Recommendations for flat oyster restoration in the North Sea - ARK ...
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Hein Sas
Karin Didderen
Tom van der Have
Pauline Kamermans
Karel van den Wijngaard
Emilie Reuchlin
Recommendations for
flat oyster restoration
in the North Sea
Synthesis of lessons learned from the Dutch Voordelta experiments,
with additional observations from flat oyster pilots in Borkum Reef and
Gemini wind farm, modelling exercises and literature
April 2019
Commissioned by:
Karel van den Wijngaard (ARK)
Emilie Reuchlin-Hugenholtz (WWF)
Suggested citation: Sas, H. et al., Recommendations for flat oyster restoration in the North Sea, 2019. Report can be downloaded at: ark.eu/schelpdierbanken The shellfish reef restoration projects described in this report are sponsored by:
3
Recommendations for
flat oyster restoration
in the North Sea
Synthesis of lessons learned from the Dutch Voordelta experiments,
with additional observations from flat oyster pilots in Borkum Reef and
Gemini wind farm, modelling exercises and literature
April 2019
4
Table of contents
Summary 6
1 Introduction 10
1.1 Background and history 10
1.2 Objectives and scope of the report 11
1.3 Brief description of contents 13
2 Objectives and overall strategy of flat oyster reef restoration 14
3 Flat oyster life cycle and reef restoration steering factors 16
3.1 Flat oyster life cycle 16
3.2 Steering factors in oyster reef restoration 16
4 Pilots: locations and design 18
4.1 Introduction 18
4.2 Voordelta experiments and pilots 18
4.3 Borkum Reef pilot 21
4.4 Gemini wind farm pilot 21
5
5 How to restore a flat oyster reef: lessons from the Voordelta, Borkum Reef,
Gemini wind farm pilots and other sources 22
5.1 Habitat requirements to restoration pilots 22
5.2 Oyster sources and treatment prior to deployment 25
5.3 Seed population: age structure, condition, test population and extension 26
5.4 Stimulation of settlement 29
6 Standard restoration pilot design and monitoring 30
6.1 Introduction 30
6.2 Standard restoration pilot design 30
6.3 Monitoring 31
6.4 Recommended activities in subsequent pilot phases 32
References 33
6
Summary
Figure S.1 Locations of the three
pilots which are the basis of this
report, projected onto the oyster
reef areas (orange shadings) as
indicated by Olsen (1883).
The main objective of this report is to provide The key objective for these pilots is to The recommendations are limited to general
recommendations for flat oyster restoration ‘kick-start’ autonomously growing, or at least (i.e. non pilot-specific) factors governing flat
pilots in the North Sea. These recommendations self-sustaining, flat oyster reefs in the North oyster reef restoration.
are derived from three years of pilots and Sea. Of course, we are not there yet, so pilots
experiments with flat oyster (Ostrea edulis) are still necessary. The reason for restoring flat The recommendations are:
restoration in the Voordelta area, with additional oyster reefs is to stimulate biodiversity of the For location selection of flat oyster restoration
information from the Borkum Reef and Gemini North Sea ecosystem. pilots within the Dutch part of the North Sea,
wind farm restoration pilots, plus other relevant in probable order of importance:
studies and model exercises. Various sponsors The recommendations are based on our • Bottom disturbance caused by human
have supported these projects, but the most current knowledge of factors governing flat activities, such as bottom-trawling fishery,
important among these are WWF Netherlands oyster restoration. An important knowledge sand or gravel extraction, and construction
and ARK Natuurontwikkeling. gap is the critical surface area and density of work should be completely absent. Possibly,
a self-sustaining flat oyster start-population even noise and/or vibrations caused by these
Figure S.1 shows the location of the three pilots. in the North Sea. This can only to be solved activities are detrimental to flat oysters.
The underlying reports can be downloaded by more pilots and experiments at various • The sediment should be firm enough to
from www.ark.eu/schelpdierbanken. locations, accompanied by monitoring support flat oyster beds, i.e. fine or coarse
(‘learning-by-doing’). sand and gravel are suitable, whereas silt is
probably less suitable.
7
• Prolonged periods of oxygen depletion, as its success, as long as they are in good • Transport and storage before deployment
for example caused by thermal stratification, health at the moment of deployment. should be done in cool conditions and
should be absent. • In principle, young oysters (‘spat’) produced minimised in time.
• The sea floor should be sufficiently stable in a hatchery should be used for the seed • In any case: oysters should be tested for
and bottom shear stress should be sufficiently population, in order to avoid overharvesting health (e.g. by measuring the shell weight to
low since these factors will disturb oyster and introduction of invasive alien species. flesh ratio of a sample) shortly before they
settlement and growth. Areas with proper However, if these are not available, employing are deployed.
conditions can be found in the northern and adult oysters may be considered, but then
southernmost part of the Dutch North Sea. strict protocols against overharvesting and If there is doubt about the appropriateness
• Net water current should be as low as introduction of alien invasive species must of the location, it is important to install a flat
possible to stimulate larval retention in be adhered to. oyster test population of 300-600 specimen
the area. These areas are located in the when starting the restoration pilot. The test
southernmost and northern part of the It appeared that the supply of Bonamia-free population should be regularly checked for its
Dutch North Sea. young flat oysters in Northern Europe was survival, growth, condition, and development
• Primary production should be sufficient too scarce to satisfy the demand for the pilots of reproduction organs (gonads). The oysters
enough to provide food for the oysters. discussed in this report. Fortunately, several in this population therefore must also be
There is a high to low productivity gradient hatchery/nurseries are being created (among measured before deployment and contained in
roughly perpendicular to the Dutch shore. others, supported by WWF). These need to be robust racks, with meshes as wide as possible
However, it is not clear how far from shore scaled up in order to bridge the gap between in relation to the size of the deployed oysters.
this becomes limiting to the oysters. supply and the rapidly growing restoration-
based demand. A cost-efficient method for extension of the
Presence of empty shells (‘shelliness’) is pilot may be to employ ‘spat-on-shell’ around
probably important too, since this may facilitate Oysters from a Bonamia-free area have not the seed population: young oysters which
settlement of oyster larvae. We did not identify developed resistance against the disease, are allowed to settle on shell substrate. This
distribution information of shelliness on the so that mortality will probably be high if the practice is widely used in other oyster restora-
North Sea floor, but a bottom survey will easily Bonamia infection reaches the population. tion projects, particularly in the USA. However,
reveal the amount of shelliness present before This circumstance can never be completely we do not know the survival rate of flat oyster
a pilot. excluded. That is why the development of spat-on-shell in North Sea waters, so this must
a disease-free but resistant or tolerant be tested in practice.
