background to native oyster populations in Scottish waters including distribution, historic trends and drivers of change. This shows that scattered and, in many cases, small populations remain around the west coast and islands. The evidence and knowledge base for the extant west coast populations is incomplete and there are ongoing efforts to improve our understanding of their distribution, abundance and health. The assessment also shows that our east coast populations have been extirpated. The University Marine Biological Station UMBSM (2007) report gives further detail on the status of native oysters at that time, including considerations for restoration. Shelmerdine & Leslie (2009) investigated suitable areas for restocking native oysters around Shetland. As well as identifying areas for restoration, the study found that historical evidence of fishing grounds was not a suitable basis for locating present day stocks.

In Scotland, the responsibility for Ostrea edulis and Mytilus edulis (blue mussels) passed from the Crown Estate to Scottish Ministers in 2014. Confirmation of the requirements for permissions or licensing with respect to collecting, cultivating, relaying or restoring either of these species must therefore be sought from the Marine Directorate.

Conservation of native oysters in Scottish waters

As a result of concerns about the trends in their distribution and abundance in Scotland and across Europe, the native oyster is a species of conservation importance. This status is reflected in Scottish waters in a number of ways.

Native oysters and their habitat are a Scottish Priority Marine Feature (PMF) and are included in a list of the 11 PMFs regarded as being most vulnerable to seabed disturbance. The National Marine Plan General Policy GEN 9b provides protection to PMFs through a requirement to ensure that development and use of the marine environment does not have a significant effect on their national status. In practice, this means that a PMF Assessment needs to be completed with NatureScot during the consenting of activities that may impact on wild native oyster populations, irrespective of whether that activity is taking place within or outwith a Marine Protected Area (MPA).

Native oysters are also protected as part of the Scottish MPA network, for example as a protected feature of the Loch Sween Nature Conservation MPA. The MPA Network in Scottish inshore waters is comprised of European Marine Sites (originally designated under the Habitats Directive (92/43/EEC) and the Birds Directive (79/409/EEC) and transposed into Scottish Law via The Habitats Regulations, together with Nature Conservation Marine Protected Areas (NCMPAs) and Sites of Special Scientific Interest (SSSIs). Under Part 5 of the Marine (Scotland) Act 2010, “Marine Protection and Enhancement: The Marine Protection Area”, conserving a feature may include “enabling or facilitating its recovery or increase”.

Any activities within MPAs, including native oyster restoration work, need to be assessed in relation to their potential impacts on the designated features of the site. The type of assessment required depends on the type of designation. For example, activities within European Marine Sites require a Habitats Regulations Appraisal (HRA). Restoration practitioners would need to ensure the project management plan monitors both the restored native oyster population and the protected features of the site. Further information on MPAs and individual sites within the network can be found via NatureScot MPAs and SiteLink.

The Wildlife and Countryside (Scotland) Act 1981, as amended by the Wildlife and Natural Environment (Scotland) Act 2011, prohibit the release of non-native species into the wild. The legislation therefore provides a mechanism to protect native oysters from non-native species that could be detrimental to native oyster populations, such as American oyster drills (Urosalpinx cinerea) and slipper limpets (Crepidula fornicata). There are also situations under this legislation and associated with conservation translocations where the reintroduction of native oysters may require licensing from NatureScot. All restoration practitioners should seek advice from NatureScot to check if this is needed for their project and follow the Scottish Code for Conservation Translocations as best practice.

Within the wider North-East Atlantic context, native oysters are included in the OSPAR list of Threatened and/ or Declining Species and Habitats as the European flat oyster, Ostrea edulis. This recognises the ecological significance of native oysters and the habitat they create for themselves and other species. This includes providing important nursery and foraging habitat for mobile species, together with provision of habitat for a wide range of sessile organisms and infauna. OSPAR define Ostrea edulis beds as having densities of ≥ 5 per m² on shallow, mostly sheltered sediments (typically 0 to 10 m depth, but up to 50 m). The OSPAR recommendations for the conservation of native oysters include investigating the habitat’s distribution, extent and quality; increasing the knowledge base and awareness of O. edulis; and addressing and minimising adverse impacts on this feature. In areas where pressures have caused the decline or disappearance of O. edulis, and where practical and adequately managed, OSPAR recommends restoration through seeding or translocation of adults from pathogen and parasite free donor populations.

