The Shellfish Association of Great Britain
Fishmongers‟ Hall, London Bridge, London EC4R 9EL
Tel: 020 7283 8305
Director: Dr Tom Pickerell
York YO41 1LZ
15th December 2009
UK NON-NATIVE ORGANISM RISK ASSESSMENT SCHEME Version 3.3
Crassostrea gigas - Pacific oyster
I am writing on behalf of the Shellfish Association of Great Britain (SAGB) the UK
industry trade association, with regards your recently published risk assessment on
the Pacific oyster (Crassostrea gigas).
Firstly I would like to begin by noting that this economically important species was
introduced in 1965 to Conwy, North Wales by MAFF from the USA and British
Columbia. In 2007 the total UK production of Pacific Oysters was around 1300
tonnes (approx 15m oysters). In England and Wales about 64% of this production
takes place in the South East of England, with a further 30% from farm sites in the
South & West. All Pacific oyster production in England and Wales is from registered
shellfish farms with about 27% of the total Pacific oyster production in England and
Wales coming from Fishery Order areas.
We are deeply concerned that the Pacific oyster is even being considered as an
invasive non-native species; legislative control of the Pacific oyster on mainland
Europe does not appear to match the UK stance. For example, a Spanish pilot study
carried out by their Department of Land Action and Environment (DOTMA) looking at
the pressures and impacts that would place water bodies at risk of failing to achieve
ecological good status, if appropriate programmes of measures were not designed
and implemented, actually states:
Some species, introduced between the end of the 19th century and the middle of the
20th century (such as Crassostrea gigas or Asparagopsis armata) have not been
considered as alien, because they are naturalized1
Furthermore I understand Member States can elect how to apply the new
Aquaculture Alien Species regulation to „long used‟ non-native species such as the
Pacific oyster. Why has the UK not considered this with this species? The UK is the
only member state, to our knowledge, that has (by UKTAG) placed Pacific Oysters
on the Water Framework Directive „red‟ list. Either way, the high density of Pacific
oyster stocks around mainland Europe and a long planktonic larval phase may
potentially undermine any nationally imposed control. For example, UK production of
Pacific oysters in 2006 amounted to 1,290 tonnes while France produced around
115,000 tonnes. Reports from France suggest that wild Pacific oyster biomass levels
Angel Borja, Ibon Galparsoro, Oihana Solaun, Inigo Muxika, Eva Marıa Tello, Ainhize Uriarte,
victoriano Valencia (2006) The European Water Framework Directive and the DPSIR, a methodological
approach to assess the risk of failing to achieve good ecological status. Estuarine, Coastal and Shelf
Science 66 (2006) 84e96
greatly exceed cultured biomass. In the case of the Bay of Brest 2,000 tonnes of
cultivated oysters compares with an estimated 15,000 tonnes of wild oysters. Does
this vast pool of larvae from the French coast settle in UK waters? The only genetic
study2 carried out to date shows that spatfall in the River Teign, South Devon,
originated from French stock, although it is unclear whether this was from adult
specimens discarded at English sites or from larvae that crossed from the French
side of the channel. Ultimately, we are concerned that our „Fortress Britain‟ approach
to the Pacific oyster will prove expensive and ultimately wholly ineffective and UK
cultivators will be disadvantaged by ineffectual, yet well intentioned, UK management
We should point out that Pacific oyster is on general release under a General licence
granted by the Minister of Agriculture, Fisheries and Food of the Wildlife and
Countryside Act 1981. The wording states Pacific or Japanese Oysters … and
Portuguese Oysters… or any eggs of those species.
Finally, currently there are breeding populations as far north as Denmark in the East
and Ireland & Northern Ireland in the West. We are in the middle of this wild
settlement and we remain unconvinced that any UK measures will result in any
With regards your risk assessment we have the following observations:
1. Firstly, the evidence base used to generate this assessment seems to be mainly
based on the Natural England publication “The impacts of Pacific Oysters in
European Marine Sites”. This publication has not been peer-reviewed and the UK
industry has substantial concerns over the accuracy of the majority of the
assumptions made. We sent our concerns to Natural England in the summer but
have received no response.
2. Question 2 – why is the Wadden Sea being considered in a UK risk assessment.
3. Question 2 - is it acceptable for an official risk assessment, with the potential to
have serious consequences for the nation and for the shellfish industry in
particular, to be using Wikipedia as an information source?