For source selection of the flat oysters as population is crucial.
a seed population in a restoration pilot: Flat oyster settling can be stimulated by
• Since the North Sea is considered to be To ensure that the oysters are in good health deploying clean and empty bivalve shells
free from the Bonamia disease, the source when deployed: around the seed population, with the
population should originate from areas free • Due to reproduction, flat oyster condition following provisions:
from this disease. is low between May and October. Therefore, • The deployed shell substrate can consist
• The source location of flat oysters used for avoid this period when harvesting as well as of various types of bivalve shells (mussel,
restoration pilots is of limited relevance to deploying them. cockle, oyster).
8
Ca. 100 m
Reef dome or other
artifical reef structure
Rack (cages with oysters of
different age classes inside)
Ca. 100 m Spat collecter (with clean mussel,
Pacific oyster and cockle shells)
Empty shells, adult oysters
and/or ‘spat on shell’
Figure S.2 Recommended basic design of a flat oyster
restoration pilot.
• The shells should be clean and free of live Settlement can be determined by putting consist of oysters loosely strewn on the sea
epibionts. ‘spat collectors’ into the water in or around floor. A combination is possible too: loosely
• The cleaned shells should be deployed the pilot at regular time intervals during the strewn oysters around the racks with the
within two weeks of the first peak in larvae summer period. This is relatively costly and test population.
swarming. The latter can be determined by time-consuming, so an alternative method is • If not already present, distribute empty and
regular water sampling in and around the to survey substrate in and around the pilot relatively clean shell material around the
test population and subsequent laboratory by an ROV with camera, by the time the pilot, within two weeks of the first peak in
analysis for larval identification and counts. young oysters have grown to at least one larvae swarming (to be determined by larvae
This peak may also be predicted with a cm in size. sampling).
temperature-sum model, but this must be • Deploy artificial reef structures in or around
calibrated per location. Based on these recommendations, we have the pilot, in particular for enhancement of
• If there are already many bivalve shells developed a standard design for a restoration general biodiversity, protection against
present on the sea floor at the pilot pilot (schematically depicted in Figure S.2): disturbance, and traceability of the pilot.
location it may not be necessary to deploy • If it is not certain that the conditions at the • Deploy and collect spat collectors for
extra shells, but this must be tried out in pilot location are appropriate for flat oysters, monitoring of flat oyster settlement in the
practice. try this out with a test population, as first pilot years or, when newly settled
described above. If the conditions are known oysters have grown, use an ROV with
to be appropriate, the seed population can camera.
9
Once successive analysis of the test population • Survey development of biodiversity in and more substrate. Monitoring emphasis can shift
shows that pilot conditions are appropriate for around the pilot, among others to investigate away from survival, growth, and reproduction,
flat oysters, large-scale extension of the pilot ecosystem services provided by the focussing more on recruitment and the reef’s
can be undertaken by deploying extra oysters oyster reef. spatial distribution around the pilot, as well
(possibly in the form of spat-on-shell) and as the development of biodiversity.
substrate. These guidelines are in line with the Phase 4 Reef development phase: years ca.
monitoring recommendations by the Native 6 to 10 (or longer)
Given the various uncertainties inherent to Oyster Restoration Alliance (NORA, 2017). Gradually decrease measures aimed at
a flat oyster restoration pilot, it is essential extension of the reef, to investigate whether
to accompany it with monitoring. We present Given the uncertainties about flat oyster the reef maintains or grows by itself. Continue
a full monitoring programme. Whether this restoration, we recommend a pilot duration environmental, recruitment, and biodiversity
is achievable depends on the available budget of about 10 years (if achievable). This period monitoring. Consider measures to enhance
in combination with the specific pilot objec- may be roughly divided into four phases, with ecosystem functions and structure if keystone
tives. the following recommendations per phase: species are missing (especially those which
Phase 1 Preparation and deployment: year 1 are functional in maintaining the reef).
Our recommendations for a full monitoring T0 monitoring, prior to deployment. Deploy-
programme of flat oyster pilots are: ment period: well before or after the summer Mainly due to expensive ship time, working
• Regularly inspect the installation and oyster period. Start with a test population if it is not in offshore conditions is extremely costly.
racks (if used) for damage and remove known whether local conditions are suited for Hence we recommend combining monitoring,
growth of attached organisms (if necessary, flat oysters (go to phase 2), otherwise start maintenance, and extensions visits to the pilot
clean and/or repair). with higher amounts of oysters and substrate as much as possible.
• Continuously record environmental data (go to phase 3).
at the pilot location near the sea floor: Phase 2 Test phase: years 1 to ca. 3
temperature, food availability, oxygen level, Perform monitoring and maintenance of the
and current speed. test population. Monitoring focuses on survival,
• Measure survival, growth, gonad develop- growth, reproduction, and the occurrence of
ment, and condition of the test population, recruitment (SGRR) as such, and on environ-
i.e. the flat oysters in the racks, and if mental parameters. Extra oysters and/or
possible also oysters on the sea floor. settlement substrate can also be deployed,
• During the reproductive period (June- but this entails risks as long as SGRR are not
August) sample water in the pilot, near the yet proven to be sufficient. Based on the SGRR
bottom, and analyse for concentration of results in this phase, a stop/go decision can be
flat oyster larvae when planning to deploy made.
settlement substrate. Phase 3 Extension phase: years ca. 3 to 6
• Deploy and collect spat collectors, and If it is decided to continue, pilot extension
later use ROV with camera, to determine flat can be undertaken by deploying more oysters
oyster settlement in and around the pilot. (possibly in the form of spat-on-shell) and/or
10
1 Introduction
1.1 Background and history Bottom-trawling fishery is destructive to flat oyster reef near the Brouwersdam
epibenthic flora and fauna, including flat that separates Lake Grevelingen from the
In their feasibility study of flat oyster (Ostrea oyster reefs. Voordelta (Christianen et al., 2018). The
edulis) restoration in the Dutch North Sea, flat oysters are probably the offspring of
Smaal et al. (2015) concluded that the time was Almost simultaneously with the Dutch study, aquaculture flat oysters in the Grevelingen.
right to start flat oyster restoration in the Dutch a German feasibility study (Gercken & Schmidt, This discovery confirmed that flat oysters
part of the North Sea. The main reasons behind 2014) was undertaken, coming to similar are spreading autonomously beyond their
this conclusion were, and still are: conclusions. original culture area.