Conservation aims for Scotland’s native oysters

Existing populations of native oysters on the West coast and islands of Scotland are of critical value for the survival of the species in Scottish waters and are a resource of fundamental importance for future enhancement and restoration efforts. Therefore extant populations of native oysters require protection from actions that may negatively impact them.

Whilst the precise natural range for this species is difficult to define due to the lack of robust, long-term monitoring data, where it is understood that native oysters have been extirpated (mainly on the East coast of Scotland), restoring them to their natural range may therefore be appropriate.

Based on the conservation legislation outlined in the previous section, there are therefore two conservation outcomes for native oysters in Scottish waters:

1. To protect and enhance the remaining populations on the west coast and islands.
2. To restore the species to its natural range on the North Sea coast.

These conservation aims are focused on strengthening native oyster populations and building their resilience to human threats and climate change throughout Scottish waters. They also assist with meeting wider aims through OSPAR, for example.

Native Oyster Restoration Guidance

Setting out clear aims, objectives and goals for any active restoration project is key to its potential for success. In addition to this specific guidance for native oyster enhancement, The Marine and Coastal Enhancement Framework Guidance materials provide generic information on policy, licensing and biosecurity aspects, as well as details on existing native oyster restoration projects in Scotland. These can be accessed via Marine Enhancement | NatureScot.

The Scottish Marine Environmental Enhancement Fund (SMEEF) has developed a Restoration Toolkit which draws on the Marine and Coastal Enhancement Framework and provides a resource for enhancement practitioners, including the ability to create and tailor a checklist of the steps to be followed and contacts for their particular project.

When is it appropriate to restore native oysters?

The primary aim of enhancement/ restoration of native oysters in Scottish waters should be to assist with meeting the conservation aims for the species/ habitat, along with building resilience for native oyster beds and their associated biodiversity. It is important that any interventions are appropriate, necessary and responsible.

Where there is an existing population of native oysters in a proposed enhancement project area, this represents a decision point for the project officers. The first priority must be to conserve these. Leaving these populations undisturbed may therefore be the most appropriate conservation action. Retaining some undisturbed populations is important to help understand population dynamics and other aspects such as effects of climate change, species resilience regarding risks of the introduction of disease and Invasive Non-Native Species (INNS), retaining genetic/ local adaptations, and providing reference populations against which the success of enhancement attempts elsewhere may be measured.

Promoting natural recovery through, for example, removing pressures could assist native oyster populations. This approach can also enable research into gaining a better understanding of the processes related to recovery. However, in some areas the existing native oyster beds may be unable to recover naturally. This could be due to extirpation, low population densities, distance between individuals and from other populations, changes in environmental conditions, or the availability/ quality of suitable oyster settlement habitat, for example. The need for intervention in these cases must be carefully assessed, and thorough baseline surveying is key to deciding on the best course of action to be taken. In areas where native oysters are no longer present, then restoration may be an appropriate action to take and would be consistent with the aims of the second conservation outcome of reintroduction to their natural range.

It is important to factor in when considering any actions that recovery is expected to be slow, even in areas where there are oysters present. In all cases, long-term monitoring will be needed. Active restoration can require repeated interventions and significant investment of time and funds if it is to have the best chance of success. Recovery or establishment of a native oyster habitat is expected to take decades.

Restoration in the past was largely aimed at shoring up dwindling fisheries. If regenerated populations do eventually become self-sustaining, some projects may consider supplying stock to other restoration projects or sustainably harvesting native oysters for market, for example to fund further restoration efforts. Whilst this can be a valid objective for restoration, it is worth considering that the value of native oysters in terms of the provision of ecosystem services such as nursery habitat, habitat for biodiversity, sediment stability, water quality improvement, nutrient cycling and cultural services, could be much greater than that from restored commercial harvesting. Claims around carbon sequestration within shellfish habitats need to be approached with caution but there may be some benefits to be derived from these habitats. Commercial concerns therefore should not be the primary motive for restoring native oysters but rather might be in certain circumstances a secondary, long-term project objective of some community projects, for example.

