Interest in restoring the native oyster in the Dutch Wadden Sea dates back to 2005 when the cooperating nature organisations included the native oyster as part of their ambition of an “ecologically sound Wadden Sea” in the booklet ‘Het Tij Gekeerd’. In 2010 the “Programme towards a Rich Wadden Sea” (PRW, www.rijkewaddenzee.nl) was established.
The PRW-approach is to identify with stakeholders the most appropriate native oyster restoration strategy. A first step was taken in 2011 with the ‘Biobouwersdag’, an expert meeting about the role and improvement challenges of bio-engineers. At the end of 2015 PRW held an workshop to assess the potential use of artificial reefs in the Dutch Wadden Sea. A feasibility study for native oyster restoration projects followed in 2016, through which a factsheet was also developed. In October 2016 there was a discussion, organized by the University of Applied Science Van Hall Larenstein about the possibilities of commercial use of re-introduced native oyster. The feasibility study resulted in the development of a strategy: Get a good insight in the potential hotspots for native oyster restoration, earn from restoration projects e.g. in the Voordelta, be aware of the ecological relationships with e.g. the Pacific oyster and risk management e.g. Bonamia. In the meantime a commercial Crassostrea gigas collector reported findings of living Ostrea edulis specimens on mud flats in the western Wadden Sea. And researchers of the NIOZ-institute initiated some first surveys.
During the summer of 2017, a survey was conducted by Bureau Waardenburg, Wageningen Marine Research and Altenburg & Wymenga, resulting in the location of ca. 51 live Ostrea specimens. In addition, Ostrea larvae were found in the water nearby. The origin of the oysters is unknown, but they may have been the result of a recent accidental introduction. In the winter 2017/2018 the rangers of the Waddenunit ship Asterias found a population of native oyster in the Vliesloot, south of the island of Vlieland, however, winter conditions prevented detailed investigation.
With the results of this survey the strategy was developed further in order to address two issues. Firstly, developing production of native Wadden Sea oysters for local restoration projects, and secondly to research Bonamia resistance. As a result, PRW became co-funder of a NIOZ initiative for a hatchery and nursery to get ‘Dutch Wadden Sea’ stock of spat, and PRW also now co-funds research into Bonamia-resistance in native oyster of Zeeland and the Wadden Sea stock.
Between 2012 and 2016 a big practical ecosystem research project was done on the littoral part of the Dutch Wadden Sea; Waddensleutels. Together with the project Mosselwad there was interest in the restoration possibilities of litoral musselbeds. In 2019 the sublittoral research project Waddenmozaïk (www.waddenmozaiek.nl) started. This project aims to understand to the potential to restore the area, including native oysters.
2019 also saw the initiation of PAGW, a large programme for nature improvement of the ‘grote wateren’, (the Southwest Delta waters, the main rivers, the Lake Ijsselmeer and the Wadden Sea), of which native oyster restoration is a part. In 2020 three additional initiatives started; a small practical survey about the survival possibilities of young native oysters in the Western Dutch Wadden Sea (report will be published soon), an exploration of the presence of the native oyster in the Eastern part of the Dutch Wadden Sea and a practical survey of survival, growth and reproduction of native oysters in the (Eastern part of the) Dutch Wadden Sea. The first two financed by PAGW, the latter by PRW. The results of the exploration and the practical survey will come in 2021.
Again, adjustment of the strategy seems to be necessary. If there are specimens of native oyster in the Eastern part of the Dutch Wadden Sea, then restoration or dispersion projects are not the optimal restoration stratefy. Recovery may in that circumstance be best achieved by protection of actual or potential habitats, as opposed to active restoration. As it seems that the Wadden Sea native oyster is Bonamia-free, it is critical that the the Wadden Sea population is separated from populations where Bonamia is present.
Concluding, since the potential to restore the native oyster to the Wadden Sea was first raised (Het Tij Gekeerd, 2005) the interest and awareness has grown. Debates, surveys, detailed research-projects and adaptive strategies have given the Dutch Wadden community a practical roadmap.
Programme towards a Rich Wadden Sea / PAGW