• Probably due to improved health, the flat
oyster population shows signs of autono- 2015 also saw the start of the Haringvliet During the past three years, co-created and
mous extension in the Dutch Delta area (in Dreamfund Project, aiming at restoring an mainly funded by ARK Natuurontwikkeling
the extreme southwest of The Netherlands), estuary, which is part of the Delta area, in the and WWF Netherlands, we have worked on
where it has been kept in culture since about southwest of The Netherlands. The project a method for extending and creating shellfish
a century ago. includes flat oyster reef restoration in the reefs in the Voordelta, with an emphasis on
• The areas without bottom-trawling fishing Voordelta, which is the seaward extension of flat oysters. We did this by studying the
activities will probably increase in the North the Delta. While looking for a suitable location structure of this reef, by performing various
Sea, due to the current extension plans of to start restoration pilots, we discovered a experiments, and by starting an additional
wind farms and Marine Protected Areas. mixed Pacific oyster (Crassostrea gigas) and restoration pilot in the area (Sas et al., 2017,11
2018a; Didderen et al, 2019a). This has helped et al., 2019b,c). WWF is the main driver behind
Picture: Onderwaterbeelden.nl
to identify several of the steering factors of the Borkum Reef pilot. The Gemini pilot is an
flat oyster reef development and restoration. initiative of Jan Groot, WWF and the Gemini
In a sense, the Voordelta has become a ‘field ‘Buitengaats’ consortium. Monitoring in these
laboratory’ for flat oyster reef restoration. pilots yielded important additional results,
This is an essential asset, since the factors which we have therefore used for this synthesis,
influencing development and sustainability of together with results of relevant modelling
wild flat oyster reefs in the marine environment exercises and scientific literature.
are largely unknown. Moreover, the reefs in the
North Sea disappeared before anyone realised
that they were worth studying. Fortunately, 1.2 Objectives and scope
oysters have been kept in culture, in the Delta of the report
area as well as in other inlets and estuaries
around the North Sea, which gives us some The main objective of this report is to present
basic information about the flat oyster life- recommendations for flat oyster restoration in
cycle and the initial population for restoration the North Sea (offshore as well as nearshore),
attempts (in the Delta only in that very area, based on the research results over the past
due to Bonamia infection; see Chapter 4). three years. In this, we do not distinguish
However, knowledge of how to maintain and between restoration ‘pilots’ and ‘attempts’.
Oyster Reef at Brouwersdam, Voordelta
extend an oyster culture in the sheltered coastal A pilot is intended to try out certain locations
zone is something different from restoring wild and methods, whereas a true attempt is aiming
oyster reefs in the open North Sea. at actually restoring a population at a certain know how extensive, in terms of oyster
location. But in both cases, the initiator aims density per surface unit and total surface
The Voordelta oyster reef discovery and the to get insight in the critical success factors for area, a reef should be in order to sustain itself
results of the experiments have drawn wide- the population to survive and proliferate, for and/or expand in the open North Sea. These
spread attention and have stimulated various which our recommendations are intended. uncertainties can only be filled in through more,
organisations to start surveying other areas for well-monitored experiments and restoration
flat oyster presence and restoration potential The recommendations should be considered attempts, at different locations, with differing
(e.g. Fariñas-Franco et al., 2018). This resulted as ‘work in progress’ towards guidelines for areas, oyster densities, and time scales.
in the discovery of a small population in the native oyster reef restoration in the North Sea,
western Wadden Sea (Van der Have et al., since there still are uncertainties. Firstly, the In this circumstance, it is important to mention
2018a). After successful pilots nearshore, and various factors behind flat oyster recruitment that the number of flat oyster restoration pilots/
drawing on the experience of these pilots, in (i.e. spat settlement and growth to a size at attempts in the North Sea is still increasing. For
spring 2018 two new and relatively large oyster which most oysters can survive and reproduce) instance, in autumn 2018 a pilot was started in
reef restoration pilots were started in the deeper are not all known. Recruitment is crucial to the Luchterduinen offshore wind farm, initiated
waters of the Dutch North Sea: in the Borkum maintenance and growth of an oyster reef. by a consortium of NGO’s and companies (The
Reef area and the Gemini wind farm (Didderen Secondly, and related to this, we do not yet North Sea Foundation, Natuur & Milieu, Eneco,12
Wadden Sea
Flat oysters found in 2017 Wind farm Gemini
Hatchery/nursery started in 2018 Started in 2018
Borkum Reef
Started in 2018
Wind farm Luchterduinen
Started in 2018
Overarching projects:
Hatchery/nursery of Bonamia
free flat oysters
Voordelta project
Started in 2016
EcoFriend: innovative monitoring
in wind farm pilots
Figure 1.1 Current flat oyster restoration projects in the Dutch North Sea area, projected onto the flat oyster reef map by Olsen (1883).
The orange areas indicate the historic flat oyster reefs.13
ASN Bank, and Van Oord). In addition, there Southampton, Ben Ainslie Land Rover BAR, 1.3 Brief description of contents
are various hatchery/nursery initiatives in order and MDL Marinas. See: www.bluemarine
to produce the growing number of oysters foundation.com/project/solent In Chapter 2, we describe the overall strategy
required. • The Essex Native Oyster Restoration of flat oyster restoration we are currently
Initiative (ENORI) on the UK east coast aims following. We have already set important steps,
See Figure 1.1 on page 12 for a complete over- at supporting self-sustaining populations of but a much larger effort is required before we
view of current pilots and other relevant flat native oysters within several estuaries there. attain the objective of self-sustaining flat oyster
oyster initiatives in the Dutch North Sea and See: www.essexnativeoyster.com reefs in the North Sea.
coastal waters, projected onto the flat oyster
reef map produced by Olsen (1883). As can be The increased interest in flat oyster restoration Chapter 3 describes the flat oyster life cycle
seen, most projects are within, or close to, the led to the establishment (in 2017) of the Native and the steering factors we humans can employ
historical oyster reefs. Oyster Restoration Alliance (NORA), a European to achieve reef restoration.
network aiming at reinforcement and restoration
Other important flat oyster restoration of the native European flat oyster. Network Chapter 4 describes the pilots and experiments
examples in the North Sea area and nearby members are representatives of governmental in Voordelta, Borkum Reef, and Gemini wind
Channel are: agencies, science, non-governmental farm.