4. Question 5 – Actually Crassostrea gigas and Crassostrea angulata (the
Portuguese oyster) are considered to be the same species3. How does this affect
the risk assessment?
5. Question 11 onwards – No answers are given on the risk assessment yet we
proceed to Section B. Why did this happen? Where is the evidence that a
detailed risk assessment is required?
1.1 “Larvae can travel up to 1300km”? Where is the evidence for this? We have
evidence4 that suggests a distance of between 5 – 15 km with a maximum of
Child, A.R., Papageorgiou, P., & Beaumont, A.R. 1995. Pacific oysters Crassostrea gigas
(Thunberg) of possible French origin in natural spat in the British Isles. Aquatic Conservation: Marine
and Freshwater Ecosystems, 5: 173-177.
Menzel R.W., 1974. Portuguese and Japanese oysters are the same species. Journal of the
Fisheries Research Board of Canada, 31: 453-456.
Smith, S.M., Heppell, D., & Picton B.E in Howson, CM and Picton, BE. 1997. The Species
Directory of the Marine Fauna and Flora of the British Isles and Surrounding Seas. Ulster Museum /
Marine Conservation Society, Belfast.
Brandt, G., Wehrmann A., & Wirtz K.W., (2008) Rapid invasion of Crassostrea gigas into the
German Wadden Sea dominated by larval supply. Journal of Sea Research 59, 279–296
50 km. Much less than 1300km but still a risk for continental European origin
larvae settling in UK waters however and further illustrates the amateurish
nature of the risk assessment.
1.2 “C. gigas is brought to the UK and bred as a commercial species within the
open marine environment.” Actually they are bred in hatcheries with three
commercial hatcheries in the UK supplying both the UK and Ireland. Pacific
oyster seed production for England and Wales in 2006 was estimated at 708
million as compared to 63 million seed oysters in 2000 (CEFAS, 2007).
1.3 “C. gigas are farmed in high densities.” This is incorrect. The industry in the
UK is low density and well dispersed. In 2007 the UK produced just under
1300 tonnes compared to:
3.6m tonnes from China
261,000 tonnes from Japan
238,000 tonnes from South Korea
115,000 tonnes from France
43,000 tonnes from USA
23,000 tonnes from Taiwan5
1.9 “…with a female potentially spawning 50-60 million eggs (NIMPIS, 2002).”
This is correct BUT from these several millions eggs only very few larvae
develop and survive the hazardous planktonic phase to settle6. After
successful metamorphosis and settlement, recruitment processes may
significantly alter the larval supply pattern and cause mortalities exceeding
90% during the first year7.
1.10 “Spawning, whilst temperature dependant, it is annual and occurs in the
summer months.” It would be more correct to state that the Minimum
temperature for spawning in this species is 19°C8.
1.11 The answer to this question notes “A variety of biotic and abiotic factors will
affect the successful spawning and recruitment of Crassostrea gigas, but the
most important of these seem to be temperature, and where temperature has
increased the oysters have spread more readily.” We disagree that
temperature is the most important factor and we draw your attention to Annex
A which details other factors that will influence both conditioning and whether
viable larvae survive and develop to achieve settlement in a particular area.
1.13 “Introduction of C. gigas for aquaculture will allow the further spread.” Where
is the evidence for this?
1.16 “other abiotic factors are unlikely to limit distribution” See Annex A for peer-
reviewed evidence which disputes this (non-evidence based) claim.
1.21 “Unlike blue mussels, oysters are only consumed by a few species and suffer
little predation”. According to ICES9 C. gigas is consumed by a variety of
organisms including seastars, boring gastropods and bivalves, spionid
polychaetes, crabs, benthic feeding fish such as stingrays and sea breams,
FAO Fishery Statistics, 2006
Rumrill, S.S., 1990. Natural mortality of marine invertebrate larvae. Ophelia 32 (1–2), 163–
Gosselin, L.A., Qian, P.Y., 1997. Juvenile mortality in benthic marine invertebrates. Mar. Ecol.
Prog. Ser. 146, 265–282
Hunt, H.L., Scheibling, R.E., 1997. Role of early post-settlement mortality in recruitment of
benthic marine invertebrates. Mar. Ecol. Prog. Ser. 155, 269–301.
