The demise of the European Native Oyster, Ostrea edulis, from that of a keystone species to an obscure Bivalve, throughout the majority of its natural range has been well documented. A number of factors have been associated with the demise of global estuarine Oyster populations between the mid-1700s and late-1800s; industrial pollutants, coastal development, increases in sewage outflow and growing urbanisation. However, the most prominent drivers related to historical losses in Native Oyster populations have been identified as overfishing and disease. The combination of these stressors devastated stocks to such an extent that the Oyster still remains extinct from many of its historically prolific sites more then 100 years after its disappearance. Instances whereby the Native Oyster has returned unaided are rare and those which have been documented were often the result of an aquaculture spawning event. Pro-active interventions using stock augmentation are therefore considered vital if Ostrea edulis is to make a return to its historic locations. As a result, numerous Ostrea edulis restoration programmes are currently underway throughout the UK and Europe in an attempt to address the Oysters decline.
Belfast Lough in Northern Ireland once accommodated a substantial population of Native Oysters. The first report of a recognised commercial oyster fishery in the Lough was in 1780 when it was stated that 'the Oyster is dredged from September to May by 27 boats and 123 fishers all of whom can read with the exception of two'. The report also implied that Oyster stocks were in a state of decline as a boat could dredge 800 to 1200 large Oysters a day at a price of 4–7 shillings per 100 in the year of 1800. However, by 1819, a boat was dredging between 100 and 300 Oysters a day at a price of 8–18 shillings per 100. Since the official closure of the Belfast Lough fishery in 1903 there have been no reports of wild Ostrea edulis, no aquaculture stocks and no restoration attempts.
An investigation into the 1897, 1898 and 1901 enteric fever epidemics of Belfast City by the physician Dara Mair stated that, ‘the working class of Belfast were heavy consumers of shellfish Including Periwinkles, Cockles, and Mussels but not Oysters as these have been practically extinct for many years’’. The Irish Fisheries Commission considered the Belfast Lough Oyster fishery officially closed in their report of 1903. The renowned malacologist Nora Fisher McMillan did not record any living Ostrea edulis specimens during meticulous surveys of the Lough between 1928 and 1929. John Gee and Kieth Wilson some fifty years later still failed to document the Oyster on a comprehensive Molluscan species list and more recently, in 1999, malacologists from the Ulster Museum Belfast could not detect any living native Oysters during an extensive subtidal and intertidal investigation. The 2002 Joint Nature Conservation Committee survey of the Lough was also unsuccessful in locating any live specimens and in 2017 the Sanitary Review of Belfast Lough only documented the presence of sub-fossil shell.
These historical accounts and surveys confirm the European Native Oyster as being absent from Belfast Lough for over 100 years. However, there have recently beeb unconfirmed reports of solitary Ostrea edulis along the intertidal zone of the Lough.
In a paper published in the journal Regional Studies in Marine Science in December 2020, David Smyth, Maria Hayden-Hughes, Jenna Alexander, and Philippa Bayford of the School of Ocean Science at Bangor University Wales, and Louise Kregting of the School of Natural and Built Environment at Queen’s University Belfast, present the results of a study which investigated these unsubstantiated sightings of individual Ostrea edulis to ascertain if an unassisted recovery had indeed occurred after more than a century.
Belfast Lough is a fully marine inshore body of water located on the east coast of Northern Ireland. The Lough is a relatively shallow marine bay roughly 21 km long and 11 km wide, with a max depth of 23 m covering an area of 130 km² with annual seawater temperatures of between 2 to 21°C. The intertidal zones of both shores are characterised by a sandy mud substrate with high volumes of overlaying shell material. The three most abundant contributing species to substrate mixes are; Mytilus edulis, Cerastoderma edule and Artica islandica. The main freshwater input into Belfast Lough is via the River Lagan, at a mean flow of 8.521 cubic metres per second.
The River Lagan is impounded by a floodgate, the Lagan Weir, which typically only allows water exchange for 2 hours either side of high tide. According to the water balance index there is a clear dominance of the tidal dynamics against river dynamics in the Lough. Belfast Lough is also subject to intensive anthropogenic stressors. The major industrial shipping port at the head of the Lough manages over 80% of Northern Ireland’s petroleum and oil imports. Furthermore, Belfast Port handles more than 7000 vessels per year with an average freight through flow of 24.6 million tonnes. The inner lough accommodates 21 licenced Blue Mussel, Mytilus edulis, mariculture beds which are fished by dredge. Fishing activity in the outer Lough focuses on pot fishing, Scallop dredging and bottom-trawling for Dublin Bay Prawns (Langoustine), Nephrops norvegicus.