• The pilot near Helgoland, undertaken by organizations, as well as oyster growers and
the German Alfred Wegener Institute (AWI) other private enterprises. Chapter 5 provides the recommendations we
at Bremerhaven (Pogoda, 2019), based on See www.noraeurope.eu for general information have derived from these pilots and experiments,
the positive conclusions by Gercken & and www.noraeurope.eu/restoration-projects model results, and literature data.
Schmidt (2014). for information on various restoration initiatives.
See: https://www.awi.de/en/science/ Given the growing interest in flat oyster Chapter 6 shows how a flat oyster pilot can be
biosciences/shelf-sea-system-ecology/ restoration, we assume that more flat oyster set up, monitored, maintained, and expanded.
main-research-focus/european-oyster. restoration pilots will be initiated in the North
• The Dornoch Environmental Enhancement Sea area. This synthesis and the related
Project (DEEP), an initiative to restore Native recommendations are intended for all those
European oysters to the Dornoch Firth by who want to give it a try, and for those who are
the Glenmorangie whisky distillery, Heriot- trying. Its scope is to give guidance based on
Watt University, and the Marine Conservation our key field work results, with emphasis on the
Society. See: www.theglenmorangiecompany. general (i.e. not pilot-specific) factors governing
com/about-us/deep flat oyster reef restoration. Such factors are
• The Solent Oyster Restoration Project mostly of a biological and ecological nature,
(SORP), aiming at restoring the native oyster though some information on costs and legal
to the Solent, the strait that separates the factors are also general and therefore included.
Isle of Wight from mainland England, by A description of costs, permits, design, etc.
the Blue Marine Foundation in partnership specific to an offshore wind farm pilot is given
with the Universities of Portsmouth and in Sas et al. (2018b).14
2 Objectives and
overall strategy
of flat oyster reef
restoration
Flat oysters once covered a large area of the number of epibenthic species was 60% Other offshore shellfish reefs (e.g. horse
North Sea, as depicted in Figure 1.1. The map higher compared to adjacent sand patches mussels Modiolus modiolus) were, as far as
illustrates that flat oysters can survive and (Christianen et al., 2018). we know, never so extensive in the southern
reproduce in offshore areas, up to 60 metres North Sea. Flat oyster reefs are therefore
water depth and maybe even more. The For North American waters, field surveys and considered to have the highest potential for
importance from the viewpoint of nature ecological modelling showed that fish and enhancing marine biodiversity.
restoration is that flat oyster beds provide mobile crustaceans thrive on oyster beds (with
a natural hard-substrate habitat in a pre Virginia oysters, zu Ermgassen et al. (2015)). Given adequate conditions, shellfish reefs
dominantly soft-bottom marine environment We assume that this is the case for the North should be able to maintain themselves and
and influence other environmental conditions Sea too. Hence, North Sea marine life, including expand, which gives them an extra
as well by filtering, providing, and recycling fish and mobile crustacean production, is characteristic that artificial hard substrate
nutrients, providing shelter and possibly also expected to increase when flat oyster reefs are does not have.
by storing carbon. restored. Of course, this must be checked by
monitoring, once a reef is established. Taking Our key objective in flat oyster restoration is
An oyster reef is a species assemblage with measures to enhance biodiversity should even to ‘kick-start’ self-sustaining reefs in the open
higher species richness than a sandy or silty be considered if keystone species are missing North Sea, with all the naturally associated
sea bed. In the Voordelta mixed oyster reef (especially those which are functional in species and ecosystem services. In other words,
we observed that in quadrats with oysters the maintaining the reef). after an initial stimulation phase, it should not15
Picture: Onderwaterbeelden.nl
Business case>2020?
Break- even Ecosystem services
Number of oysters
Oyster bed restoration
€€€ 2018-2022
Oyster pilots 2016-2020
€€
Figure 2.1 Illustration
of the time-line and
scaling up of oyster € Feasibility study 2015
reef restoration in the
Dutch North Sea.
Oyster reef size
be necessary to add new or other specimens around 1000 ha (with sufficient oyster density,
and biodiversity should develop. Obviously i.e. at least five individuals per m2; see par. 3.3).
we are not there yet, but this is the direction This, in turn, will require major investments.
restoration practices should go and for which But ultimately flat oysters create important
the recommendations of this report are i ecosystem services, so in the long run gains
ntended. probably outweigh costs. One way to approach
this is to monetize ecosystem services, such as
The overall strategy of scaling up flat oyster offshore oyster culture and increased stocks of
restoration attempts is depicted in Figure 2.1 fish and large mobile crustaceans (Figure 2.1).
below. We are currently in the second phase,
in which restoration attempts of the scale of Costs aside, we consider oyster reef restoration
1 ha are initiated and in which we also attempt as a part of good stewardship of the North Sea.
to extend the existing Voordelta oyster reef
(which spreads across about 40 ha).
Intuitively, we suspect that at the single ha
scale, flat oyster reefs are not yet self-sustaining.
Flat oysters
The required dimension lies probably somewhere16
3 Flat oyster life cycle
and reef restoration
steering factors
3.1 Flat oyster life cycle females are larviparous, that is, the eggs are develop if there is sufficient survival, growth,
present in the cavity and after fertilization the and reproduction of mature oysters and
Flat oysters have a complex life cycle shared by larvae are brooded by the female, 8-10 days recruitment of young oysters into the local
all members of the genus Ostrea (Figure 3.1). in flat oyster. After brooding, the larvae swarm population.
