The Marine Life Information Network

Temperature increase (local)

Benchmark. A 5°C increase in temperature for one month, or 2°C for one year. Further detail

Evidence

Filtration rate, metabolic rate, assimilation efficiency and growth rates of adult Ostrea edulis increase with temperature.  Growth was predicted to be optimal at 17°C or, for short periods, at 25°C (Korringa, 1952; Yonge, 1960; Buxton et al., 1981; Hutchinson & Hawkins, 1992).  Hutchinson & Hawkins (1992) noted that temperature and salinity were co-dependent so that high temperatures and low salinity resulted in marked mortality, and no individuals survived more than 7 days at 16 psu and 25°C, although these conditions rarely occurred in nature.  No upper lethal temperature was found but Kinne (1970) reported that gill tissue activity fell to zero between 40-42°C, although values derived from single tissue studies should be viewed with caution.  Buxton et al. (1981) reported that specimens survived short-term exposure to 30°C.  Ostrea edulis occurs from the Mediterranean to the Norwegian coast and is, therefore, unlikely to be adversely affected by long-term changes in temperatures in Britain and Ireland.

Spärck's (1951) data suggest that temperature is an important factor in the recruitment of Ostrea edulis, especially at the northern extremes of its range and Korringa (1952) reported that warm summers resulted in good recruitment.  Spawning is initiated once the temperature has risen to 15-16°C, although local adaptation is likely (Korringa, 1952; Yonge, 1960).  Davis & Calabrese (1969) reported that larvae grew faster with increasing temperature and that survival was optimal between from 12.5 - 27.5°C but that survival was poor at 30°C.  Therefore, recruitment and the long-term survival of an oyster bed is probably affected by temperature and may benefit from both short and long-term increases.

Sensitivity assessment.  The resistance of Ostrea edulis to the pressure at the benchmark is assessed as ’High’ with a consequent resilience of ‘High’.  Therefore, this species is recorded as ‘Not sensitive’ to the pressure at the benchmark level.

Temperature decrease (local)

Benchmark. A 5°C decrease in temperature for one month, or 2°C for one year. Further detail

Evidence

Hutchinson & Hawkins (1992) suggested that Ostrea edulis switched to a reduced, winter metabolic state below 10°C that enabled it to survive low temperatures and low salinities encountered in shallow coastal waters around Britain.  Davis & Calabrese (1969) also noted that larval survival was poor at 10°C.  Korringa (1952) reported that British, Dutch and Danish oysters can withstand 1.5°C for several weeks.  However, heavy mortalities of native oyster were reported after the severe winters of 1939/40 (Orton, 1940) and 1962/63 (Waugh, 1964).  Mortality was attributed to relaxation of the adductor muscle so that the shell gaped, resulting in increased susceptibility to low salinities or to clogging with silt.

Low temperatures and cold summers are correlated with poor recruitment in Ostrea edulis, presumably due to reduced food availability and longer larval developmental time, especially at the northern limits of its range. Therefore, a reduction in temperature may result in reduced recruitment and a greater variation in the populations of Ostrea edulis.

Sensitivity assessment.  Decreases in temperature experienced in a severe winter are more extreme than at this pressure benchmark.  However, long-term decreases in temperature could potentially effect overall recruitment.  Resistance is assessed as ‘Medium’, and resilience has been assessed as ‘Low’ so that sensitivity of this species is recorded as ‘Medium’ to the pressure at the benchmark.

Salinity increase (local)

Benchmark. A increase in one MNCR salinity category above the usual range of the biotope or habitat. Further detail

Evidence

Ostrea edulis is found in full to variable salinity waters and is unlikely to experience increased salinity waters.  Hypersaline effluent may be damaging but no information concerning the effects of increased salinity on oyster beds was found.  Therefore, an assessment of ‘No evidence’ is recorded.

Salinity decrease (local)

Benchmark. A decrease in one MNCR salinity category above the usual range of the biotope or habitat. Further detail

Evidence

Ostrea edulis is euryhaline and colonizes estuaries and coastal waters exposed to freshwater influence (Yonge, 1960).  Yonge (1960) reported that the flat oyster could not withstand salinities below 23 psu.  However, Hutchinson & Hawkins (1992) noted that scope for growth was severely affected below 22 psu, probably because the oyster's valves were closed, but that 19 -16 psu could be tolerated if the temperature did not exceed 20°C.  At 25°C animals did not survive more than 7 days at 16 psu.  Hutchinson & Hawkins (1992) noted that at low temperatures (10°C or less) the metabolic rate was minimal.  This may help Ostrea edulis survive in low salinities associated with storm runoff.

Sensitivity assessment.  Ostrea edulis can be found in both fully marine and variable salinity regimes.  If a salinity regime were to become 'reduced' then Ostrea edulis may be adversely affected by the decrease in salinity.  Ostrea edulis could probably tolerate short-term acute reductions in salinity due to runoff.  However, a decrease in the salinity regime for a year is likely to have a negative impact on the species.  Therefore, resistance has been assessed as ‘Medium’ and resilience as ‘Low’, so that a sensitivity of ‘Medium’ is recorded at the benchmark level. 

Water flow (tidal current) changes (local)

Benchmark. A change in peak mean spring bed flow velocity of between 0.1 m/s to 0.2 m/s for more than one year. Further detail

Evidence

Ostrea edulis is found in areas with' Very Weak' (negligible), and 'Weak' < 1 knot (<0.5 m/sec.) tidal flows.  An increase in water flow rate could cause oysters to be swept away by strong tidal flow if the substratum to which they are attached is removed.  Therefore, a proportion of the oysters may be lost, depending on the nature of the substratum. 

Increased water flow can affect the ability of oysters to feed.  An increase in water flow could reduce the time oysters are able to feed but could improve the availability of suspended particles on which oysters feed.  The latter of which is most likely to have the greatest impact on individual oysters.  With increased water flow rate the oyster filtration rate increases, up to a point where the oysters are unable to remove more particles from the passing water, after which any further increase is not of any benefit.

