Invasion
Invasion Description
1st Record: Netherlands/Oosterschelde (1964, 1st spawning 1975, beds by 1980, very extensive by 2000, Troost 2010). Introductions in northern European waters were initially dependent on hatcheries, but now natural spawning and recruitment occurs in many regions (Eno 1997; Reise 1998; Troost 2010) [We are excluding introductions of M. angulata (Portuguese Oyster), which began in this region as early as 1850 (Wolff and Riese 2002).
Geographic Extent
Conwy, North Wales/Wales/Irish Sea (Walne and Helme 1979; Utting and Spencer 1992); Northern Ireland/Strangford Lough (1970s, Guy and Roberts 2010, limited natural settlement and spread by 2008; continuing spread, Zweeschke et al. 2016); Ireland/Irish Sea (1969, Minchin 2007); Port Edgar/Scotland, Whitehouse Bay, Firth of Forth (2007, Simith et al. 2015, established population); Zeeland, Belgium/Voor Delta (1969, Kerckhof et al. 2007); Netherlands/Oosterschelde (1964, 1st spawning 1975, natural beds by 1980, very extensive by 2000, Troost 2010); Netherlands/Lake Grevelingen (1987, Troost 2010); Amsterdam/Netherlands/Amsterdam Harbor (2007, Troost 2010); Texel/Netherlands/Wadden Sea (1983, Wolff 2005; Gittenberger et al. 2010); Balgzand/Netherlands/Wadden Sea (2001, Beukema and Dekker 2011, increase in abundance and distribution, 2001-2007); Thomton Bank Wind Farm, 30 km offshore/Belgium/North Sea (2011-2012, de Mesel et al. 2015, 51 32 52.73500 N, 2 55 46.27300E, occasional); Sylt/Germany/Wadden Sea (1986, natural spatfall seen since 1991, Reise 1998); Emden-Cuxhaven/Germany (2014, Rohde et al. 2017); Denmark/Wadden Sea (1970s, Wrange et al. 2010, natural spatfall seen in mid-1990s, 'The biomass was estimated to 1,056 tonnes in 2005 increasing to 3,289 tonnes in 2006. In 2007, the biomass had almost doubled, to a total of 6,264 tonnes with local densities of up to 55.8 kg m-2 Wrange et al. 2010)'; Denmark/Limfjorden (1972, Wrange et al. 2010, natural spatfall seen since 2002; Groslier et al. 2014, limited population growth, suboptimal conditions); SW England/Dart, Teign and Exe Rivers (1989, Utting and Spencer 1992)
Vectors
Level | Vector |
---|---|
Probable | Fisheries Intentional |
Regional Impacts
Ecological Impact | Habitat Change | |
On the Wadden Sea Coast of Germany and the Netherlands, M. gigas has been settling on mussel (Mytilus edulis) beds growing on mudflats, since at least 1991, resulting in overgrowth of mussels and attached barnacles, converting mussel beds to extensive oyster beds (Reise 1998; Diederich 2005; Gittenberger et al. 2010; Walles et al. 2015; Herbert et al. 2016). Oysterbeds provide potential habitat for attached algae, but native forms are outcompeted by the introduced Sargassum muticum (Lang and Buschbaum 2010). Overall, C. gigas beds supported greater abundance and diversity of native epi-and infauna than mussel beds (Markert et al. 2010). Oyster reefs are stablilizing the sediment, but also increasing the deposition of organic material (as pseudofeces), forming anoxic layers (Troost 2010). In experimental plantings, the polychaete Lanice conchilega was more abundant on oyster rings and the oligochaete Tubificoides benedeni on mussel rings (Kochman et al. 2013). Settling of spat of C. gigas on shells of Littorina littorea (Common Periwinkle) had adverse impacts on the movement, growth, and reproduction of this snail in the Wadden Sea (Germany) (Eschweiler and Buschbaum 2011). While invasion of mudflat and mussel bed habitats altered the density and diversity of epifauna, benthic assemlages were similar between C. gigas and native Ostrea edulis communities in Strangford Lough, Northern Ireland (Zwerschke et al. 2016; Zwerschke et al. 2018). However, a later study in Strangford Lough found that epibiota were more diverse on O. edulis than M. gigas, possibly because of the flakier nature of the M. gigas shell (Guy et al. 2018). The development and consolidation of Pacific Oyster beds in the Wadden Sea has had mixed effects on shorebirds. Eurasian Oystercatchers (Haematopus ostralegus) fed more easily when oysters successfully recruited, while as young oysters grew, and the reef consolidated, feeding was more difficult, but birds were able to maintain a steady intake. Eurasian Curlews (Numenius arquata) were favored by increased density of Green Crabs (Carcinus maenas), while the feeding of Herring Gulls (Larus argentatus) was hampered by the replacement of mussel beds with oysterbeds (Markert et al. 2013). Waser et al. (2016) found that 46 of 50 species of shore- and waterbirds were not affected by the replacement of mussels with oysters. However, the abundances of 4 birds, Common Gulls (Larus canus), Common Eiders (Somateria mollissima), Eurasian Oystercatchers (Haematopus ostralegus), and Red Knots (Calidris canutus) was reduced when oysters were dominant. On the whole, the authors considered that negative impacts from oyster removal exceeded the oysters' negative impacts on bird populations (Waser et al. 2016). On a mudfalt in southeast England, areas colonized by oysters were ustilized by greater numbers of Eurasion Oystercatchers and Curlews, but smaller numbers of smaller shorebiurds (Herbert et al. 2018). Markert (2020) has published a detailed study of the structure of oyster reefs in the Wadden Sea, and comparisons with native reefs of the Blue Mussel (Mytilus edulis), as habitats for nstive and non-indigenousspecies. | ||