Diederich, S., 2005.Differential recruitment of introduced Pacific oysters and native mussels at
the North Sea coast: coexistence possible? J. Sea Res. 53, 269–281
Miossec, L., Le Deuff, R‐ M., and Goulletquer, P. 2009. Alien species alert: Crassostrea gigas
(Pacific oyster). ICES Cooperative Research Report No. 299. 42 pp.
ducks and wading birds. In addition, parasites such as the copepod Mytilicola
spp., boring sponges (Cliona spp.), and sea worms (Polydora spp.) affect
oyster physiology and, thereby, can significantly affect oyster culture.
1.23 Regards the use of triploids, Seafish commissioned the report Development
of a Pacific Oyster Aquaculture Protocol for the UK (FIFG Project No:
07/Eng/46/04) which notes the relative reproductive potential for both
chemically induced and mated triploids have been assessed based on
fecundity values calculated in recent research. These calculations show that
whilst triploid Pacific oysters are not absolutely sterile their reproductive
potential when compared to diploids is reduced. Furthermore, the most
effective use of triploid Pacific oysters for biological containment is where
those oysters are placed in areas free from diploids as the presence of diploid
oysters increases the chances of the production of viable embryos from
triploid x diploid crosses. Accordingly, please be aware that triploidy is not a
1.25 Please refer to our response to 1.1.
1.27 “C. gigas can survive in a wide range of temperature, salinity, dissolved
oxygen, and pH conditions (Eno et al. , 1997; NIMPIS, 2002).” This refers to
the adult organism and not applicable to larvae.
1.28 “There are a number of founder populations and it is thought that natural spat
from France has settled in UK waters (River Teign) (Child et al. , 1995).”” This
begs the question what is the UK going to do about the vast French
populations and the reefs in the Wadden Sea. Even if we developed a risk
management system for UK cultivators would it make any difference to wild
settlement originating from these areas?
1.30 “The majority of eradication campaigns would involve destruction of the
organism in the environment and it is likely that this would result in
environmental degradation, including non target species.” We have posed this
question to Natural England and are yet to receive any answer. The industry
cannot accept any restrictions on cultivation if reefs (in or out of UK waters)
are spawning and negating any „best practice‟ measures.
2.2 “This is dependant on the licensing of further oyster farms and movement of
seed beds etc.” We are not aware of any movement of seed beds.
2.3 Please refer to our response to 1.2.3.
2.5 “some evidence has been found that C. gigas alters both reefs and
substrate”. Firstly, the natural state of the sea bed, pre human, almost
certainly had a much higher coverage of biogenic beds than is currently the
case. Therefore increasing the number of biogenic beds and stabilising soft
sediments should be seen as a good thing. More stable sediment leads to
clearer water better light penetration, better growth of macroalgae and more
oxygenated water. Secondly, the HARBASINS10 study of the Wadden Sea
showed: (i) Pacific oyster reefs do not exhibit lower species diversity than
blue mussel beds (ii) the presence of these reefs has not led to any species
loss. Furthermore, Hily and Lejart (2007)11 found both species diversity &
abundance increased significantly in mud flat & rocky shores when Pacific
oysters were present.
2.6 These figures assume that the Pacific oyster will wipe out all other
commercial species in an area and is not based on any evidence. The risk
Nehls, G., and Büttger, H., 2007. Spread of the Pacific Oyster Crassostrea gigas in the
Wadden Sea. Causes and consequences of a successful invasion. HARBASINS report on behalf of The
Common Wadden Sea Secretariat.
Hily, C. and Lejart, M., 2007. Ecological consequences of the Pacific oyster (Crassostrea
gigas) invasion on coastal habitats in Western Europe. In „Proceedings of the 10th International
Conference on Shellfish Restoration‟, Netherlands, November 12-16, 2007.
assessment actually quotes the Diedrich (2006)12 report which noted “in terms
of community structure no species losses were observed”.
2.8 What evidence exists that consumers are demanding native oysters? We
doubt if the majority of consumers would know there is a difference.
Furthermore, walking across any shellfish bed is likely to be uncomfortable –
why is the Pacific oyster being targeted?
