ECOFRIEND context and objectives
The ECOFRIEND project (2019-2023) is funded by the TKI Wind op Zee, a Dutch governmental funding program that aims to “stimulate, connect and supports Dutch organisations and knowledge institutes with the development and deployment of innovations to help speed up the transition to sustainable, reliable and affordable energy system, focussing on offshore wind.” (https://www.topsectorenergie.nl/en/about-tki-wind-op-zee). The project partners are Wageningen Marine Research, Waardenburg Ecology, Deltares and Sas Consultancy. The industry partner is the Gemini Offshore Windfarm.
Offshore wind is increasingly important in the Dutch energy transition. Its development is not only influenced by environmental factors (e.g. distance to coast, water depth, seabed structure and dynamics) and technical developments, but also by ecological considerations. The Dutch Ministry of Agriculture, Nature and Food Quality intends to steer offshore wind farm design towards strengthening the North Sea ecosystem, by enhancing policy-relevant marine species. This is part of the North Sea 2050 Spatial Planning Agenda (Min I&M and Min EZ, 2014), the North Sea Agreement (OFL, 2020) and and is being operationalized trough permit-obligations. Offshore wind licence holders are required to make demonstrable efforts into this direction (e.g. wind energy tenders IJVER and HKZ).
European flat oyster (Ostrea edulis) reefs constitute a potential keystone habitat for the North Sea ecosystem, enhancing biodiversity. But a combination of diseases, pollution, cold winters and overfishing have caused their disappearance circa a century ago, so it is widely attempted to restore these reefs. The ECOFRIEND project aimed to develop and study new methods to re-introduce and monitor flat oyster reefs and related biodiversity in offshore wind farms, in cooperation with the wind industry.
The expected outcomes of the ECOFRIEND project (2019-2023) were a proof-of-concept for active reintroduction of offshore flat oyster beds, to show whether there would be a viable population of flat oysters in an offshore wind farm and to develop novel methods, including (predictive) models, to help monitoring the effectiveness of restoration initiatives. Thereby assisting restoration attempts in Dutch marine waters, but also providing competitive advantage for Dutch research organisations and industry and to increase the knowledge base for research organisations and industry that can be applied to initiatives in other North Sea countries. This report provides a summary of the project’s main findings. Detailed findings are described in scientific articles and/or in technical reports which are being published separately and also listed in this report.
Proof of concept of active reintroduction and demonstration of a viable population
To set a first step in creating a flat oyster reef, an initial population of mostly adult oysters has to be introduced at the location where the reef is intended. In Gemini Offshore Windpark a flat oyster population was introduced in 2018 by Gemini, before the ECOFRIEND project started in 2019. The population appeared to be viable as shown by monitoring: the oysters showed a high level of survival, growth and also larval production. It is recommended to develop adequate settlement detection techniques. Further, it is recommended to develop a standardized monitoring protocol (for sampling larvae, growth, and condition), since this will enable comparison of monitoring results with other oyster restoration projects.
Development and testing of innovative monitoring methods
The ECOFRIEND project’s objective was to develop and test a variety of innovative relevant monitoring techniques which can be applied in the - usually harsh - offshore conditions to follow several aspects of reef development. As test sites, flat oyster pilots in the Gemini wind park and Borkum Reef Ground (a WWF Netherlands pilot) were used.
Innovative oyster cage: WERC-dock
Usually, flat oyster restoration projects start with deploying an initial oyster population to the sea floor in a contained form, which is regularly hoisted up to monitor whether they remain alive and active. Traditionally, large cages are deployed for this, requiring heavy (hence costly) deployment and retrieval equipment, and environmental risks. Therefore, we developed a much lighter alternative, in the form of the so-called WERC-dock. This is a compact, but stable and robust device that holds oyster baskets which can be deployed to and lifted relatively easily from the seafloor. Hence, WERC-dock is a cost-effective alternative for the original large cages.
Measuring oyster activity: valve gape monitor
As stated above, cages can be used to inspect survival and condition of an oyster population. However, there is scientific and operational benefit if their condition can be monitored continuously and in situ. Therefore, the valve gape monitor was developed. This is essentially a frame which stands stably on the sea floor and to which oysters are attached, connected with electrodes for monitoring and recording their opening and closing. Combined with a Chlorophyll A sensor, turbidity and temperature, the valve gape monitor provides continuous insight in the oyster’s survival and daily activity, e.g. in relation to food availability and other environmental parameters.
Simplifying sampling and larvae analyses
It is also essential to be able to monitor oyster reproduction, i.e. to detect whether there are flat oyster larvae in the water column. However, traditional sampling techniques are difficult to use under the often rough North Sea conditions. We therefore developed an easy-to-use larval sampling technique, by which larvae were sampled by filtering 200 L (2 x 100L) of seawater per sample from 0,5 – 1 m above the seafloor with the help of a 30 m long hose and a pump. This appeared to function well.