After a short pelagic period (6-10 days for flat into the water column in June-August.
oysters), oyster larvae settle permanently by
cementing themselves to hard substrate. In the The critical success factors during the life 3.2 Steering factors in oyster
six-month period after settlement they are cycle are survival rate (larvae, spat and mature reef restoration
called ‘spat’ (Walne 1974). oysters), growth rate, sex organ (gonad)
development, larvae production, and recruit- Restoration can be defined as: “The process
In the first three years they function as males ment of new oysters into the population by of establishing or re-establishing a habitat that,
(protandrous life cycle) and in subsequent successful settlement of larvae as spat, as in time, can come to closely resemble a natural
years they can function alternately as females already indicated in Figure 3.1. condition in terms of structure and function”
or males. This sex change depends on (Baggett et al., 2014). Shellfish (flat oyster)
environmental conditions (temperature, food). These success factors are summarized as reefs can be defined as intertidal or subtidal
The males produce sperm clumps (spermato- the Survival, Growth, Reproduction, and biogenic structures formed by shellfish (flat
zeugmata; Ó Foighil et al., 2000) which after Recruitment (SGRR) conceptual model: oysters) living at high densities and building
spawning are inhaled by the females. The a self-sustaining or growing oyster reef will a habitat with significant surface complexity.17
larvae
Recruitment Reproduction
Addition of
spat substrate
(cultch)
>3y
spat Oyster
substrate oysters
Natural
shelliness?
Survival Growth
1-3y
oysters
Figure 3.2 Important steering factors of flat oyster bed
development (in green): deployment of live oysters (of both
sexes and different ages) to produce larvae and deployment
of substrate for settling of these larvae. Blue arrows indicate
that oysters (1-3 y and >3y) and natural shelliness also
Figure 3.1 The general life cycle of an Ostrea oyster. provide settlement substrate.
Several experts insist on using the term oyster This can be accomplished, in principle, by • Both if oysters and suitable substrate are
‘bed’ instead of ‘reef’. We stick to the term influencing key steering factors in the oyster absent.
‘reef’ however, to underline that flat oysters life cycle (see Figure 3.2). These include
create a three-dimensional structure on the (Beck et al., 2009): Choice of a presumed optimal location is an
sea bed. • Providing or adding a ‘seed population’, i.e. additional steering factor.
a mix of oysters of both sexes to ‘kick-start’
Shellfish reef restoration is successful if high a population, by producing larvae, if the In Chapter 5 we will elucidate more specific
densities with substantial structural complexity population is recruitment limited (which is recommendations for these steering factors,
occur and maintain themselves. In the case of the case in the North Sea). based on the Voordelta, Borkum Reef, and
flat oysters, such a reef should have a minimal • Providing settlement substrate for spat if Gemini pilots, as well as additional sources.
density of five individuals per m2 (OSPAR, the population is substrate limited (which
2000). may not be the case in some areas of the
North Sea).18
4 Pilots: locations
and design
Figure 4.1 Locations of the three
pilots which form the basis of
this report, projected onto the
Olsen (1883) map
4.1 Introduction - Sas H., Kamermans P., Have T. Van Der These reports can be downloaded from
& Christianen M. (2018a) Shellfish reef https://www.ark.eu/natuurontwikkeling/dieren/
This synthesis is based on the results of restoration pilots Voordelta: Annual schelpdierbanken
experiments and pilots in the Voordelta report 2017.
(2016-18), with additional information from the - Didderen, K., P. Kamermans, A. van den
Borkum Reef and Gemini oyster restoration Brink, T.M. van der Have, J.H. Bergsma, 4.2 Voordelta experiments
pilots (2018). The locations of these pilots are H. van der Jagt, W. Lengkeek, M. Maathuis, and pilots
presented in Figure 4.1. H. Sas (2019a) Shellfish bed restoration
pilots Voordelta, Netherlands, Annual The Voordelta is a nature conservation area and
Full information about these pilots and report 2018. part of the EU Natura 2000 network. A map of
experiments is presented in the annual reports. • Borkum Reef: Didderen, K., P. Kamermans, the designated Natura 2000 area is presented
These are: W. Lengkeek (2019b) Borkum Reef oyster in Figure 4.2.
• Voordelta: pilot, results 2018.
- Sas H., Kamermans P., Have T. van der, • Gemini wind farm: Didderen, K., P. We performed our main pilots and experiments
Lengkeek W. & Smaal A. (2017) Shellfish Kamermans, W. Lengkeek (2019c) Gemini near the Blokkendam location (see Figure. 4.3).
reef restoration pilots Voordelta: Annual wind farm oyster pilot, results 2018, BuWa The main characteristics of this part of the
report 2016. report 19-012. Voordelta are:19
Figure 4.2 Map of the Voordelta Natura-2000 area (Rijkswaterstaat & Royal Haskoning/DHV).20
• The area is shallow, with water depth Figure 4.3 Map with relevant pilot
varying from 1 to 6 metres. and experiment locations in the
Voordelta area.
• There is a mixed flat oyster and Pacific
oyster (Crassostrea gigas) reef present near
Blokkendam location. Shrimp fishery is the
only type of bottom-trawling fishery in the
Voordelta area, but it is even absent near
the Blokkendam, due to the shallow water
depth and the presence of large stones.
• The oyster density varies strongly within the
reef. Its area is at least 40 ha. Most probably,
the flat oyster larvae originally came from
nearby Lake Grevelingen (Figure. 4.3), a
saline lake with limited tidal range and in
open connection via the Grevelingen outlet
to the Voordelta since 1998. In this lake, flat
oysters occur naturally and on culture plots.
As a result, in early summer, flat oyster larvae
can be detected swarming in the Voordelta
area.
• Other shellfish species also occur in the area,
such as razor clam (Ensis sp.), surf clam
(Spisula sp.), soft shell clam (Mya arenaria),
and blue mussel (Mytilus edulis).