2.9 Please refer to our response to 2.6. Furthermore, there could well be a
developing export market for wild-harvested Pacific oysters which is not being
2.10 Is there any evidence from France to show that this is the case. They have
been breeding in the wild there for 40 years.
2.11 “Other non-native species have been introduced as a result of C. gigas
Introduction” Please can you direct us to any evidence of this. The Pacific
oyster was introduced into the UK (in 1965) under strict quarantine.
2.12 Cultivating oysters in dredged or hand picked beds increased eelgrass growth
2.17 So what do we do then? Surely this is the point we have been raising with
Natural England since 2007.
2.19 No evidence has been presented which supports any of these claims. The
introduction of more than half the total number of species described in Eno et
al (1997)14 are considered to have been introduced to Britain in association
The studies that are most referred to in the risk assessment are those from the
Wadden Sea, there is also work carried out in French coastal lagoons. These sites
are enclosed waters to all intents and not coastal estuaries such as the UK, the
effects predicted do not take into account the movement of water down estuaries
taking seed out to open water most of the time and away from the areas of concern.
Furthermore, the assumptions that Pacific oysters out compete mussels (section 2.5)
seem to be based on the Natural England report which notes:
Intertidal mussel beds in the Waddenzee are mapped annually using aerial
photography and ground-truthing. In the northern Waddenzee (Schleswig-Holstein
area) intertidal mussel beds decreased from about 1,500 ha in 1988 to 500ha in 2004
and total biomass decreased from 60,000t in 1989 to 12,000t in 2004.
Most authorities attribute this to the decrease in anthropogenic input nutrients and
especially phosphates in washing powders and nitrogen fertilisers.
We‟d like to end on a positive and bring to your attention a recent study15 which
found that the presence of Pacific oysters reduced the parasite load in mussels by
65-77% and 89%. They act as decoys for trematodes thus reducing the risk of hosts
to become infected. This dilution effect was density-dependent with higher reductions
at higher oyster densities. As parasite infections have detrimental effects on the
Diederich, S., 2006. High survival and growth rates of introduced Pacific oysters may cause
restrictions on habitat use by native mussels in the Wadden Sea. Journal of Experimental Marine
Biology and Ecology 328 211-227.
Tallis, H.M., Ruesink, J.L., Dumbauld, B., Hacker, S., and Wisehart, L.M. (2009) Oysters and
Aquaculture Practices Affect Eelgrass Density and Productivity in a Pacific Northwest Estuary. Journal
of Shellfish Research 28(2):251-261
Eno., N.C., Clark, R.A. & Sanderson, W.G. (Eds) (1997) Non-native marine species in British
waters: a review and directory. JNCC.
Thieltges, D.W., Reise, K., Prinz, K., Jensen, K.T. (2009). Invaders interfere with native
parasite-host interactions. Biological Invasions, 11 pp. 1421-1429.
mussel hosts, the presence of the Pacific oysters may elicit a beneficial effect on
I hope the above is useful, and should you have any further questions, please do not
hesitate to contact me.
Dr Tom Pickerell
Shellfish Association of Great Britain
Tel: 020 7283 8305
Fax: 020 7929 1389
Mob: 07507 339156
Annex A – Factors affecting spawning & settlement of larvae
Nutrition: Nutrition during the larval phase influences both the speed of development
and the overall survival to metamorphosis.
Adverse hydrodynamics: Eno (1994) showed that the Fleet Lagoon, which is a
shallow tidal enclosure in Dorset containing a Pacific oyster farm, had a suitable
temperature regime for successful recruitment, yet no wild settlement had been
observed. Eno postulated that the flushing regime and the Pacific oyster‟s extended
larval life might have resulted in the removal of potential recruits from the area.
Unfavourable biotic factors: King et al. (2006) recorded a number of potential
confounding factors that strongly influenced Pacific oyster growth and performance
away from the conventionally accepted norms. A comparison was undertaken in
North Wales between a warm „Inland Sea‟ area against „high‟ intertidal and „low‟
intertidal culture sites. Conventional culture wisdom, as defined in the MAFF culture
manual (Spencer, 1990), would indicate that the best growth and conditioning
performance would be achieved in the Inland Sea where the stock was continually
immersed and received the highest temperatures. This would be followed by the
Menai Strait „low‟ stock which was immersed and feeding for most of the time
whereas the Menai Straits „high‟ stock which feed for the shortest period should
perform the poorest. King et al. showed that the expected pattern was in fact
reversed with the theoretically best site exhibiting stock that performed the worst.