Reproductive success and successful recruitment of Ostrea edulis may also be affected by a change in water flow.  Recruitment is known to be sporadic and dependent on the hydrographic regime and local environmental conditions but enhanced by the presence of adults and shell material (Cole, 1951).  An increase in water flow rate may interfere with the settlement of spat and it is thought that growth rates of Ostrea edulis are faster in sheltered sites than exposed locations, although this is thought to be attributed to the seston volume rather than flow speed or food availability (Valero, 2006).

Sensitivity assessment.  A change in water flow at the benchmark of this pressure it is unlikely to cause any effect on this species.  It may remove fine sediments that accumulate in the bed but otherwise leave the hard substrata (gravel shell, pebbles, cobbles, etc.) to which the oysters are attached in place.  Both the resilience and resistance of this species are assessed as ’High’ so that the species is assessed as ‘Not sensitive’ at the benchmark level. However, an increase above the benchmark of this pressure could have a negative impact. 

Emergence regime changes

Benchmark.  1) A change in the time covered or not covered by the sea for a period of ≥1 year or 2) an increase in relative sea level or decrease in high water level for ≥1 year. Further detail

Evidence

Beds of the native oyster Ostrea edulis may occur low on the shore and are exposed for a proportion of the tidal cycle.  Ostrea edulis is known to be able to survive aerial exposure at low temperatures during storage and is known to be capable of anaerobic respiration (Korringa, 1952; Yonge, 1960), which suggests that they can tolerate aerial exposure.  In addition, in the mariculture of oysters (native and introduced species), oyster trays are positioned in the low intertidal, and regularly exposed to the air.  Therefore, an increase in the emergence of Ostrea edulis is unlikely to result in the death of the oysters at the level of the benchmark.  However, exposure to the air prevents feeding, and anaerobic respiration usually results in an oxygen debt, an energetic cost that the organism must make upon return to aerated water, resulting in reduced growth and reproductive capacity.

Sensitivity assessment.  Ostrea edulis is likely to resist an increase in emergence at the benchmark level.  Both resistance and resilience are assessed as ‘High’ so that Ostrea edulis is assessed as ‘Not sensitive’ at the level of the benchmark.

Wave exposure changes (local)

Benchmark. A change in near shore significant wave height of >3% but <5% for more than one year. Further detail

Evidence

Ostrea edulis occur at wave exposures ranging from very sheltered through to wave exposed conditions.  The species is found from 0 –80 m in depth.  The wave action in shallow water results in oscillatory water flow, the magnitude of which is greatest in shallow water and attenuated with depth.  While the oysters' attachment is permanent, increased wave action may result in erosion of its substratum and the oysters with it.  In areas where sufficient shell debris has accumulated may be less vulnerable to this disturbance.  Ostrea edulis may be replaced by other species characteristic of stronger wave action and coarser sediments. 

Sensitivity assessment.  At the benchmark of this pressure, it is highly unlikely that the change will cause any effect on Ostrea edulis.   Both the resilience and resistance of Ostrea edulis are assessed as ’High’ so that the species is assessed as ‘Not sensitive’ at the benchmark level. However, an increase above the benchmark of this pressure could have a negative impact. 

Transition elements & organo-metal contamination

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

The results of the Rapid Evidence Assessment on the effects of 'Hydrocarbons and PAH' contaminants on oyster species (Crassostrea spp., Ostrea spp., Saccostrea spp. and Magallana gigas). are summarized below. The full 'Oyster species evidence review' should be consulted for details of the studies examined and their results. A sensitivity assessment is provided for each type or source of 'transitional metal or organometal' contaminant examined, together with an overall pressure assessment. 

Transitional metals. In adults and juveniles, 12% of the results reported ‘severe’ mortality and 33% reported ‘significant’ mortality but 43% reported sublethal effects.  However, ‘severe’ mortality was reported in 31% of the results from early life stages, and ‘significant’ mortality in 57% of the results.  There is considerable evidence to suggest that exposure to copper, cadmium, zinc, silver, mercury and lead could result in ‘severe’ or ‘significant’ mortality, although experimental designs and exposure concentrations vary.  Several other metals were only included in a few studies. Overall, all life stages were reported to experience mortality after exposure to transitional metals, with the exception of cobalt.  The evidence agrees with His et al. (2000) who ranked the toxicity of metals and organometals to bivalve larvae as follows: tributyltin >mercury >silver >copper >zinc >nickel >lead >cadmium.  Therefore, resistance to transitional metals exposure in adult, juvenile, and early life stages of oyster species is assessed as ‘None’, resilience as ‘Very low’ and sensitivity as ‘High’.

Curiously, CCA (copper-chrome-arsenic) affected the swimming of Crassostrea gigas larvae swimming but did not result in mortality when used as an antifoulant mixture (Prael et al., 2001).  Similarly, copper-oxide paint was reported to have only sublethal effects on adult C. gigas (His et al., 1987).

Ostrea edulis was the least studied oyster species.  The adults and juveniles of Ostrea edulis were reported to experience ‘significant’ mortality after exposure to nickel, mercury, and chromium.  The early life stages of Ostrea edulis were only studied in one article that only examined the effects of mercury, which in turn resulted in ‘significant’ mortality.  Bryan (1984) reported a 48-hour LC50 for Hg of 1-3.3 ppb in Ostrea edulis larvae compared with a 48-hour LC50 for Hg of 4,200 ppb in adults.

Ostrea edulis was able to survive in the lower reaches of Restronguet Creek, one of the most heavy metal polluted estuaries in the world, where metals from mining wastes reached concentrations several orders of magnitude above normal (Bryan et al., 1987).  Bryan et al. (1987) noted that O. edulis in the creek were highly polluted, that is, specimens were reported to be 'green' since the 1880s, in 1927, and in specimens collected in 1980 (Bryan et al., 1987).  Ostrea edulis from the Falmouth estuary were shown to be able to detoxify metals (Cu and Zn) in amoebocytes.  Bryan et al. (1987) noted that Cu and Zn were accumulated in the tissues of Ostrea edulis, with estimates ranging from ca 1,000 to ca 16,500 µg/g dry weight.  Bryan et al. (1987) concluded that this detoxification mechanism allowed O. edulis to survive in the lower reaches of the creek.