We also showed that larvae can be detected by DNA-analysis (qPRC), which is easier to use than the traditional visual analysis by microscope. We compared the results of DNA-analysis with the traditional (and reliable) visual inspection by microscope, and found high variability, hence it is recommended to further improve the DNA-technique.
Flat oyster produce larvae during relatively short peak moments (several weeks), hence it is important to be able to predict when these are present in the water to perform the sampling in the right period. Again, this is important for the work in offshore conditions (window of opportunity for sampling), since a fruitless sampling expedition bears heavily on a project’s budget. The common prediction method employed is measuring the local seawater temperature, but this is usually too cumbersome in offshore situations. Therefore, we developed a method in which the 3D Dutch Continental Shelf Model in Flexible Mesh (3D DCSM-FM) can be used to predict the temperature development in a given year. This model is very versatile and can for example predict currents, temperatures, and other abiotic variables at any giving location in the North Sea. In this project we used and improved the model, among others with temperature data measured by logging devices attached to harbour seals, to produce the temperature predictions.
What causes oyster motion on the sea floor?
An important other application of the 3D DCSM-FM model in the ECOFRIEND project was the determination of flat oyster mobility on the sea floor. To create an oyster reef, it is important that the initial population (a number of young and/or adult oysters introduced from elsewhere) deployed at the intended restoration site remains in place there. However, of a relatively large population (80,000 oysters) loosely deployed oysters at the Borkum Reef Grounds (in a pilot undertaken by WWF Netherlands) only a few could be retraced in later monitoring. We therefore conducted an experiment with ‘dummy oysters’ at Deltares. Hundreds of vividly coloured oyster shells doublets, of different weights and sizes, were subjected to various wave regimes representative of the Borkum Reef Ground seafloor conditions, in a Deltares test basin. Then they were taken to the Borkum Reef Grounds and dropped at the seafloor. One year later a scuba-dive and ROV operation was organised to see whether they could be found back, but that was not the case. On the basis of the 3D DCSM-FM model and the results from the basin experiments, it could be shown that the turbulence caused by regular strong storm events indeed causes dispersal of flat oysters in the seafloor in water depths less than ca. 40 meters, as is the case in Borkum Reef Ground. For a variety of practical reasons, all other flat oyster pilots in the North Sea occur at less than 40 meter water depth. Hence, deploying loosely strewn flat oysters at the North Sea floor will not be conducive to reef formation in such projects. It is recommended to investigate and develop methods by which the initial population can be kept together on the sea floor, or measures to stabilise the seabed.
Where do oyster cages go?
We attempted to make 3D DCSM-FM model predictions of the movement of oyster cages if these, for one reason or the other (inadequate design, unforeseen turbulence etc.), happen to move over the seafloor. It is important to be able to trace these, since a moving cage can cause damage to the wind farm infrastructure and one does not want to lose the monitor population either. The model predicts at what level of hydrodynamic forcing the oyster cages start to move. The rough movement paths of such cages could indeed be predicted, but probably not precise enough to be able to find them back. However, the model can be used to design future deployment cages that will stay stable under the prevailing metocean conditions.
Oyster larvae detected far away from restoration projects
During ECOFRIEND fieldwork, larvae were sampled at 2 locations where oysters restoration pilots were located (Gemini wind farm and the Borkum Reef Ground), for which Norwegian oysters were used, as well as at a control location roughly halfway between these sites (the ‘Halfweg’ location). Surprisingly, in two consecutive years, the control location yielded higher larval concentrations than at the restoration sites. Modelling showed that the larvae at the Halfweg location were unlikely originating from either of these two sites. This would indicate to the presence of a relic population, capable of reproduction, of which the scale and position is currently not identified. Genetic analyses of the larvae should yield more insight in their potential origin.
Since flat oyster reef restoration is intended to enhance the North Sea biodiversity, it is important to be able to monitor whether local biodiversity around an initial reef is indeed increasing. To get an impression of the (fish) biodiversity of the environment, we used environmental DNA (eDNA) sampling using Niskin bottles. This proved to work really well and is now commonly used in many other nature restoration pilot projects in the Dutch North Sea. We also conducted trials with bait cams and a consumer type underwater drone (ROV). Both methods proved to work (after some adaptations in the bait cam set-up) to get an impression of the fish and benthic community present. For example, fish species recorded on the bait cam also appeared in the eDNA results.
These results and recommendations have already been disseminated widely, via various communication methods and channels. This enables the flat oyster restoration community and other interested parties, such as the offshore industry, to use them for design, execution and monitoring of flat oyster restoration pilots in the offshore marine environment.