To assist with deciding if it is appropriate to attempt to actively restore native oysters in a particular location, please refer to the following flow diagrams. The first is relevant to all locations, irrespective of designation. The second is specifically tailored for potential projects within MPAs. Further information on MPAs and individual sites within the network can be found via NatureScot MPAs and SiteLink.

We suggest that to decide if the location has a large population of native oysters, would-be restorers should survey and establish the following:

• are the oysters only scattered individuals over a wide area or < 10 oysters in the survey area (small population)? or
• are the oysters more clumped, densely aggregated within the survey area (large population)?

When determining if the oysters are close together during a survey, we suggest assessing whether the oysters in the area are scattered (> 1m between individuals) or clumped (aggregated) together (< 1m apart). This information is important as widely dispersed populations of few individuals are more likely to be impacted by Allee effects and suffer from impaired recruitment, negatively influencing the population’s ability to recover on its own.

When determining if there is connectivity, would-be restorers will need to know if there are any other native oysters in the same bay or local area. An understanding of local hydrography would also be helpful in determining this. For example, are there currents that might bring larvae from another nearby population into the restoration area or could a restored population feed larvae into other populations in the area? This is also an important consideration when assessing risk from INNS and diseases. When surveying, any suspicion of their presence should be identified, recorded and reported (see section on Biosecurity). Small, isolated populations may be recruitment-limited and less able to recover on their own. In that case, it would be necessary to consider the possible impacts on local genetic structure from bringing in oysters of other origin as this may affect local adaptations that have occurred as a result of genetic drift.

Further discussion and guidance on assessing a potential restoration site follow in the next section (see section on Site Selection).

Full size image of the diagram

Site Selection

As can be seen from the flow diagrams, the location of a potential native oyster restoration project will depend on a number of factors. Circumstances will vary depending on aspects such existing activities and relevance to protected sites and species. Particularly important will be the proposal’s implications for any existing native oyster populations. Consequently, the initial phase of any proposal would be expected to include an understanding of the site. This may involve a review of existing native oyster information and/ or field survey results. Such information is essential for developing suitable objectives for the restoration proposal, to inform the consenting process (see section on Consents, Licences and Planning Permission), and to help identify the most appropriate enhancement approach. As already highlighted, an active restoration project may often not be the most appropriate conservation action to take. 

Site selection within the proposed location is an important stage of any native oyster restoration project. Research conducted on historical and existing native oyster populations throughout their natural range and within the area can provide useful information. Understanding native oysters’ biology and requirements and the wider ecosystem, together with gathering as much baseline data as possible are key to the success of a project. The growth, survival and reproduction of native oysters are highly influenced by a range of physical, chemical and biological factors; many of which are subject to daily, seasonal, annual and inter-annual variation. All these factors need to be taken into consideration, along with the risks posed by diseases, INNS, predators, pollution and disturbance. Where there are no oysters present, even though they may have been present historically, it is not sufficient to assume that a location will still be suitable. Ground work will still be needed to determine the suitability of present day conditions.

NORA have published a native oyster restoration site selection checklist including the consideration of abiotic and biotic factors and providing logistical advice. The web resource NMPi contains a lot of the necessary information about existing activities and protected sites. NatureScot and the Marine Directorate can advise on information needs and potential sources of additional data.

Please note that publicly available location data for wild native oyster populations are redacted for their protection from illegal harvesting and access to the un-redacted data is restricted to minimise risk. Access to the un-redacted data may be granted by NatureScot under licence for specific purposes. Applications for this are evaluated on a case-by-case basis.

When selecting a site for restoration, relevant factors to take into consideration include:

1. Protected status of the area. SiteLink provides information regarding a protected site and its features:

• Is the potential project to take place in a site that is part of the Scottish MPA network?
• If so, what are the protected features of the site?
• What management measures are in place that may be relevant to native oyster restoration?
• Are there any other PMF species or habitats recorded in the proposed site? PMF Assessments are required if so.
• What impacts could enhancement of native oysters have on these features, especially if the project will involve introduction?
• Is there connectivity with other protected areas in the wider region?
• In addition to landowner permission for access to the proposed site, NatureScot may need to provide permissions for example for activities in an SSSI. For NCMPAs, a MPA Assessment would be required. For European Marine sites (SACs and SPAs), an HRA would be required. NatureScot Area staff will be able to guide projects on these aspects and therefore early contact with the appropriate Area Office relevant to the proposed site is recommended.