Ostrea edulis is, therefore, resistant to high levels of Cu and Zn and is able to survive in the lower reaches of Restronguet Creek, where other species are excluded by the heavy metal pollution.  Larval stages may be less resistant, but larval recruitment must be high enough for a population of oysters to survive for ca 123 years in the lower reaches of Restronguet Creek and the Falmouth estuaries.  Bryan et al. (1987) do not clarify their abundance/density in the creek.  However, it appears that Ostrea edulis is capable of localized adaption to transitional metal contamination, in particular, from copper and zinc. Although the adult Ostrea edulis may be tolerant of heavy metal pollution the larval effects suggest that recruitment may be impaired resulting in a reduction in the population over time, and hence a reduction in the associated fauna.

Therefore, the resistance of Ostrea edulis to transitional metals is assessed as ‘Low’, with the possible exception of Cu and Zn and the understanding that long-term exposure could result in localised adaption.  Hence, resilience is assessed as ‘Low’ and sensitivity as ‘High’, albeit with ‘Low’ confidence. 

Little information on the tolerance of ascidians or sponges was found. However, polychaetes are thought to be relatively tolerant of heavy metal pollution, even though some heavy metals may suppress reproduction (Bryan, 1984). Similarly, Bryan (1984) suggested that adult gastropod molluscs were also relatively tolerant of heavy metal pollution. Therefore, most other characteristic species in this biotope may be relatively tolerant of heavy metal pollution.

Organometals. Overall, 12% of the results of exposure to organotins reported ‘severe’ mortality, 49% ‘significant’ mortality, 6% ‘some’, 8% no mortality and 24.5% sublethal effects (Table 4.1).  In adults and juveniles, ‘severe’ mortality was reported in 6% of results, ‘significant’ in 31% and sublethal in 43% of results. However, in early life stages, 11% of the results of exposure to organotins reported ‘severe’ mortality, but 66.7% ‘significant’ mortality, 16.7% no mortality and 5.5% sublethal effects.  The evidence suggests that early life stages are more sensitive than adults.  In the five studies that examined Ostrea edulis, organotin exposure was reported to result in ‘significant’ or some ‘mortality’.  Therefore, the resistance of oyster species to organotins is assessed as ‘None’, resilience as ‘Very low’ and sensitivity as ‘High’. 

Rees et al. (2001) suggested that TBT contamination may have locally reduced population sizes of Ostrea edulis. In Ostrea edulis, TBT has been reported to cause reduced growth of new spat at 20 ng/l, a 50% reduction in growth at 60 ng/l. Although older spat grew normally at 240 ng/l for 7 days, larval production in adults was prevented by exposure to 240 and 2,620 ng/l for 74 days (Thain & Waldock, 1986; Bryan & Gibbs, 1991). Adults bioaccumulate TBT. Thain & Waldock (1986) and Thain et al. (1986) noted that TBT retarded normal sex change (male to female) in Ostrea edulis.

While the loss of predatory neogastropods (which are particularly intolerant of TBT) may be of benefit to Ostrea edulis populations, TBT has been shown to reduce reproduction and the growth of spat. Rees et al. (1999; 2001) reported that the epifauna of the inner Crouch estuary had largely recovered within 5 years (1987-1992) after the ban on the use of TBT on small boats in 1987. Increases in the abundance of Ascidiella sp. and Ciona intestinalis were especially noted. Ostrea edulis numbers increased between 1987 -1992 with a further increase by 1997. However, they noted that the continued increase in Ostrea edulis numbers and the continued absence of neogastropods suggested that recovery was still incomplete at the population level.

Nanoparticulate metals. Short-term (2-6 hours) exposure of Crassostrea virginica to titanium dioxide (TiO₂) did not result in negative effects. However, 48-hour exposure of C. virginica embryos to silver nanoparticles significantly impaired development (Ringwood et al., 2010). Therefore, the resistance of oyster species to TiO₂ is assessed as ‘High’ but exposure to silver nanoparticulates may be ‘Low’.  Hence, resilience is assessed as ‘Low’ and sensitivity as ‘High’ but with ‘Low’ confidence due to the lack of evidence.

Overall sensitivity assessment. The evidence suggests that oysters are highly sensitive to transitional metal (except cobalt) and organometal (organotin) exposure depending on the concentration and duration of exposure and the life stage of the oysters. The evidence on the effects of nanoparticulate metals is limited to two studies but reported larval sensitivity to nanoparticulate silver.  Therefore, the worst-case resistance of native oyster (Ostrea edulis) beds to transitional metals and organometals is assessed as 'Low'. Hence, resilience is assessed as 'Low' and sensitivity as 'High'. Ostrea spp. was the least studied species in the evidence review but as several studies provided direct evidence confidence in the assessment is 'Medium'. 

Hydrocarbon & PAH contamination

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

The results of the Rapid Evidence Assessment on the effects of 'Hydrocarbons and PAH' contaminants on oyster species (Crassostrea spp., Ostrea spp., Saccostrea spp. and Magallana gigas). are summarized below. The full 'Oyster species evidence review' should be consulted for details of the studies examined and their results. A sensitivity assessment is provided for each type or source of 'Hydrocarbon' contaminant examined, together with an overall pressure assessment. 