Ecological Impact | Competition | |
On the Wadden Sea Coast of Germany, M. gigas has been settling on mussel (Mytilus edulis) beds growing on mudflats, since at least 1991, resulting in overgrowth of mussels and attached barnacles, converting mussel beds to oyster beds (Reise 1998; Baird 2012). However, year to year variation in oyster spawning and settlement, the steadier recruitment of mussels, and the poor settlement of oysters on mussels covered with the seaweed Fucus vesiculosus allow for the co-occurrence of oysters and mussels (Diederich 2005). | ||
Economic Impact | Fisheries | |
In northern Europe, overfishing and pollution led to a great decline in stocks of the native Flat Oyster (Ostrea edulis). The native oyster was partially replaced by cultured seed of the Portuguese Oyster (M. angulata), imported from Portugal or Spain. However, diseases in the 1960s and 70s ended this trade, and led to searches for a new oyster. Magallana gigas was introduced into waters of the United Kingdom in 1965 (Walne and Helm 1979; Utting and Spencer 1992). Similar introductions took place in the Netherlands (in 1965), Belgium (in 1969, Kerckhof et al. 2007), Ireland (in 1969, Minchin 2007), Germany (in 1986, Reise 1998), Denmark (in 1972, Wrange et al. 2010). Initially, aquaculture was dependent on hatcheries, but natural spawning and recruitment was seen at some locations in the late 1980s to the present, with extensive oyster beds forming in the Wadden Sea (Netherlands-Germany-Denmark) (Reise 1998; Gittenberger et al. 2010; Troost 2010). Negative impacts of fisheries include reduction of areas where fishnets can be used, declines in biomass of Blue Mussels (Mytilus edulis) and Common Cockles (Cerastoderma edule) (Troost 2010). The expansion of Pacific Oyster aquaculture in the Netherlands, as well as the expansion of wild beds, and increasing populations of the American razor clam Ensis leei, has been associated with a decrease in yeilds of cultured mussels (Mytilus edulis) and Edible Cockles (Cerastoderma edule) (Smaal et al. 2013). | ||
Ecological Impact | Parasite/Predator Vector | |
Parasite-Predator vector- The introduction of M. gigas has been a possible/probable vector for a number of oyster foulers or predators in northern European coastal waters, including the seaweed Sargassum muticum, many other macroalgal species, Pteropurpura (=Ocinebrellus) inornata (Japanese Oyster Drill), the parasitic copepod Mytilicola orientalis, and the tunicates Botrylloides violaceus, Didemnum vexillum and Perophora japonica (Eno et al. 1997; Reise et al. 1999; Wolff and Resie 2002; Gittenberger 2010). Mytilicola orientalis, on the coast of the Netherlands, infected Pacific Oysters (Magallana gigas at 2-43% frequency, but also were found in Blue Mussels (Mytilus edulis, 3-63%), Common Cockles (Cerastoderma edule, 2-13%), and Baltic Tellins (Macoma balthica, 6-7%) (Goedknegt et al. 2016). | ||
Ecological Impact | Food/Prey | |
Conversion of mussel (Mytilus edulis) beds in the Wadden Sea to Pacific Oyster beds may have adverse effects on some bird species which have difficulty detaching and opening oysters, particularly Common Eiders (Somateria mollissima). Other species such as European Oystercatchers (Haematopus ostralegus) and Herring Gulls (Larus argentatus) may be able to adjust feeding habits to the new prey (Scheiffarth et al. 2007; Baird 2012). Pacific oysters showed different patterns in the concentration of trace metals (lead, copper, cadmium, zinc), compared to native Blue Mussels (Mytilus edulis, potentially affecting the accumulation of these metals in the foodweb. However, shell thickness and predation rates may have a greater effect than metal concentrations on how these metals enter the food web, as C. gigas replaces M. edulus (Bray et al. 2015). Predation by the native Green Crab (Carcinus maenas dod not provide biotic resistance to M. gigas invasion, since the crabs preferred the native Blue Mussel (Mytilus edulis (Joyce et al. 2020). | ||
Ecological Impact | Herbivory | |
The expansion of Pacific Oyster aquaculture in the Netherlands, as well as the expansion of wild beds, and increasing populations of the American razor clam Ensis leei, has resulted in an increase of filtering biomass, and a decrease in phytoplanktion concentrations, and a shift towards an increassing proportion of picoplankton (very small, poorly grazed cells) (Smaal et al. 2013). | ||
Ecological Impact | Trophic Cascade | |
The expansion of Pacific Oyster aquaculture in the Netherlands has had indirect impacts on the predation of the Green Crab (Carcinus maenas on the Blue Mussel (Mytilus edulis, reducing predation on juvenile mussels, by providing refuges in the interspaces ibetween the oysters (Waser et al. 2015). Another indirect effect of M. gigas involves the effect of its parasite Mytilicola orientalis, which also infects the native Blue Mussel Mytiulus edulis. The longeer-lived planktonic larvae of this copepod are more likely to infect mussels at the top of the bed, more exposed to current, while a native trematode, with a short lived larva is more likely to infect mussels at the bottom of the reef, less exposed to current (Goeknecht et al. 2020). | ||