2. Is there any current, historical, and/ or recent information about native oysters in the area? This can range from anecdotal observations to written reports or survey data. Evidence of native oysters may include the presence of old or recent native oyster shell or larval settlement (be aware that the saddle oyster, Anomia ephippium, can easily be mistaken for native oyster spat). If there is an existing native oyster population in the area, then advice should be sought as to what, if any, enhancement action is appropriate.
    
3. What activities and pressures occur in the area?

• Are there any existing marine planning or management measures that may be relevant to the project, including those that may assist with removing pressures from the proposed enhancement site?
• What activities take place in and around the proposed site? Examples include commercial and pleasure boat traffic, moorings, anchoring, fishing (mobile and static gear, sport), fish farms, shellfish and seaweed aquaculture.
• Do any of these activities place pressure on existing native oyster populations in the area?
• Could any of these activities impact positively or negatively on any proposed enhancement works?
• The Feature Activity Sensitivity Tool (FeAST) can assist in identifying pressures in protected areas

4. Are the site conditions suitable for native oysters? Historical evidence of native oyster presence in an area does not necessarily mean that a site is still capable of supporting them.

• Abiotic factors include depth; temperature; salinity; turbidity (amount of suspended particles in the water); substratum (seabed composition e.g. muddy, sandy, sandy gravel, shell, etc.); current speed; exposure (in the context of wind and wave action e.g. sheltered, exposed; location (e.g. intertidal, low intertidal, subtidal); emersion time (if intertidal).
• Biotic factors include food supply (e.g. plankton, detritus, dissolved and particulate organic matter – turbidity measurements can assist with evaluating these aspects); species including native oysters present in the proposed site; presence of native oyster predators and scavengers (for example starfish, oyster drills and other predatory marine snails); and presence of INNS or pathogens.

5. What is the shellfish disease status of the area? Are there any restrictions that may apply to the area, for example because of known presence of Bonamia, a notifiable disease-causing pathogen of Ostrea edulis? Information on this can be found on the Fish Health Inspectorate (FHI) website or by contacting the FHI.
    
6. Are there any known INNS already present in the area? Do any of the baseline site surveys identify the presence of INNS? If there are, the project must seek advice from the Marine Directorate and NatureScot before carrying out any activities. The presence of some species may limit/ diminish the chances of success of enhancement work or even make it unfeasible to carry out work in that location. The GB Non-Native Species Secretariat and National Biodiversity Network can assist in finding information on species that may be present around/ within the area of the proposed site.
    
7. Site access – there might be some constraints on access to a potential site and therefore projects need to investigate this early on in the planning stages.

• Is the site accessible and safe for carrying out planned activities and long-term monitoring?
• What permissions does the project need to access the land and shore at the site? (e.g. from the landowner (who may be Crown Estate Scotland) or a private estate/ individual)

Consents, Licences and Planning Permission

Most native oyster restoration projects will require some form of licence/ consent. It is the responsibility of project personnel and best practice to check with the competent authorities if any planned activity requires specific permissions/ licences. The requirement for consents, licences or planning permission will vary depending on the location and the methods to be employed. Projects also need to plan in sufficient time for these processes. Early contact with the relevant authorities is therefore advised. It is always better to seek permission for an activity even if it turns out not to be needed.

It should also be noted that this applies not only at the start of a project but also throughout the life of the project as activities and scale develop. Other than where indicated, the Marine Directorate is the main contact for licensing of activities in Scottish waters.

The following points cover the most common requirements: 

• A native oyster enhancement project may require authorisation and registration as an aquaculture business in Scotland.
• A project may also need to lease the seabed from the Crown Estate Scotland.
• Projects should also check if plans involve a ‘licensable marine activity’ under the Marine (Scotland) Act 2010. Depositing materials such as shell cultch from a vessel, for example, requires a marine licence under this Act. Some activities, however, are exempted from requiring a marine licence and are described in The Marine Licensing (Exempted Activities) (Scottish Inshore Region) Order 2011 . The Marine Directorate’s Licensing Team (MD-LOT) evaluates the need for a marine licence on a case-by-case basis. Guidance for restoration projects is available via their Marine Licensing Restoration Project Supplementary Guidance webpage. Once a project has details of the proposed site, methods to be employed, etc., then they should contact MD-LOT to confirm whether or not a marine licence is required for the activity.

