Return of the native facilitated by the invasive? Population composition, substrate preferences and epibenthic species richness of a recently discovered shellfish reef with native European flat oysters (Ostrea edulis) in the North Sea

Abstract

van den Wijngaard & T. M. van der Have (2018) Return of the native facilitated by the invasive? Population composition, substrate preferences and epibenthic species richness of a recently discovered shellfish reef with native European flat oysters (ABSTRACT After being ecologically extinct for almost a century, the discovery of a shellfish reef with native European flat oysters (Ostrea edulis) in the Dutch coastal area of the North Sea by the authors of this study called for an extensive survey to better understand some of the key requirements for the return of the native oyster in coastal waters. We assessed habitat conditions, its potential for increasing biodiversity, and the role of substrate provision by other bivalves such as the invasive alien Pacific oyster (Crassostrea gigas). Using underwater visual census, O. edulis size-frequency distributions and attachment substrate was investigated, as well as the composition of the epibenthic community and substrata types inside quadrats that were distributed across the reef. This reef was found to be composed of native European flat oysters, invasive alien Pacific oysters and blue mussels (Mytilus edulis), alternated with sandy patches. The O. edulis population (6.8 ± 0.6 oysters m −2 ) consisted of individuals of different size classes. In quadrats with native and non-native oysters the number of epibenthic species was 60% higher compared to adjacent sand patches within the reef. Notably, our results showed that the native oyster predominantly used shell (fragments) of the invasive Pacific oyster as settlement substrate (81% of individuals). Our results optimistically show that conditions for native oyster restoration can be suitable at a local scale in the coastal North Sea area and suggest that the return of native oysters may be facilitated by novel substrate provided by invasive oysters at sites where their distribution overlap. ARTICLE HISTORY

Wild gregarious settlements of Ostrea edulis in a semi‐enclosed sea lough: a case study for unassisted restoration

“…However, rejuvenation within wild O. edulis assemblages is rare with only a few remote sites displaying tentative recovery. Nevertheless, those which do exist could offer valuable indicators as to what in situ conditions are required to reinstate or support a restored population (Laing et al ; Christianen et al ). One such location worthy of investigation is the small, semi‐enclosed sea Lough of Strangford on the northeast coast of Northern Ireland.…”

Ostrea edulisat shipwrecks in the Dutch North Sea

“…To date, the European flat oyster was considered extinct offshore in the northern area of the Dutch part of the North Sea. Only in southern Dutch North Sea waters and in the Wadden Sea flat oysters had been observed recently (Bouma and Lengkeek, 2013; Christianen et al, 2018; Kamermans et al, 2018; Kerckhof et al, 2018). During visits of over 100 ship wrecks between 2010 and 2019 by the same group of wreck divers, no other live flat oysters were observed offshore (personal observations by the authors), with exception of locations in the south of the Netherlands and Belgium (Kerckhof et al, 2018).…”

“…Medium sized white dot with black centre: unconfirmed sighting offshore (Christiaan Huygens wreck; Kerckhof et al [2018]). Medium sized light grey and intermediate grey dots: sightings during dedicated one-time inventories (Bos et al, 2019; Christianen et al, 2018; van der Have et al, 2017). Dark grey medium sized dots: presence observations from fish and shell fish surveys extracted from FRISBE-database (data 2007-2017), Wageningen Marine Research.…”

“…Dark grey medium sized dots: presence observations from fish and shell fish surveys extracted from FRISBE-database (data 2007-2017), Wageningen Marine Research. Small grey (background) dots: absence sightings from fish and shellfish surveys (Bos et al, 2019; Christianen et al, 2018; van der Have et al, 2017). Striped line: Borders of exclusive economic zones of Belgium, the Netherlands, Germany, Denmark and United Kingdom.…”

“…The European flat oyster Ostrea edulis once formed an extensive population in offshore Dutch waters (Olsen, 1883), that has been overfished and was decimated around 1900 (Kamermans et al, 2018). Nowadays, the flat oyster is considered extinct offshore in the Netherlands (Berghahn and Ruth, 2005; Christianen et al, 2018; Kamermans et al, 2018). Although coastal populations have been reported recently in the south of the Netherlands, near shore and offshore in Belgium and in the Dutch part of the Wadden Sea (Bos et al, 2019; Christianen et al, 2018; van der Have et al, 2017; Kerckhof et al, 2018), no living O. edulis of natural origin have recently been recorded in Dutch offshore waters north of the Offshore Wind Farm Egmond aan Zee (OWEZ) where the most northern recent living offshore oyster in the Netherlands was found (Bouma and Lengkeek, 2013, but see Kerckhof et al, 2018).…”