As stated in Chapter 1, the main objective of
the overall Voordelta project (2016-2018) is to Specific objectives and related activities for Therefore, it was decided to stop monitoring
develop a method for extending and creating the experiments are (see Sas et al., 2017 and at this location.
shellfish reefs. In order to attain this objective, 2018a; Didderen et al., 2019a): • To unravel the dynamics of spat settlement
secondary objectives are: • To measure survival, growth and repro as a basis for recruitment of flat oysters,
• Understand the critical factors for formation duction of flat oysters. This was done by since this constitutes the ‘Achilles heel’ of the
and development of these reefs. deploying racks with a defined population establishment of flat oyster reefs. The main
• Identify suitable locations for shellfish reefs at two different locations: Blokkendam and question here is: How can flat oyster spat
in the Voordelta. Hinderplaat. Conditions for flat oyster settlement be stimulated and how can this
survival near Hinderplaat appeared to be knowledge be used to create new flat oyster
unsuitable, probably due to large and reefs? A related question is: Which substrate
sudden fresh water inflows from Haringvliet. is most suitable for flat oyster spat settle-21
ment? These questions are addressed by We also performed a survival and condition (up to three years of age) are all males, while
monitoring the occurrence of flat oyster test with Norwegian oysters, from Hafrsfjord older oysters can both be male and female
larvae in the water column and deploying (see below for source description). (see par. 3.1).
substrate when flat oyster larvae appear
to be present, as well as by mapping the Water depth at the pilot location is about 25
substrate used by flat oysters in the 4.3 Borkum Reef pilot metres. The pilot is located about 20 km from
Blokkendam reef. shore. The artificial reefs have various functions:
• Based on the results of the above The Borkum Reef pilot, which was started protection and references points for the racks,
experiments: in spring 2018, is a larger-scale restoration settlement substrate and elevation form the
- To attempt to extend the reef, by attempt. It consists of: seabed for oysters, substrate and/or shelter for
timely deploying substrate at locations • A small number of oysters in racks, to other species. Local fishermen have agreed to
considered suitable. allow monitoring of growth, health, and avoid the area with bottom trawling equipment
- To develop a new pilot, on the lee (east) development of gonads (reproduction and work together with WWF and ARK regard-
side of the tidal flat Bollen van de Ooster. organs) of this subset. ing monitoring and maintenance.
This location was chosen because shellfish • Artificial sandstone reefs with and without
surveys conducted earlier showed the oysters.
presence of living flat oysters and because • About 80,000 oysters loosely distributed 4.4 Gemini wind farm pilot
fishery or other bottom-disturbing on the sea floor, within an area of
activities are not allowed there. The pilot approximately 1 ha. The pilot in the Gemini windfarm (see Didderen,
was started in 2018. 2019c), which was also initiated in spring 2018,
• To map the biodiversity associated with the See Didderen et al. (2019b) for full description is of smaller scale than the one in Borkum Reef.
oyster reef, in order to demonstrate its value of the pilot. It is performed with Norwegian It is performed with about 15,000 Norwegian
for marine nature development. oysters. The same regulation as mentioned in oysters from the same batch as used for the
par. 4.2 prohibits the deployment of Bonamia- Borkum Reef project. A small part of these was
The oysters employed for the Voordelta infected oysters in the open North Sea because also put in racks, while the majority was loosely
pilots usually came from the region itself: this is considered as a Bonamia-free zone. The strewn in and around wind farm monopile scour
Lake Grevelingen, Eastern Scheldt, and some- Norwegian oysters are free from the Bonamia protection stones. Water depth varies between
times from the reef in the Voordelta. These disease. They originate from wild oyster reefs, 28 and 36 metres. The wind farm is 85 km from
are infected with Bonamia. So, we tested for located in relatively shallow water (up to shore.
Bonamia in the Blokkendam reef and it is 5-metre depth) and are harvested by divers.
indeed present in that population (Sas et al., A sustainable harvest protocol was set up and
2018). Therefore, in this case, oysters infected maintained by WWF and the oyster company.
by Bonamia can be used for reef restoration.
A Dutch regulation is being prepared limiting The oyster population contains a naturally
the deployment of Bonamia infected oysters occurring variety in age classes. The use of
to this part of the Voordelta only. different age classes is important, since such
a population is productive. Young oysters22
5 How to restore a flat oyster
reef: lessons from the
Voordelta, Borkum Reef,
Gemini wind farm pilots
and other sources
5.1 Habitat requirements for extraction. Possibly, even noise and/or enough to support flat oyster beds, i.e.
restoration pilots vibrations caused by these activities are fine or coarse sand and gravel are suitable,
detrimental to flat oysters (Tamis et al., whereas silt is probably less suitable. This
Smaal et al., (2015) argued that abiotic 2019). is the case in most parts of the Dutch North
conditions (including bottom shear stress Sea. Possibly with the exception of (large
and stability, current velocity, salinity, In model studies, Smaal et al. (2017) and parts of) the Oyster Grounds. Explanations
sediment composition, temperature, oxygen Kamermans et al. (2018a,b) investigated other for this could be that the organic part of
availability, water depth, and absence of relevant conditions for flat oyster restoration this silt was produced by the oyster reefs
bottom disturbance) and biotic conditions pilots in the Dutch part of the North Sea. We that were once present there, and/or that
(diseases, food availability, predators) should recapitulate these below, with additional infor- currently sedimenting algae are no longer
allow flat oyster growth and reproduction in mation from the pilots in Borkum Reef, Gemini consumed by oysters.
large parts of the North Sea nearshore and and some German experiments, to provide • Presence of empty shells (‘shelliness’) or
offshore. recommendations for optimal conditions and other hard substrate probably facilitates
locations for restoration attempts/pilots. settlement of oyster larvae. The type of shell
A primary condition for oyster reef develop- is probably of little importance (see par. 5.4).
ment is the total absence of seabed disturbance The relevant conditions are: We did not find information on spatial
in the designated reef area, including trawling • Sediment composition without too fine distribution of such substrates on the North
fishery, construction works, and gravel/sand fragments: the sediment should be firm Sea floor, but a bottom survey will easily23
par. 5.3). But if there is a choice, the areas
Picture: Ester van den Doel/Duiken in Beeld
with lowest possible net current should be
selected for restoration pilots. A tidal current,
moving to and from the pilot (hence: without
a net current) will be less problematic and
occurs everywhere in the North Sea anyway.
Such locations do exist in the Dutch part of
the North Sea, as the model studies show.
In particular, there is a very low net current
in the Gemini wind farm. At Borkum Reef,
modelling shows a moderate net current to
the east.