If the area covered by a marine licence exceeds 1000 m², this may trigger a pre-application consultation, under The Marine Licensing (Pre-application Consultation) (Scotland) Regulations 2013. This is a minimum 12-week process which takes place before the actual marine licence application can be submitted. A period of up to 14 weeks should then be allowed for marine licence application processing. If the site is located within a European marine site, then this process would also trigger an HRA.

Projects should also consider what is outlined in the OSPAR Guidelines on Artificial Reefs and treat it as part of the guidelines for completing an application.

• Movements of shellfish, including native oysters, require consent from the Fish Health Inspectorate.
• Wild native oysters should never be collected for any purpose without a permit from the Marine Directorate.
• In cases where re-introduction/ translocation of native oysters is part of the project plan, especially when relocating native oysters outwith their native range, licensing from NatureScot may also be required. NatureScot’s Licensing Team can advise when this is necessary and please also refer to the Scottish Code for Conservation Translocations. As mentioned earlier, this code should be adopted as best practice by all projects involved in conservation translocations.
• It may also be necessary to contact your Local Authority to enquire whether Planning Permission is required for the proposed project (for example, where oysters are to be kept in baskets/ nurseries at sea or on the shore).
• If any of the project’s activities include extraction or emission of water or plans for weathering/ storing cultch on land, then SEPA should be contacted regarding advice and licensing requirements.
• Other permissions e.g. SSSI, protected sites assessments. For advice around this, please make early contact with your local NatureScot area officers or, alternatively, our Marine Enhancement team can assist with putting you in contact with them.

Biosecurity

Biosecurity must be an integral part of any native oyster restoration project. A large part of the value of native oyster habitat is its ability to support a large diversity of other species. An inherent risk of moving oysters is that, without appropriate biosecurity protocols, “passengers” can be transported with them. Whilst the above photograph shows native species which form part of the typical community associated with native oyster habitat, live oysters (and other shellfish) and shell (cultch) can also easily transport INNS and pathogens.

Biosecurity risks should be assessed on a case-by-case basis with local considerations taken into account. Advice should be sought from the competent authorities as early as possible during project planning.

For general information and good practice advice for biosecurity considerations, see the European Guidelines on Biosecurity in Native Oyster Restoration and the NatureScot Marine Biosecurity Planning guidance

The Scottish Code of Practice on Non-Native Species should also be consulted.

• A detailed Biosecurity Plan is an essential part of the development of a native oyster restoration project, and with respect to disease risk may be a requirement for consent. The biosecurity plan should outline how the risk of spreading shellfish diseases or INNS will be mitigated and/ or avoided.

For INNS, a key consideration is their presence at a proposed site. Surveying for INNS should form part of the initial baseline data collection and ongoing monitoring of any project.

Another key risk area is the source of the stock. The biosecurity plan should set out how stock is handled and how risks are mitigated throughout the process from source to deployment. Projects should satisfy themselves and be able to provide evidence/ confidence that sources such as hatcheries have good biosecurity.

The biosecurity plan will need to be adapted throughout the life of the project, for example to reflect changes in activities or methods.

• Native oysters from disease-free status areas are in demand for supplying stock for restoration projects around the UK. One of the contributing factors historically to the decline of native oysters throughout their range was the taking of stock to replenish populations elsewhere. It is therefore necessary to protect these donor populations from overexploitation and it is not possible for them to provide stock for all projects. Projects also need to be mindful that INNS may be present in these areas and there are considerable biosecurity considerations to be overcome in any translocations. Developing a robust, biosecure, reliable supply chain is key to active restoration, especially for scaling up from pilot to wider scale projects.
• Whilst much attention is focused on the risks of transferring diseases and INNS between geographical areas, restorers also need to be aware of the risks within the planned project area. For example, if INNS are present at the proposed restoration site, deployment of cultch and increased vessel movements or other disturbance (e.g. due to monitoring) could increase the risk of non-native species spreading to wider areas. Cultch and structures could act as a stepping-stone for these species, and vessels and personnel may become vectors for their movement. This therefore also needs to be factored into developing the biosecurity plan for a project.