In order to ascertain if Ostrea edulis had settled along the intertidal zone of the Lough, both east and west shores were surveyed between May and June 2020 using belt transects of 5 m carried out 1 m from the low water mark on tides lower than 0.8 m below datum chart. Ten × 1 km transects were completed along the west shore starting at Shellbank and finishing at Kilroot. Dangerous sheer rock terrain and fine sand substrates limited the east shore survey to six × 1 km transects starting at, Fujitsu Shore and finishing at Rock Beach.
If an Oyster was recorded along a transect, a 20-minute timed search was carried out at the site of settlement within a 5 × 20 m survey plot. Substrate type was recorded at all sites and all oyster height measurements recorded in-situ using Vernier callipers. Specimens were photographed using a Canon Powershot G16 on auto setting:19 mm focal length, ISO 100, 1/50 s at f/5.6. 16:9 aspect ratio with a reference in frame for scale. Images were calibrated using Coral Point Count to the suggested known overhead distance of (1 m). The image measuring application in Coral Point Count was applied to each image to obtain morphometric data which could be applied to the shell age associations for Ostrea edulis.
As this was a baseline investigation, statistical analysis was limited. However, a t-test was carried out to examine the total number of Oysters recorded on the east and west shores to determine if settlement was governed by location. In order to ascertain if a difference existed in the size of Oysters recorded at each site a one-way permutational multivariate analysis of variance was carried out between site and size of individual oysters using PAST vr3.4.
Within the intertidal zone of Belfast Lough, live Ostrea edulis were recorded at six sites. The west shore had five sites and the greatest number of Oysters with 32 individuals, the smallest being 27 mm in length and the largest 112 mm. The size and age variations between sites indicated that recruitment has been on-going within the Lough over the past 8–10 years. The average age of Oysters on the west shore were between 3–4 years with an average shell height of 64 mm. The east shore had one settlement site with nine individuals, the smallest being 33 mm and the largest 72 mm. The average age within the assemblage was 1–2 years with an average shell height of 50 mm. However, it must be emphasised that there may be variations within these age determinations these estimates were based on Ostrea edulis specimens from Essex which sits three lines of latitude below the sites in Belfast Lough.
Intertidal Ostrea edulis assemblages: (4) Gideon’s Green, (5) Hazelbank, (6) Whiteabbey, (7) Jordanstown, (8) Greenisland and (13) Kinnegar Barracks. Map generated using Arc Map 10.7.1 spatial analysis WGS84 coordinate geometry used throughout. Smyth et al. (2020).
A t-test was carried out using PAST vr3.4 between the total Oyster abundance of west and east shore assemblages, no significant difference was detected. Furthermore, permutational multivariate analysis of variance analysis did not detect any significant differences between Oyster size and site. A basic substrate description was assigned to each survey transects and live Oyster sites.
Unsubstantiated reports of solitary European Flat Oysters on the shores of Belfast Lough suggested that Ostrea edulis had returned to the waterway after more than a century of absence and the findings presented by Smyth et al. have categorically shown that this is indeed the case. A total of 42 Oysters were recorded with sizes ranging from 28–112 mm indicating a population consisting of both 0–1-year juveniles and adults of over 8 years. These variations in age are significant and indicate that recruitment had been taking place unnoticed within the Lough for at least 8-10 years.
While the number of Oysters does not signify a sustainable population, it does raise questions as to; where did they come from and what allowed the settlement of larvae to occur? There have been no attempts at Ostrea edulis aquaculture within the Lough’s catchment and therefore the recent settlements must have been incidental. An understanding of recent abiotic and biotic changes within the Lough may offer some explanations as to how Ostrea edulis has managed to make a return. An, important recent development within the Lough to be noted before assumptions are made occurred in 2016 with the dredging of the central channel. This was carried out to accommodate an increase in shipping traffic which had been steadily growing since 2000 when 5336 vessels visited Belfast Port to more than 6800 in 2018. As a result, over 400,000 m³ of dredge spoil were removed to widen the main shipping routes and accommodate deeper drafts.
The hypothesis that shipping ballast water spreads Molluscan species has been postulated since the late 1800s and is now a well-documented route for invasive and native species into new areas. The transportation of the Pacific Oyster, Crassostrea (or Magallana) gigas, larvae has been periodically detected in ballast water sources. It has been hypothesised that ballast water transfer in the 1990s was the cause of Crassostrea gigas introductions into the east coast of Scotland as there had been no aquaculture ventures for the species in the region.