Oil spills. Only two studies reported on the direct effect of oil spills on oyster beds.  Levings et al. (1994) reported that the Galeta oil spills resulted in a significant reduction in C. virginica beds along the mangrove fringe, which lasted at least five years.  Powers et al. (2017) reported a ‘severe’ reduction in the abundance of C. virginica in intertidal beds after the Deepwater Horizon spill.  However, any effect on subtidal beds was obscured by mass mortality caused by freshwater runoff.  Therefore, the evidence suggests that direct oiling of oyster beds could cause ‘severe’ or ‘significant’ mortality amongst the oysters.  Hence, resistance is assessed as ‘None’, resilience as ‘Very low’ and sensitivity as ‘High’ for oysters as a group.  However, evidence of the direct effect of oiling on Ostrea edulis was not found.  In the UK, Ostrea edulis beds occur in the shallow subtidal (0-20 m) and rarely in shallows exposed at low tide. But in sheltered areas, oil is likely to persist, and reach the shallow sea bed adsorbed to particulates or in solution. Therefore, the resistance of Ostrea edulis beds is assessed as ‘Medium’ to represent the chance that the most shallow extent of the biotopes might be exposed to an oil spill that coincided with the lowest tides.  Hence, resilience is assessed as ‘Medium’ and sensitivity to oil spills as ‘Medium’ but with ‘Low’ confidence. 

Petroleum hydrocarbons – oils and dispersed oils.  The effects of various petroleum hydrocarbons as oils (e.g. crude, fuel, and diesel) and dispersed oils (e.g. chemically enhanced water accommodated fractions, CEWAFs) were examined by 31 separate articles on adult, juveniles, early life stages and gametes of oyster species. There was considerable variation in the types of oil or oil and dispersant mixtures studied, experimental design and, hence, the results. For example, 3.6% of the results from the studies of the effects of complex hydrocarbons (crudes oils, WAF/WSF/HEWAF) on oysters reported ‘severe’ mortality, 47% reported ‘significant’ mortality, 4.8% ‘some’, 9.6% no mortality and 35% reported only sublethal effects.  However, if only early life stages are included, 5.4% report ‘severe’ mortality, but 65% report ‘significant’ mortality, 7% ‘some’, 14% no mortality and only 7% report only sublethal effects. The effects of dispersed oils (dispersant and oil mixtures) are similar. For example, 72% of the results of the effects of dispersed oils reported ‘severe’ or ‘significant’ mortality but 100% of the results from early life stages reported ‘severe’ or ‘significant’ mortality. However, only one article examined the effects of dispersed oil on adults and reported only sublethal effects. Similarly, only two articles examined the effects of oils on oysters, both of which reported sublethal effects.

His et al. (2000) also noted that oils, detergents, and their mixtures were usually toxic to the early life stages of bivalves at concentrations in the order of 1 ppm or higher.  His et al., 2000 also noted that refined oils were more toxic than crude oils but rarely at environmentally realistic concentrations. Oils and detergents also inhibited settlement in Crassostrea virginica while oil inhibited settlement in Ostrea edulis at 1-2 ppt (Renzoni, 1973b; Smith & Hackney, 1989; His et al., 2000).

Therefore, the resistance of the early life stages of oyster species to oils (WAF/WSF/HEWAFs) and dispersed oils is assessed as ‘None’.  Although limited evidence suggests that the adults may be resistant, we may assume that loss of larval stages would result in a decline in resident populations that are dependent on recruitment for their abundance (Bayne, 2017).  No evidence of the effects of oils and dispersed oils on Ostrea edulis was found.  Hence, the resistance of oyster beds is assessed as ‘Low’ to represent the resultant loss in annual recruitment and potential population decline.  Therefore, resilience is assessed as ‘Low’ and sensitivity as ‘High’ but with ‘Medium’ confidence due to the lack of evidence from adult populations.

Dispersants. The majority (72%) of the results of the effects of dispersants on oyster species reported ‘significant’ mortality, 5.5% ‘severe’ mortality, 5.5% no mortality and 16.7% reported only sublethal effects.  The proportion of ‘severe’ and ‘significant’ mortality results increase to 7.7% and 80.7% respectively in early life stages.  However, none of the results from adults and juveniles reported ‘severe’ mortality, but 66% reported ‘significant mortality and 33% reported sublethal effects.  Therefore, it appears that dispersants alone are more toxic to oyster species as adults, juveniles, or early life stages than complex hydrocarbons and dispersed oils.  This conclusion is consistent with the finding of Woelke (1972; cited in His et al., 2000).  Therefore, the resistance of oyster species to dispersants is assessed as ‘None’, resilience as ‘Very low’ and sensitivity as ‘High’.  However, Ostrea edulis may be an exception as only ‘significant’ mortality was reported in this species.  Hence, the resistance of Ostrea edulis and its beds to dispersants is assessed as ‘Low’, resilience as ‘Low’ and sensitivity as ‘High’.

Polyaromatic hydrocarbons (PAHs). The results of exposure of oyster species to PAHs were split evenly between ‘severe’ and sublethal effects, albeit based on only 12 articles.  However, early life stages were more sensitive, with 64% of the results reporting ‘severe’ mortality’, 18% ‘significant and only 9% either no mortality or sublethal effects.  PAH exposure was reported to result in reduced scope for growth in adults, reduced sperm motility and reduced fertilization rate and abnormal larval development (His et al., 1997; Jeong & Cho, 2005; Choy et al., 2007; Kim et al., 2007; Wessel et al., 2007; Nogueira et al., 2017; Xie et al., 2017b).  The toxicity of PAHs also increased in light (UV exposure) (Lyons et al., 2002).  Therefore, the resistance of oyster species to PAHs is assessed as ‘None’, resilience as ‘Very low’ and sensitivity as ‘High’.  However, as no direct evidence of the effects of PAHs on Ostrea spp. was found, confidence is assessed as 'Low'. 

Nonylphenol.  Nice et al. (2000; 2003) examined the effect of nonylphenol on Crassostrea gigas larvae or gametogenesis (Nice, 2005).  Nonylphenol reduced sperm production, caused hermaphroditism in some specimens after 48-hour exposures, and had transgenerational effects in which offspring had reduced survival if one parent was exposed to nonylphenol during larval development.  However, 72-hour exposure of larvae to 1.0 mg/l nonyphenol resulted in 100% larval mortality.  Therefore, the resistance of oyster species to nonylphenol is assessed as ‘None’, resilience as ‘Very low’ and sensitivity as ‘High’.