This study showed that although the warm Inland Sea experienced high chlorophyll a
content (indicative of good primary productivity) the dominant species present was
non-optimal and may have retarded development. Furthermore, the inversion of the
expected Menai Straits site performance was attributed to a heavier infestation with
mudworm (Polydora) of the lower placed oyster stock which may have resulted in
retarded growth rates. Steele and Mulcahy (1999) compared the reproductive biology
of Pacific oysters from two sites on the south coast of Ireland for variations in
maturation rate and condition indices. Qualitative data were compiled by examining
gonadal development using histological sections. Temperatures were sufficient to
allow spawning at one site and yet spawning did not occur within Cork Harbour stock
despite a higher temperature regime. The Cork Harbour oysters reached ripeness
and then started a process of gametic resorption. This lack of spawning was not
attributed to the monitored levels of temperature, dissolved oxygen and chlorophyll
but was tentatively attributed to levels of pollutants in the water. Oyster condition in
Cork Harbour was significantly affected by the presence of blistering due to tributyltin
(TBT) levels in the water and also by Polydora sp. Infestation in the shell. In addition
to the potential impact of TBT upon adult development this highly toxic molluscicide
has also been shown to kill and deform Pacific oyster larvae thus reducing
recruitment potential. Champ and Seligman (1996) included a report of the impact of
TBT levels on Pacific oyster larvae from both field observations and tank
experiments. Following the introduction in 1971 of the Pacific oyster to the Arachon
basin in south west France there followed a number of years with significant wild
settlement within this warm semi-enclosed bay where summer temperatures
regularly exceed 22 ºC. From 1976 to 1981 however there was greatly reduced
settlement or spat failure along with depressed growth rates and shell thickening –
classic signs of TBT contamination. This change in oyster settlement and growth
occurred at the same time as an observed increase in the number of pleasure craft.
Tank studies showed that at concentrations above 20ng/l TBT the majority of Pacific
oyster larvae died within 14 days with the rate of abnormalities also correlating to
Biological resource debt: Child and Laing (1998) showed that although the Pacific
oyster was able to feed and assimilate food at 6°C and 9°C it underwent loss of body
mass in the form of metabolised protein at 3°C. In consequence, prolonged winter
exposure below this point would lead to loss of body mass and a loss of biological
reserves. Shellfish need to boost their biological reserves in the spring prior to
spawning as gametogenesis and subsequent conditioning require considerable
energy resources. It is therefore probable that exceptionally cold winters in cold
regions (such as North Sea coasts) could leave a resource debt that will need to be
replaced prior to conditioning. Such debt repayment would delay reproductive timing.
Champ, M.A. and Seligman, P.F. (Ed.), 1996. In: Embryogenesis and Larval
Development in Crassostrea gigas: Experimental Data and Field Observations on the
Effect of Tributyltin Compounds. Organotin: Environmental Fate and Effects. His, E p.
Child, A.R. and Laing, I., 1998. Comparative low temperature tolerance of small
juvenile European, Ostrea edulis L., and Pacific oysters, Crassostrea gigas
Thunberg. Aquaculture Research, Vol. 29 (2), p. 103-113.
Eno, N.C., 1994. Monitoring of Temperature Regimes to which the Pacific Oyster
Crassostrea gigas is Subject to in Coastal Inlets in Britain (the Fleet Lagoon and
Teign and Dart Estuaries) in Relation to Their Reproductive Success. JNCC Report,
King, J.W., Malham, S.K., Skov, M.W., Cotter, E., Latchford, J.W., Culloty, S.C. and
Beaumont, A.R., 2006. Growth of Crassostrea gigas spat and juveniles under
differing environmental conditions at two sites in Wales. Aquatic Living Resources,
Vol. 19, p. 289–297.
Spencer, B.E., 1990. Cultivation of Pacific oysters. MAFF Directorate of Fisheries
Research, Lowestoft. Laboratory Leaflet No. 63, 47 p.
Steele, S. and Mulcahy M.F., 1999. Gametogenesis of the oyster Crassostrea gigas
in southern Ireland. Journal of the Marine Biological Association of the United
Kingdom, Vol. 79, p. 673-686.