The increases in shipping to Belfast Port over the last decade offer the possibility for a ballast water induced spread via the transfer of pelagic larvae through ballast deposition. However, the reality of this is doubtful as the global sources of planktonic Ostrea edulis larvae are extremely rare and mortalities within the pelagic stage of the life cycle are considerable.
A more probable explanation is that adult Oysters were introduced through the commercial Blue Mussel fishery. The Lough has 21 licenced subtidal Mytilus edulis mariculture sites spread over a number of plots located either side of the shipping channel. The fishery plots are seeded with juvenile Mussels dredged from various locations in the Irish Sea. It may be that within some of the dredged seed that a number of fecund Ostrea edulis were collected and relayed. However, the translocation of seed Mussel has been carried out for over 30 years with no Ostrea edulis settlements recorded. Therefore, if the mariculture plots have been the source of Oyster larvae, it must have been from seed deployed within the last ten years. This is significant as records of the seed Mussel site locations for the 2008, 2009 and 2010 relays may reveal a possible undiscovered Irish Sea population of Native Oysters.
In addition, abiotic or biotic changes within Lough over the last 10 years could have created conditions conducive to promoting the pooling and settlement of Oyster larvae. Recent bathometric changes through the deepening of the central shipping channel and the resulting subsequent alterations to fine-scale hydrodynamics, may offer an explanation. However, fine-scale particle tracking and hydrodynamic modelling of the watercourse would be required to confirm this theory and unfortunately this was economically beyond the resources of Smyth et al.'s short survey.
Belfast Port sits at the head of the Lough and is the largest commercial harbour in Northern Ireland. Anthropogenically introduced disturbances such as dredging are often associated with detrimental changes in marine habitats. However, in the case of Belfast Lough the increases in shipping and the subsequent dredging induced bathymetrical changes may have created a situation whereby the hydrodynamic regimen is now in a position to once again permit Ostrea edulis larval settlements.
Increases in vessel activity and their subsequent wakes in particular have been shown to be instigators of environmental change within soft sediment benthic communities. Wake generated water velocity has led to instances of increased siltation in shallow estuarine systems and the subsequent smothering of bivalve beds and the burying of hardshell substrates. However, this is not always the situation and a number of factors need to be considered when assessing the effects of vessel wakes. Sediment composition, channel geometry, distance to shore, the number of successive vessels and their speed of passage can all influence wake effects.
In 2012 Kyle Demes, Rebecca Kordas, and Jennifer Jorve, showed in British Columbia that primary production actually increased on rocky shore sites which lay favourable distances from intermittent ferry wake pulses. The distance from the wake’s origin can have a significant influence on the degree of environmental stress. In some instances, Crassostrea virginica Oyster reef systems in shallower estuarine regions were displaced by wakes with heights as shallow as 2 cm. However, wakes were shown to have had no impact on recruitment. Indeed, the dislodgement of Oyster assemblages may have actually aided the geographical expansion of Crassostrea virginica within estuaries.
The combined effect of regular wake travel to and from the shore can result in cleaning substrate surfaces of sediment through a washing effect. This process can lead to increases in species abundance and richness along intertidal zones which are situated sufficiently far enough from the initial high velocity water pulse.
Smyth et al.'s studty has revealed no Oyster settlement close to the Port entrance even though shell substrate was substantial. Therefore, the speculation that the mariculture plots were a source of larvae becomes ever more likely as settlements were localised to sites within the boundaries of the Mussel fishery lays and between 3–7 km from the head of the Lough. A distance which appears to have been suitably far enough from initial wake amplitude to allow shell substrates to remain clear of siltation, while tidal water retention remained sufficient to enable Ostrea edulis pediveligers the opportunity to undergo cementation. The provision of clean settlement material and sufficient water retention have been recognised as optimal for larvae to settle and metamorphose.
The substrate mixes along Lough’s intertidal zones provide some excellent settlement areas, however the findings of the survey identified assemblages in close proximities to each other. Suggesting, that the assumptions as to how the Native Oysters returned is not confined to one factor but to a combination of the proposed hypothesises. The unassisted reoccurrence of Native Oysters in Belfast Lough is undoubtably a unique event with many questions needing addressed before a pathway of re-establishment can be confirmed.
The 2020 documented settlements of Ostrea eduli in Belfast Lough have raised a number of interesting hypotheses. Further research as to how the return of the Oyster was induced is required. In depth investigations into the effects of pollution and dredging impacts were practically and financially beyond the scope of Smyth et al.'s study. The main aim of this manuscript was to document that Oysters are once again present in Belfast Lough after a century of absence. The initiation of future studies should be considered a matter of urgency, as a better understanding of abiotic and biotic parameters within the Lough could greatly benefit numerous European Native Oyster restoration projects which are currently underway.
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