Dioxin. Cooper et al. (2009) investigated the effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on the embryonic development of Crassostrea virginica.  They reported 97-99% mortality in larvae at 2-10 µg/l TCDD (‘severe’ mortality) but only sublethal effects on adults.  Cooper et al. (2009) suggested their results might explain the lack of self-sustaining populations of bivalves in estuaries contaminated by TCDD.  Therefore, the resistance of oyster species to TCDD is assessed as ‘None’ in early life stages but ‘High’ in adults.  However, if Cooper et al. (2009) suggestion is correct, and TCDD contamination might result in population decline, the resistance of oyster beds may be assessed as ‘Low’, resilience as ‘Low’ and sensitivity as ‘High’ but with ‘Low confidence as the evidence is based on a single study.

Others. The results for ‘other’ petrochemicals are dominated by aromatic hydrocarbons and detailed in the ‘evidence summary spreadsheet’.  Exposure of Crassostrea virginica embryos for 48 hours to 25 mg/l 2,4-Dimethylphenol was reported to result in ‘severe’ mortality in one study.  Exposure to biphenyl was reported to result in sublethal effects in one study.  However, exposure to one or more of 19 aromatic petrochemicals (individually) was reported to result in significant mortality in five of the studies reviewed.  Therefore, the sensitivity of the early life stages of oyster species to 2,4-Dimethylphenol is probably ‘High’ (resistance is ‘None’ and ‘resilience ‘Very low’), albeit at high concentrations.  But oysters are probably ‘Not sensitive’ to biphenyl, albeit based on limited evidence.  Overall, the early life stages and hence populations of oyster species are probably of ‘High’ sensitivity (resistance and resilience are ‘Low’) to aromatic petrochemicals depending on the individual chemical, the exposure concentration, exposure duration and life stage exposed.  

Polychaetes, bivalves and amphipods are generally particularly affected by oil spills in infaunal habits, and echinoderms are also particularly intolerant of oil contamination (Suchanek, 1993).

Overall sensitivity.  The evidence suggests that oyster species, especially in their early life stages, are highly sensitive to a range of hydrocarbons and some PAHs. This conclusion agrees with the findings of His et al. (2000). Limited evidence suggests that the adults may be resistant to hydrocarbon contamination, but we may assume that loss of larval stages would result in a decline in resident populations that are dependent on recruitment for their abundance (Bayne, 2017). Therefore, the worst-case resistance of native oyster (Ostrea edulis) beds to hydrocarbon or PAH contamination is assessed as 'Low'. Beds may be protected from oil spills due to their depth depending on local conditions. Nevertheless, resilience is assessed as 'Low' and sensitivity as 'High'. However, Ostrea spp. was the least studied species in the evidence review so confidence in the assessment is 'Low'. 

Synthetic compound contamination

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

The results of the Rapid Evidence Assessment on the effects of 'Synthetic compound' contaminants on oyster species (Crassostrea spp., Ostrea spp., Saccostrea spp. and Magallana gigas). are summarized below. The full 'Oyster species evidence review' should be consulted for details of the studies examined and their results. A sensitivity assessment is provided for each type or source of 'synthetic compound' contaminant examined, together with an overall pressure assessment. 

Pesticides/biocides. Adults and juveniles were more resistant to pesticide/biocide exposure than early life stages. For example, 5.8% of the results of exposure of adults and juveniles to pesticides/biocides reported ‘severe’ or ‘significant’ mortality compared to 88% that reported sublethal effects, whereas 89% reported ‘severe’ or ‘significant mortality after exposure of early life stages, compared to only 6% that reported sublethal effects.  Over 200 different pesticides/biocides, their metabolic or degradation products were catalogued, and divided amongst 22 different functional (e.g. herbicide, insecticide) or structural (e.g. organohalogen, organophosphate) groups. Therefore, it is not possible to discuss the sensitivity of each pesticide/biocide reviewed and the 'Oyster species evidence summary' should be referred to for details. 

Overall, there is considerable evidence to suggest that adults and juveniles are resistant to most pesticides, with the exception of some insecticides, organophosphates and organohalogens but that early life stages (e.g. larvae) are sensitive to a wide range of pesticides/biocides. Therefore, the resistance of the early life stages oyster species to pesticides/biocides is assessed as ‘None’. Hence, the resistance of oyster beds is assessed as ‘Low’, resilience as ‘Low’ and sensitivity as ‘High’ on the assumption that loss of recruitment would lead to population decline.

Pharmaceuticals. ‘Severe’ mortality was reported in 16.6% of the results of exposure to pharmaceuticals, 53% reported ‘significant’, 2.8% ‘some’, 5.5% ‘no mortality, and the remaining 22% only reported sublethal effects.  However, the five articles that examined adult and juvenile oysters did not report any mortality (22% of results) or only sublethal effects (88% of results).  Conversely, the 11 articles that examined early life stages reported ‘severe’ mortality in 22% of the results, ‘significant’ in 70%, ‘some’ in 3.7%, ‘none’ in 3.7% and sublethal effects in 3.7% of the results. Overall, 28 separate pharmaceuticals were reported in the review but most of the chemicals were only tested in a single study (see ‘Oyster species evidence summary' spreadsheet).

The resistance of the early life stages in oyster species to pharmaceuticals is assessed as ‘None’.  However, the ‘worst-case’ resistance of adults and juveniles is probably ‘High’ (no mortality and/or sublethal effects).  Therefore, the resistance of oyster beds is assessed as ‘Low’ based on the assumption that loss of recruitment would lead to population decline.  Hence, resilience is assessed as ‘Low’ and sensitivity as ‘High’. 

