Introduction
Conservation and management concerns
Methods and approach
Results
Impacts on species diversity and ecosystem functioning
Habitat | Region | Impact | References |
---|---|---|---|
Littoral rock | Southern England, Kent | Patches of C. gigas reef (200 ind. m−2) present on lower shore of chalk reef (Fig. 1) | Herbert et al. (2012) |
France, Bay of Brest (moderately exposed shore) |
C. gigas reef at all tidal levels (Mean High Water to Mean Low Water); Biomass and species richness significantly higher on C. gigas reefs compared to adjacent rock; Deposit and detritus feeders occurred only on oyster reefs and not on adjacent rock | Lejart and Hily (2011) | |
Ireland | Experimental addition of living or dead C. gigas. Effects varied with state and cover of oysters. Boulders with lowest cover of living C. gigas supported greatest diversity | Green and Crowe (2013) | |
USA, Pacific Northwest |
C. gigas common on sheltered rocky shores (low energy littoral rock) and rare (<10 % cover) on exposed shores | Ruesink (2007) | |
Canada, Strait of Georgia | Reef formation not reported from British Columbia, though higher densities are present in areas where warmer waters cause more frequent settlement | J. Ruesink (Pers. comm) | |
C. gigas settles within the barnacle zone where they may provide a greater surface area for settlement. In experimental manipulations, seastars and crabs reduced monthly survival rates of C. gigas by 25 % relative to caged oysters Some neighbouring species on exposed rocky sites might facilitate survival of C. gigas by reducing physical stresses | |||
Canada, British Columbia |
C. gigas was able to modify the thermal regime of its habitat and provide refugia for those species that might otherwise suffer from desiccation | Padilla (2010) | |
Argentina (1982) | Among eight epifaunal species, three occurred at higher densities within oyster beds and three were more abundant outside these areas | Escapa et al. (2004) | |
Littoral sediments | Southern England, North Sea, English Channel | Reef formation since 2007 (Fig. 2) | Herbert et al. (2012) |
Wadden Sea | Reef formation present on lower shore | Reise (1998) | |
Netherlands, Oosterschelde estuary | First natural recruitment in 1975. Reefs mapped in 1980 | ||
France, Bay of Brest, Brittany | Invertebrate species richness on mud beneath C. gigas reef was twice that of adjacent mudflats and dominated by carnivores, compared to suspension feeders in mudflats | ||
Ireland | Experimental addition of different covers of oysters in small plots in two estuaries. Diversity and abundance of species increased with cover of oysters. Effects on microbial communities and ecosystem processes varied with cover. Sediment–water fluxes and turnover of ammonium and silicate were greatest at medium cover and decreased with greatest cover | ||
Saltmarshes and saline reed beds | Southern England, Kent | No settlement observed, however stabilization of sediment by oyster shells may both facilitate further colonisation of non-native Spartina anglica and potentially create a firm habitat for oyster settlement | McKnight (2011) |
Argentina | Colonisation of C. gigas on the stems of the saltmarsh cord grass Spartina alterniflora
| Escapa et al. (2004) | |
Saline lagoons | Southern England, Fleet Lagoon (1988) | Little settlement. The special flushing characteristics of the lagoon and crab predation may provide resilience to wild settlement | Eno (1994) |
France |
C. gigas is cultivated in micro-tidal lagoons and has established wild populations in some areas | Miossec et al. (2009) | |
Blue mussel beds (Mytilus edulis) | Netherlands, Wadden Sea From Mean Tide Level (MTL) to the shallow subtidal |
Mytilus-beds have changed to mixed reefs dominated by 95 % C. gigas. Mussels recruit frequently and settle amongst the oysters, migrating to lower regions in the interspaces between the oysters to evade predation | |
Germany, Wadden Sea | Mixed mussel-oyster reef showed increased species richness, abundance, biomass, diversity and deposit feeding species, compared to mussel beds | Markert et al. (2010) | |
Sweden, Skagerrak, shallow subtidal | No significant differences in macrofaunal species richness compared to oyster beds, however species abundance in oyster beds was statistically higher in two out of three sites. Differences in macrofaunal composition were inconsistent | Hollander et al. (2015) | |
Ireland | Experimental addition of different covers of oysters in small plots in two estuaries. No effects on diversity or abundance of associated fauna, except a decrease on one sampling occasion at one site. Ecosystem processes including respiration, sediment–water fluxes and turnover of ammonium and silicate increased with increasing cover of oysters | ||
Polychaete worm reefs (Sabellaria alveolata) | France, Bay of Mont-Saint Michel | Oysters are colonising some S. alveolata reefs with densities >100 ind. m−2
| Dubois et al. (2006) |