• Absence of prolonged periods of oxygen
depletion, as for example caused by thermal
stratification. Lack of oxygen is lethal to all
epibenthic flora and fauna, although flat
oysters appear to be relatively resistant
(Smaal et al., 2015). Choosing a location
without prolonged stratification periods is
therefore preferred. The model studies show
that it may occur in a zone just to the north
Oyster reef at Brouwersdam
of the Dutch Wadden Sea. There are indeed
indications that temporarily stratification
occurs during spring and summer in the
reveal the amount of shells or other hard area. Borkum Reef is located relatively near Gemini wind farm. Fresh water stratification
substrate present before a pilot. to shore, but current velocity was measured occasionally occurs in a narrow band along
• A sufficiently stable sea floor: a mobile and not deemed a limiting factor (Reuchlin, the Dutch coast, hence that phenomenon
bottom will disturb oyster settlement and 2018). is less relevant to the open North Sea.
growth. The model studies mentioned • A low net sea water current, in any direction: • Sufficient primary production, to provide
above showed that, in the northern and current will cause dispersion of flat oyster food to the oysters: there is a high to low
southernmost part of the Dutch North Sea larvae. Hence, they will tend to drift away productivity gradient roughly perpendicular
areas, a sufficiently stable bottom can be from the parent reef, leaving it insufficiently to the Dutch shore. The mentioned model
found. rejuvenated. At present the magnitude of net studies indicate that food limitation could
• A sufficiently low sea floor shear stress: current, which causes larvae to drift away occur far from shore, though not just how far.
a high shear stress can damage oyster reefs. too far, is based on model predictions. In In illustration, the condition of the oysters in
The model studies showed that this should addition, the settling rate of flat oyster larvae Borkum Reef (close to shore) appears to be
not pose a problem outside the nearshore and size of the reef will also play a role (see slightly better than those in the Gemini wind24
model studies did not yield Dutch North Sea
Picture: Onderwaterbeelden.nl
areas with too high suspended sediment
concentrations for flat oysters.
• Sea water temperature near the bottom
between 3 and 30 °C: flat oysters require
a winter temperature not below 3 °C and
summer temperature not above 30 °C. This
is probably not exceeded anywhere in the
Dutch part of the North Sea.
• Sedimentation rate not too high: a high rate
tends to bury the oysters. There are no North
Sea data or model studies of (the influence
of) this factor.
• Low predation pressure, in particular of star-
fish and crabs: predation may pose a severe
threat, specially to young oysters. There are
no indications that parts of the Dutch North
Sea are particularly infested with these
predators, though they may of course be
attracted to flat oyster reefs and their
associated organisms. So, the influence of
this factor must be determined in practice.
The Voordelta pilots, with the Hinderplaat loca-
Deploying substrate at Voordelta
tion in particular, showed that a high freshwater
flow is lethal to flat oysters, but this is obviously
only relevant at locations very near to shore
farm (85 km from shore). Pogoda et al. (2011) the planned growth of the wind farm area, and at main river outlets.
also report a moderate oyster condition near this could possibly lead to food limitation
Helgoland, which is about 55 km from the for the oysters (Kamermans et al., 2018), In summary, we recommend the following
German mainland, while oysters at stations but that is a hypothesis that remains to be qualitative criteria for location selection of flat
more inshore showed a better condition. tested. oyster restoration attempts/pilots, in order of
A factor which may, in time, cause problems • Minimal suspended sediment concentration: probable importance:
for flat oyster reef development in wind farms a high concentration of suspended inorganic • Bottom disturbance by human activities
is the high number of mussels attached to particles (according to current estimates (such as trawling fishery, sand/gravel
monopiles. These compete with the oysters above ca. 50 mg/l) can interfere negatively extraction, and construction works) should
for phytoplankton in the water column. With with oyster filter feeding. The mentioned be completely absent.25
• The sediment should be firm enough to should originate from areas that are free secondly because alien invasive species can be
support flat oyster beds (i.e. fine or coarse from Bonamia and Marteilia. attached to them. That is why NORA strongly
sand and gravel are suitable, whereas silt • In a small section of the Voordelta (since advises against harvesting and deploying wild
is probably less suitable). Bonamia already occurs there) the oysters oysters for restoration purposes (see www.
• Prolonged periods of oxygen depletion can originate from an area which is infected noraeurope.eu/wp-content/uploads/2018/
should be absent (as for example caused by Bonamia. Berlin-Oyster-Recommendation-Part-1).
by thermal stratification).
• The sea floor should be sufficiently stable In the North Sea area, Bonamia is considered Nonetheless, ARK/WWF are convinced that the
and bottom shear stress should be to be a higher threat than Marteilia; therefore Norwegian oysters sourced from Hafrsfjord for
sufficiently low. Such areas can be found in our practical recommendations concentrate the purpose of the Borkum Stones and Gemini
the northern and southernmost part of the on the former. projects pose no risks of overharvesting or
Dutch North Sea. introduction of invasive alien species. That is
• Net water current should be as low as Oysters from a Bonamia-free area have not because:
possible. These areas are again located in developed resistance against the disease, so • The Norwegian government, IMR research
the southernmost part and in the northern mortality will probably be high if the Bonamia institute, and the company selling the oysters
part of the Dutch North Sea. infection reaches the population. This circum- state there is no overharvesting and that the
• Primary production should be sufficient; in stance can never be completely excluded. harvest method (by hand, with divers) does
far offshore areas it may become limiting to That is why the development of a disease-free not damage the population. A sustainable
flat oyster growth. but resistant or tolerant population is crucial. harvest protocol was developed and is
maintained.