Other synthetics. Overall, 13% of the results of exposure to ‘synthetics (other)’ reported ‘severe’ mortality, 59.4% reported ‘significant mortality, 4.3% ‘some’ mortality, 4.3% no mortality and 18.8% reported sublethal effects.  No mortality or only sublethal effects were reported for adults and juveniles.  However, the early life stages were less resistant to their effects.  For example, 17.6% of the results reported ‘severe’ mortality, 72.5% reported ‘significant’ mortality, 5.8% ‘some’ mortality, 1.9% reported no mortality and 1.9% reported sublethal effects.

Detergents and surfactants were the most toxic to larvae, which agrees with the finding of His et al. (2000).  The exposure of Ostrea edulis larvae to the detergents Kudos, Slix, Polyclens, Gamlen, Teepol, and Houghtosol halved the normal rate of larval development at concentrations ranging from 2.5-7.5 ppm (2.5-7.5 mg/l) (Smith, 1968).  Renzoni, (1973b; cited by His et al., 2000) also reported significant mortality in Ostrea edulis larvae exposed to tetrapropylene benzene sulphonate (with an LC50 of 2 mg/l). The remaining ‘Synthetic(other)’ chemicals were reported by only a few studies and varied in their sensitivity, although the effects of most types of chemicals would be assessed as ‘High’ sensitivity.

The resistance of the early life stages in oyster species to synthetic (others) is assessed as ‘None’.  However, the ‘worst-case’ resistance of adults and juveniles is probably ‘High’ (no mortality and/or sublethal effects).  Therefore, the resistance of oyster beds is assessed as ‘Low’ based on the assumption that loss of recruitment would lead to population decline.  Hence, resilience is assessed as ‘Low’ and sensitivity as ‘High’.

Polychlorinated biphenyls (PCBs). The effects of polychlorinated biphenyls (PCBs) were examined in only four studies (see above).  Exposure to either Aroclor 1016, 1254 or PCB 1254 did not result in mortality and/or only resulted in sublethal effects.  No studies on the effects of PCBs on oyster larvae were found.  Therefore, the resistance of oyster species to PCBs is assessed as ‘High’, resilience as ‘High’, and sensitivity as ‘Not sensitive’.

Flame retardants. Only two studies examined the effects of brominated flame retardants on oysters (Great Lakes Corporation, 1989; Xie et al., 2017b).  Sublethal effects (on shell deposition) were reported in immature oysters (Crassostrea sp.) but BDE-47 caused abnormal development of embryos and significant mortality in larvae.  Therefore, the resistance of early life stages to brominated flame retardants is probably ‘Low’ but immature oysters is ‘High’.  Hence, the resistance of oyster beds is assessed as ‘Low’ based on the assumption that loss of recruitment would lead to population decline.  Hence, resilience is assessed as ‘Low’ and sensitivity as ‘High’.  However, confidence in the assessment is ‘Low’ due to the disagreement in effect between the limited number of studies. 

Phthalates. The effects of phthalates were only examined in embryos and larvae.  All three of the studies reported significant mortality and/or abnormal development in embryos and larvae exposed to the phthalates studied.  Therefore, the resistance of early life stages to phthalates is probably ‘Low’.  Hence, the resistance of oyster beds is assessed as ‘Low’ based on the assumption that loss of recruitment would lead to population decline.  Hence, resilience is assessed as ‘Low’ and sensitivity as ‘High’.  However, confidence in the assessment is ‘Low’ due to the disagreement in effect between a limited number of studies.

Perfluoroalkyl substances (PFAS). Perfluoroalkyl substances (PFAS) were examined in two studies (Drottar & Krueger, 2000; OECD, 2002) neither of which specified the life stage of Crassostrea virginica examined.  Both studies reported sublethal effects at the concentrations tested.  The OECD (2002) suggested a 96-hour NOEC of 1.9 mg/l.  Therefore, resistance is assessed as ‘High’, resilience as ‘High’ and sensitivity as ‘Not sensitive’, albeit with ‘Low’ confidence due to the limited number of studies reviewed.

Overall sensitivity assessment. The effects of numerous (ca 200) pesticides/biocides, plus pharmaceuticals were examined in the literature reviewed, while PCBs, Flame retardants, Phthalates and PFAs were less studied. There was considerable variation between studies in experimental design as well as results. Nevertheless, the worst-case resistance of oyster species to 'synthetic compounds' is assessed as 'Low', especially due to the sensitivity of early life stages on the assumption that loss of recruitment would lead to population decline.  Therefore, the worst-case resistance of native oyster (Ostrea edulis) beds to 'synthetic compound' contamination is assessed as 'Low', resilience as 'Low' and sensitivity as 'High'. However, Ostrea spp. was the least studied species in the evidence review so confidence in the assessment is 'Low'. 

Radionuclide contamination

Benchmark. An increase in 10µGy/h above background levels. Further detail

Evidence

The effects of exposure to tritium were reported by one article (Nelson, 1971; not accessed).  Nelson (1971) reported mortality in Crassostrea gigas larvae after exposure to 0.000001 Ci/l to 0.01 Ci/l Tritium (Ci = Curie – a non-SI unit of radioactive decay) but did not specify the larval stage or the level of mortality observed. Another paper that examined the effects of radioactive isotopes of chromium, strontium, zinc and yttrium on oyster larvae (Nelson, 1968) could also not be accessed.  Therefore, resistance is assessed as ‘Low’ as a precaution but with ‘Low’ confidence because the level of mortality was not specified. Hence, resilience is assessed as ‘Low’ and sensitivity as ‘High’.

Introduction of other substances

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

The results of the Rapid Evidence Assessment on the effects of 'other substance' contaminants on oyster species (Crassostrea spp., Ostrea spp., Saccostrea spp. and Magallana gigas). are summarized below. The full 'Oyster species evidence review' should be consulted for details of the studies examined and their results. A sensitivity assessment is provided for each type or source of 'other substance' contaminant examined, together with an overall pressure assessment. 

Inorganic chemicals. Most of the studies examined chemicals used in chlorination as a form of disinfectant. Bellance & Bailey (1977) reported a bioassay completed by the Virginia Institute of Marine Science that established an LC50 of 5 μg/L for Crassostrea virginica larvae exposed to chlorine for 48 hours in static testing.