France, Bay of Mont-Saint Michel | Higher species richness recorded on Sabellaria reefs colonised with oysters (and with oysters and algae). Colonisation has led to damage of Sabellaria by recreational oyster harvesters. aquaculture is also thought to have contributed to habitat deterioration | ||
France, Bourgneuf Bay, Brittany | Growth and settlement of wild C. gigas has transformed areas where former S. alveolata beds had previously been recorded, so recolonization is now unlikely | Cognie et al. (2006) | |
Ireland | Oysters attached experimentally to topsides of boulders inhibited settlement of S.alveolata on undersides | Green and Crowe (2013) | |
Sabellaria spinulosa
| Southern England, Kent extreme lower shore | An area of intertidal S. spinulosa in reef formation is being overgrown by C. gigas
| |
Lanice conchilega
| Southern England, Kent Extreme lower shore | Chalk reef colonised by 50 % cover of L.conchilega worm reef is partially displaced by Pacific oysters (maximum density of C. gigas 14 ind. m−2) | McKnight (2011) |
Seagrass beds (Zostera spp.) | France, Thau lagoon, Mediterranean | Increased water clarity caused by the uptake of particulate material and phytoplankton by C. gigas and mussel aquaculture, is thought to have enabled Zostera to grow in deeper areas of the lagoon | Deslous-Paoli et al. (1998) |
USA, Washington, Willapa Bay | General pattern of reduced density and shoot size of the native seagrass Z. marina on cultured C. gigas beds | Tallis et al. (2009) | |
USA, Washington, Willapa Bay | Shoot density and cover of Z. marina declined with increasing oyster density, attributed to space competition; this competition can generate impacts above thresholds of 20 % oyster cover. At low densities, C. gigas has little impact, however oyster cover >50 % is impenetrable to seagrass | Wagner et al. (2012) | |
USA, Washington, Willapa Bay | Immediately seaward of the C. gigas zone and amongst adjacent Z. marina beds, benthic diversity was greatest below the C. gigas beds, yet fish and pelagic invertebrates were more abundant within seagrass | Kelly et al. (2007) | |
Subtidal sediments | Southern England, Thames estuary | Seen at least 3 m below Chart Datum on subtidal sediments | Herbert et al. (2012) |
Northern Ireland, Lough Foyle | Present on subtidal sediments | Herbert et al. (2012) | |
Ireland, Lough Swilly | Present on subtidal sediments | Herbert et al. (2012) | |
Netherlands, Oosterschelde estuary | Settlement observed in cultivated subtidal Pacific oyster beds (depth 2–3 m) and on adult oysters at 10 m depth | Wijsman, pers. comm. | |
Germany, Wadden Sea | Adults found at 10 m below low water, however no juveniles or recruitment observed. Are often large individuals or clusters. Most likely broken off intertidal reef structures | (Reise, pers. comm.) | |
Subtidal sediments contd. | Wadden Sea | Sublittoral stocks estimated as occupying 700 ha however it is unclear whether this is as a result of recruitment | |
Sweden | Pacific oysters found at depths from 1–9 m | Dolmer et al. (2014) | |
European native flat oyster Ostrea edulis
| Wadden Sea | Overlap with native oyster not expected as it is sublittoral | Reise (1998) |
Southern England, Poole Harbour | Found to settle on shells and living C. gigas
| Authors observation | |
Fish | USA, Washington, Willapa Bay | Immediately seaward of the C. gigas zone and amongst adjacent Z. marina beds, fish were much more abundant within the seagrass | Kelly et al. (2008) |
Birds | Wadden Sea | Species that have previously relied on mussels (e.g. Eider duck, Somateria mollissima), may not be able to feed on the oysters due to their size, shell thickness and cementation | |
Netherlands, Oosterschelde | Herring gull (Larus argentatus) and Eurasian oystercatcher (Haematopus ostralegus) are reported to feed on C. gigas
| ||
Wadden Sea | Colonisation of M. edulis beds by C. gigas had a positive impact on feeding rates of the Eurasian oystercatcher and the Eurasian Curlew (Numenius arquata) | Markert et al. (2013) | |
Argentina | Number of birds (two gulls and four wading bird species) was greater amongst C. gigas compared to control areas. Foraging rate of two species was higher amongst oysters, whereas in other two species there was no difference with control plots | Escapa et al. (2004) |
Habitat transformation and homogenisation
Policy framework and management measures
Alteration of community structure |
---|
Reduction in area of habitat/biotope or species for which the site was originally notified |
Causes on-going disturbance to species or habitats |
Presents a barrier between isolated fragments of native habitats, or reduces the ability of the site to act as a source of new native colonisers |
Causes direct or indirect change to the physical quality of the environment (including the hydrology) or habitat within the site |
Causes direct or indirect damage to the size, characteristics or reproductive ability of populations on the site |
Alter the vulnerability of populations/habitats to other impacts |