Genetic Biosecurity

The IUCN definition of biosecurity includes protecting the genetic integrity of extant populations. The biosecurity plan should therefore also consider native oyster genetics.

A genetic analysis of native oysters by Beaumont et al., 2006, examined native oysters from 10 sites in Scotland, The Netherlands, Brittany and Norway and found four main genetic groups: (1) Norway; (2) The Netherlands and Brittany; and (3) Scotland (apart from (4) Skye). However, this research used only six microsatellite markers and the distinctiveness of the Skye population could be partly due to an artefact of small sample sizes. The results suggest that human aquaculture activities over recent centuries have probably diluted any original local genetic differentiation within Scotland. However, genetic differentiation still exists at the wider scale across Europe, and O. edulis is known to develop local ecomorphs where populations are adapted to their specific local environmental conditions.

Understanding potential connectivity with other populations in the wider area around a proposed restoration site is therefore important. This helps consider whether in some isolated populations there may have been sufficient time since the last human interventions to have enabled these types of local adaptations to develop.

The origins of hatchery broodstock also need to be taken into account and projects should explore questions such as whether their offspring will be suited to the site’s conditions. It is important also with hatchery stock that broodstock is managed to reduce the risk of inbreeding. Wild native oysters must never be collected for any purpose without a permit from the Marine Directorate.

Monitoring

Baseline surveys described in the Site Selection section can feed into the long-term monitoring plan to assess change over time and the impact of restoration efforts.

Detailed guidance on monitoring a native oyster restoration project can be found in the NORA European Native Oyster Restoration Monitoring Handbook including recommended monitoring metrics, techniques and sampling designs. A native oyster restoration project in Scottish waters should have a detailed monitoring plan. Key information should include:

• The project specification, including site selection rationale.
• Details of how the project will be monitored towards achieving the restoration objectives.
• If the project is within an MPA, the plan must also include details of monitoring of the native oysters and the protected features of the site.
• The biosecurity plan addressing risks around both shellfish health/ disease and INNS species should detail how these will be monitored.

Recommendations

1. Active intervention such as restoration may not be the best course of action for native oysters at certain sites. Where native oysters exist naturally at sufficient densities to sustain the population, these areas should instead be protected and monitored.
2. Native oyster restoration and enhancement projects should have clear aims and objectives. Being clear about what the project is setting out to achieve from the start will guide the methods used to achieve these goals, and, for example, the type of permissions/ licences needed.
3. Projects should engage with the Marine Directorate, NatureScot, and other competent authorities as early as possible, and certainly before carrying out any work.
4. Native oyster restoration and enhancement projects should have a detailed biosecurity plan for all activities and stages of the project. This plan will need to be updated on an ongoing basis as the project develops.
5. It is important if translocating oysters that these are pathogen, parasite and INNS free before deployment. In addition to a biosecurity plan, projects should adopt the Scottish Code for Conservation Translocations as best practice, follow FHI guidelines, and have the required licences/ permissions.
6. Native oysters should never be moved or translocated without first seeking advice and permission from the Marine Directorate, FHI, and NatureScot.
7. Wild native oysters must not be collected for any purpose without a permit from the Marine Directorate. It is also recommended that native oysters should not be removed from existing beds for transplanting elsewhere, owing to the risk of depleting already vulnerable populations and the risk of moving diseases and INNS species to new locations.
8. A good understanding of the receiving environment is required prior to restoration activities taking place. This includes understanding the status of the species and habitats in the area (including their conservation status), with particular attention to existing native oyster populations.
9. Baseline surveys and site suitability should be assessed using a combination of historical information, modelled outputs and directed surveys of biological and physical parameters.
10. Further information on site selection, practical considerations and monitoring for native oyster restoration projects can be found on the NORA website. For further information on lessons learned and guidance specific to Scotland, please see NatureScot's Marine & Coastal Enhancement Framework Guidance and Report.