Capuzzo (1979) investigated the effects of temperature on the toxicity of chlorine and chloramine to Crassostrea virginica in a flow-through system.  The oyster larvae were exposed to chlorine or chloramine for 30 minutes at either 20 or 25℃, after the 30-minute exposure the toxicant was removed from the solution and the temperature was reduced down to the acclimation temperature.  The mortality of the larvae was assessed 48 hours after exposure.  The temperature was shown to enhance the toxic effects of the chemicals.  The LC50 and LC100 values for exposure to chlorine at 20℃ were 120 and 1400 µg/l, respectively but at 25℃ the LC50 and LC100 values for chlorine were 80 and 860 µg/l, respectively.  The LC50 and LC100 values for exposure to chloramines at 20℃ were 10 and 480 µg/l, respectively but at 25℃ the LC50 and LC100 values for chloramine were <10 and 160 µg/l, respectively. Chien & Chou (1989) examined the effects of chlorine exposure on the development of Crassostrea gigas under various temperatures and salinities, at different stages of development.  Fertilized eggs at the first polar stage and four larval stages (blastula, trochophore, veliger, and D-larva) were exposed to combinations of five concentrations (0 to 2520 µg/l) of chlorine, at four temperatures (22, 25, 28℃) and three salinities (18, 26, 34 ppt) for one hour.  Chlorine exposure to all of the tested stages had lethal impacts on the larvae.  In general, the resistance to chlorine increased with salinity, with lower LC50s observed at lower salinities.  Larval sensitivity to chlorine generally increased with higher exposure temperatures. Crecelius (1979) examined the effects of ozonization of seawater on the production of bromate.  Crecelius (1979) reported that ozonization of seawater converted of all bromide to bromate within 60 mins.  Ozonization of sodium chloride solution did not result in significant oxidants while sodium bromide solution resulted in both bromide and bromate.  Nevertheless, they concluded that the levels of bromate produced by chlorination or ozonization of power plant cooling waters were not acutely toxic to Crassostrea gigas larvae, fish, shrimp, and clams by comparison with toxicity figures determined in their laboratory.  Crecelius (1979) reported that 1.0 mg/l bromate resulted in 90% mortality in Crassostrea virginica larvae and 30 mg/l bromate caused abnormal development in 50% of Crassostrea gigas larvae during the first 48-hour of larval development (48-hour EC50/LC50 of 30 mg/l bromate).  However, no experimental details were provided (Crecelius, 1979). 

Roberts et al. (1975) examined the toxicity of chlorine in estuarine water to a range of marine species, using both flow-through and static tank systems.  They examined oyster larvae and juveniles (Crassostrea virginica), Mercenaria mercenaria larvae, Acartia tonsa (copepod), Palaemonetes pugio and fish (Menidia menidia, Syngnathus fuscus, and Gobiosoma bosci). They noted that molluscan larvae and copepods were the most sensitive species with a 48-hour LC50 of less than 0.005 ppm.  Roberts & Gleeson (1978) exposed Crassostrea virginica larvae to bromine chloride (BrCl) during 48-hour exposure assays.  The concentration that caused 50% mortality (LC50) was calculated at 210 µg/l bromine chloride.  In addition, to the larvae tests, juvenile oysters were exposed to BrCl for 96 hours to assess the impacts on shell growth.  The 96-hour EC50s for the shell growth were 100 and 160 µg/l. Roosenburg et al. (1980) examined the effects of chlorine on two larval stages of Crassostrea virginica.  Straight-hinge veliger larvae were exposed to concentrations of 10, 50, 100 and 200 µg/l chlorine for 6, 12, 24 and 36 hours, and to 50, 100, 200 and 300 µg/l for 8, 24, 48, 72 and 96 hours.  Pediveliger larvae were exposed to 50, 100, 200 and 300 µg/l chlorine for 6, 24, 48, 72 and 96 hours.  Mortality increased with increasing concentration in both larval stages.  Straight hinge veliger larvae were more sensitive than pediveliger larvae with between 83-100% mortality at the highest tested concentration at 96 hours.  Pediveliger larvae had between 32.4-46.1% mortality under the same conditions.

Scott & Middaugh (1978) investigated the seasonal toxicity of chlorination on Crassostrea virginica.  Bioassays were conducted in the fall (45-day exposure), winter (75-day exposure) and spring (60-day exposure).  Adult oysters were collected, accumulated, and exposed to sodium hypochlorite at nominal concentrations of 5.6, 3.2, 1.8, and 1 mg/L.  During each of the seasonal bioassays survival, condition index, gonadal index, and faecal production were assessed.  Total (100%) mortality occurred in all of the treatments at the highest nominal concentration.  However, the winter assay had a delay in mortality with 100% mortality occurring on day 70, compared to day 22 for fall and day 32 for spring.  The condition index of the controls was higher than the exposed oysters.  Similarly, the gonadal index was significantly higher in the control oysters. Stewart et al. (1979) investigated the toxicity of by-products of oxidative biocides on oyster larvae.  Crassostrea virginica larvae were exposed to bromate, bromoform, and chloroform, at 0.05, 0.1, 1.0, and 10.0 mg/I for 48 hours.  Mortality was observed after the 48-hour exposure period, at all concentrations of the three substances larval mortality occurred.

All but one of the studies above examined the effect of chlorination, bromination, or ozonization of oyster larvae.  All the studies reported ‘severe’ or ‘significant’ larval mortality at the concentrations tested.  In addition, adult Crassostrea virginica experienced a reduction in condition due to exposure to chlorination and 100% mortality at 5.6 mg/l.  Therefore, resistance would be assessed as ‘None’, resilience as ‘Very low, and sensitivity as ‘High’, especially in larvae.

