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

Abstract

Ostrea edulis was once prolific throughout Europe and considered as the continent's native oyster. However, O. edulis currently exists in small fragmented assemblages where natural unaided recovery is rarely encountered. This research identified the small semi-enclosed sea Lough of Strangford on the northeast coast of Northern Ireland as one of the few locations within Europe where the native oyster displayed gregarious natural rejuvenation. On close examination, four influential parameters appeared to assist in concentrated settlement; raised topographical cultch formations, shell coverage, the number of fecund in situ adults, and site protection. If these components were to be combined and managed as part of reintroduction and restoration initiatives, high-density settlements and self-sustaining populations may be possible. The research also identified that unregulated harvesting of intertidal O. edulis assemblages has the potential to seriously hinder natural recoveries. Indeed, the findings suggest that a review of policy in regards to intertidal hand gathering is necessary. However, naturally occurring high-density settlements recorded during this research should be inspirational to all involved in the restoration of the native oyster. Implications for Practice• Examination of naturally occurring native oyster expansion and the parameters which permit augmentation can provide an insight into which components could be implemented to aid successful assisted restoration.• A suite of abiotic and biotic conditions can accommodate mass gregarious Ostrea edulis settlements; hydrodynamics which allow water retention, adequate coverage of suitable substrate, intertidal topography which provides pool structures, in situ resident adults and policing of settlement sites.• Unregulated harvesting remains a constant threat to restoration and protection of the species needs to be implemented if a self-sustaining status is to be achieved.

A first record of intertidal Ostrea edulis 3D structural matrices in Strangford Lough Northern Ireland - An emergent reef?

“…When casting doubt on the reef forming potential of O. edulis, the cementation and settlement biology of its larvae should not be ignored as it has been shown, on numerous occasions, that pediveligers favour the living shell edge of conspecifics (Korringa, 1941;Cranfield, 1973;Rodriguez-Perez et al, 2019). This behavior supports previous work conducted in Northern Ireland (Smyth et al 2020). It is therefore quite plausible that in a situation when all the key settlement components are in place that multiple oyster attachments could result in 3D reef-like matrices.…”

“…While local fishermen were harvesting live oysters from the Bulgarian reefs as recently as 2002, there are no living O. edulis on these reefs today (Todorova et al, 2009). Prior to these discoveries, it was questioned as to whether O. edulis could form 3D structures as no substantial historical evidence of live formations had been recorded (Smyth et al, 2020). However, in I853 a report by Coste gives vague reference to the condition of fallowed O. edulis beds in northern France which, 'become coarse with barnacles and other parasites and adhere together in thick beds which have to be broken up' (Eyton, 1858).…”

“…The Lough once held a historically renowned Irish stock of O. edulis in both the intertidal and subtidal (Day and McWilliams, 1991). In addition, Kennedy and Roberts (2006) and Smyth et al (2020) recorded multiple O. edulis attachments of up to five oysters (known locally as 'Cloks' where traditionally fishermen said more than three joined oysters made a Clok) in remote unfished areas of the Lough.…”

“…It also benefits from a zone of approximately 90 km 2 which is closed to static and mobile fishing which is patrolled regularly by the authorities (Johnson et al, 2008). Furthermore, the north of Strangford Lough possesses a mean flow < 0.15 m/s, hydrodynamic conditions low enough to initiate larval pooling while also providing suitable intertidal settlement substrate (Kregting and Elsäβer, 2014;Smyth et al, 2016Smyth et al, , 2020. Moreover, the small resident population of O. edulis estimated at < 800,000 within the 75 km 2 northern basin of the Lough (Smyth et al 2016), could produce a substantial spawning response to high sea temperatures, such as those experienced in 2014 (MCCIP, 2017), thereby creating a situation which could be conducive to mass concentrated settlements.…”

Climate change accelerates range expansion of the invasive non-native species, the Pacific oyster,Crassostrea gigas

“…In some ways, C. gigas naturalisation may benefit or safeguard coastal ecosystems of the NWES. Historically, NWES coastlines included dense populations of native European oysters, Ostrea edulis, but overfishing and disease decimated populations resulting in a 95% decline in abundance since the 1950s (Thurstan et al, 2013;Smyth et al, 2020). This decline has resulted in altered benthic assemblages across Europe and has undoubtedly shifted ecosystem function.…”