The sea floor at the chosen location should Wild versus hatchery oysters • WWF and ARK have commissioned
be surveyed before deployment, in order to The oysters deployed in the Borkum Reef and development of a treatment protocol for
ascertain that its basic physical conditions are Gemini wind farm are harvested from Hafrsfjord wild adult oysters. This includes mechanical
met (as summarised in the above in Norway. This is a wild population, of various and chemical (bleach) treatment of the
recommendations). age classes. The principal reason for ARK/WWF oysters before transport, as well as a
employing these oysters is that the required quarantine period (Van der Have et al.
amounts of flat oysters were not available from 2018b). The protocol was applied to all
5.2 Oyster sources and treatment a hatchery at the time the pilot was starting. oysters deployed in the Borkum Reef and
prior to deployment Secondary reasons are that only a population Gemini pilots. The Dutch government has
containing various age classes is immediately granted an oyster import permit based
Bonamia free fertile (see par. 3.1) and that oysters from a wild on the adoption and application of the
Currently, a Dutch regulation is being population are genetically more diverse than protocol (Didderen, 2019b).
prepared which specifies rules for the from a hatchery.
oysters to be deployed as a start population. Test cases for Norwegian oysters
It specifies that: It is less desirable to harvest and deploy wild Before deploying the Norwegian oysters, it
• In the open North Sea (since this area is flat oysters for restoration purposes: firstly, was unclear whether they would survive and
considered to be disease-free) the oysters because wild oyster reefs are already rare, grow in the open North Sea, since they were26
transferred from a relatively shallow and quiet The recommendations we derive from this oysters are Bonamia free; a preliminary test
fjord to much deeper water, with different section are: indicates that they may indeed be free from
current speed, temperature, light regime and • In the open North Sea, Bonamia-free oysters the infection.
probably a different algae population too. In must be employed. • AWI is setting up a hatchery with
addition, the transport process from Norway to • The source location of oysters to be de- Bonamia-free oysters at its Bremerhaven
the Netherlands was deemed to be stressful. ployed for restoration attempts/pilots is laboratory.
However, monitoring two months after deploy- probably of limited relevance to its success.
ment revealed that survival rate in Borkum Reef As long as they are healthy when deployed We do not know yet which attempt will
pilot was high and surviving oysters grew well, (see par. 5.3). yield sufficient amounts of Bonamia-free/
showed gonad development and a good condi- • In principle, young oysters (‘spat’) produced Bonamia-tolerant oysters in the near future.
tion. Several female oysters had eggs and larvae in a hatchery should be used for the seed
were present in several oysters, which proves population, in order to avoid overharvesting
that spermatozeugmata were released into the and introduction of invasive alien species. 5.3 Seed population: age structure,
water column and eggs within females were However, if these are not available, it may be condition, test population and
successfully fertilized. In addition, flat oyster considered to employ adult oysters, but then extension
larvae were detected in the water column just strict protocols against overharvesting and
above the oysters. To our knowledge, there is introduction of alien invasive species must be ‘Seed population’
no other flat oyster population nearby. The AWI adhered to. As explained in par. 3.2, we call the initial
pilot in particular is located north of Helgoland population which is deployed to ‘kick-start’
(cf. www.awi.de), more than 100 km away. In the near future, the sourcing problem of an oyster reef the ‘seed population’.
Hence, it is almost certain that these larvae Bonamia free oysters may be solved by the
are the offspring of the flat oysters deployed following developments: Age structure
in Borkum Reef pilot. • Wageningen Marine Research and hatchery As set out in par. 3.1, the age structure of the
company Roem van Yerseke have developed seed population is important, since oysters
As an extra test, the Norwegian oysters were a method to grow Bonamia-free flat oysters less than 3 years old are males only. After three
deployed in the Voordelta. It appeared they from a Bonamia-infected population years they can become females. Probably the
survived rather well there too and surviving (Kamermans et al., in prep). As indicated latter produce more eggs when they grow larger.
specimen showed an equal or even better above, this is a very important development. If the restoration effort aims at immediately
condition than oysters from the region itself • WWF, ARK and Roem van Yerseke are starting a reproducing population, it must
(Eastern Scheldt and Grevelingen). setting up a hatchery with Bonamia free contain a sufficient number of female oysters.
oysters. We do not know the optimal sex ratio, but we
In summary, the experiments show that the • The Royal Netherlands Institute for Sea assume that a significant amount (estimated
deployed flat oysters rapidly adapted to a Research (NIOZ) at Texel, supported by at about 25%) should be female. Since adult
very different environment. Pogoda (2017) WWF/ARK, Jan Groot and the Programme oysters can be male as well as female, the
confirmed this finding: young French hatchery towards a Rich Wadden Sea, is also setting seed population should contain about 50%
oysters, introduced in the German offshore up a hatchery, with Wadden Sea flat oysters. adult oysters to obtain a about 25% female
part of the North Sea, also did well there. It is yet to be determined whether these population. Of course, the guidelines on27
harvest and treatment as set out in par. 5.2
Picture: Onderwaterbeelden.nl
must be adhered when using adult oysters.
Condition
The seed population should be in good
condition. Condition should be estimated (e.g.
by measuring the shell weight to flesh ratio of
a representative sample) shortly before they are
deployed. A good condition ensures sufficient
reserves for the oysters to develop gonads and
produce gametes, in particular eggs, which are
more energy demanding than sperm.
Important condition influence factors are:
• The moment of collection (if from the wild)
and deployment: during the reproductive
period and a few months afterwards, i.e.
from May to October, flat oysters are more
susceptible to stress. So, harvest as well
as deployment should be avoided in that
period.
• Transport and storage before deployment:
Brouwersdam from above
this should be done under the proper
conditions, i.e. packed without water, but
with moist paper around the shells, stored
in cooled containers at 70C. measured and weighted) and retraceable. In account for damage. We recommend each rack
order to achieve this, they should be contained, to contain 100-200 oysters.
Young oysters (less than approximately 3 years for example in robust racks, with meshes which
old) are particularly prone to stress and there- are as wide as possible in relation to the The racks should be hauled up, at least annually,
fore show less survival when they are not deployed oysters. These meshes allow water, to be cleaned and to measure survival, growth,
properly handled and stored prior to deploy- containing oxygen and algae, to enter and development of gonads and overall condition
ment (Pogoda, 2011). excrements to leave the contained population. of the oysters.
Test population The racks should be sufficiently sturdy to with- Extension of the population
If it is not certain that the conditions at the stand North Sea conditions and remain stable We do not know the critical size (e.g. surface
pilot location are appropriate for flat oysters, on the sea floor and they should be retrievable. area) and oyster density of a self-sustaining
a pilot should contain a test population, which At least three racks should be deployed per population in the open North Sea. This is
is well-defined (i.e. oysters are individually pilot, in order to have enough replicates and to related to net current speed (with respect toYou can also read