Fluoride.  Cardwell et al. (1979) (see above) examined the effects of a large number of different chemicals on the larvae of Crassostrea gigas.  Fluoride was reported to result in a 48-hour EC50 (abnormal development) of 58 mg/l and a 48-hour LC50 (larval mortality) of >100 mg/l.  Therefore, resistance to fluoride exposure would be assessed as ‘Low’, resilience as ‘Low’ and sensitivity as ‘High’.  However, this assessment is based on a single study.

Phosphoric acid.  The effects of phosphoric acid on oyster larvae were reported by Daugherty (1951) and Kunigelis & Wilbur (1987).  Kunigelis & Wilbur (1987) could not be accessed and the level of mortality was not specified.  Daugherty (1951) reported 100% mortality in C. virginica after 28-hour exposure to 1,500 mg/l but the article could not be accessed for further detail.  .  Therefore, resistance would be assessed as ‘None’, resilience as ‘Very low, and sensitivity as ‘High’, especially in larvae.  However, this assessment is based on a single study that used a high concentration (1,500 mg/l).  It might represent the effects immediately after and in close proximity to a spill and confidence in the assessment is ‘Low’.

Potassium chloride (KCl) was included in two studies.  Da Cruz et al. (2007) reported that the exposure of Crassostrea rhizophorae embryos to potassium (as KCl) for 24 hours resulted in abnormal larval development and a 24-hour LC15 of 25.13 mg/l and a 24-hour LC50 35.56 mg/l.  Nell & Holliday (1986) examined the use of KCl and CuCl₂ to stimulate larval settlement in Saccostrea commercialis larvae, in static containers in the laboratory.  Settlement was stimulated by 8-12 mM KCl (160 - 210 µg/l) and no mortality was observed.  Therefore, low concentrations of KCl were used to induce larval settlement while high concentrations (mg/l) were reported to cause abnormal larval development.  Hence, resistance would be assessed as ‘Low’, resilience as ‘Low’ and sensitivity as ‘High’.  However, the assessment is based on a single study using high concentrations of KCl so the confidence is assessed as ‘Low’.

Caldwell et al. (1975) investigated the effects of hydrogen sulphide on the survival and development of Crassostrea gigas.  The tests were run over a four-day period, the longer the oysters were exposed to hydrogen sulphide the lower the concentration was to cause 50% mortality.  The LC50 at 24, 48, and 96 hours were 3,300, 2,600, and 1,400 µg/l, respectively.  Therefore, resistance to sulphide exposure is assessed as ‘Low’, resilience as ‘Low’ and sensitivity as ‘High’.

Okubo & Okubo, 1962 (cited by His, 2000) reported a 48-hour EC50 (abnormal development) of 32-100 µg/l in Crassostrea gigas embryos after exposure to sodium cyanide.  Therefore, the resistance of the early life stages of C. gigas to sodium cyanide would be assessed as ‘Low’, resilience as ‘Low’ and sensitivity as ‘High’. 

Natural products. Daugherty (1951) reported that exposure to ‘starch’ at a concentration of 3 g/l resulted in 100% mortality in Crassostrea virginica.  Unfortunately, Daugherty (1951) could not be accessed and it is unclear how the starch was administered or how the effect was caused so no sensitivity assessment is made. Similarly, Cardwell, 1979b (cited by His et al., 2000) reported that exposure of the fertilized eggs of Crassostrea gigas to tannic acid resulted in abnormal development and a 48-hour EC50 of >10 mg/l.  Unfortunately, Cardwell, 1979b (cited by His et al., 2000) could not be accessed and it is unclear how the tannic acid was administered or how the effect was caused so no sensitivity assessment is made.

Explosives. Goodfellow et al. (1983, 1983b) examined the lethal and sublethal effects of picric acid and picramic acid on oysters as they were potential contaminants from industrial effluents and the manufacture of explosives.  Goodfellow et al. (1983) investigated the acute toxicity of picric acid and picramic acid on Crassostrea virginica.  The 144-hour LC5Os for picric and picramic acid were 254.9 and 69.8 mg/l, respectively.  No growth EC50s and shell deposition EC5Os showed that both contaminants caused adverse effects at much lower concentrations than indicated by the LC50s.  For example, the 144-hour shell deposition EC50s were 27.9 mg/l for picric acid and 5.6 mg/l for picramic acid. Goodfellow et al. (1983b) investigated the effects of picric acid and picramic acid on the growth of Crassostrea virginica.  Exposure to 0.45 and 0.05 mg/l (450 and 50 µg/l) picric acid and 0.24 and 0.02 mg/ (240 and 20 µg/l) picramic acid showed significant inhibition of shell deposition during the 42 days of exposure.  In addition, discolouration of the nacre layer of the shell and body mass was observed after exposure to both contaminants by the end of the 42-day trial.  Exposure to picric or picramic acids in the water column was reported to be lethal to C. virginica.  Therefore, resistance is assessed as ‘None’, resilience as ‘Very low, and sensitivity as ‘High’.

Lightsticks. De Araujo et al. (2015) determined the chemical composition and the toxicity of lightsticks that were recently activated, compared to lightsticks one year after activation and to lightsticks collected on beaches.  The effect of lightstick content on embryos of Crassostrea rhizophorae after 24 hours of exposure was assessed at various concentrations (0.32, 0.56, 1, 1.76, and 2.24% WSF).  The value of the WSF-effective concentration (24-hour EC50) that caused abnormal development of larvae was 0.35% for new light sticks but, after one year of activation, the toxicity of the light stick was even higher at 0.65%.  Therefore, resistance would be assessed as ‘Low’, resilience as ‘Low’ and sensitivity as ‘High’.  However, this assessment is based on a single study.

Rubber. Tallec et al. (2022) investigated the chemical toxicity of different types of new and used rubber products (tires, crumb rubber granulates, aquaculture rubber bands) on the early life stages of the Pacific oyster, Crassostrea gigas.  Leachates were obtained from the products at 0.1, 1, and 10 g/L. Sperm and embryos were exposed to leachates at 0.1, 1, and 10g/l for one hour before being assessed for viability.  The effect on fertilization was assessed by combining gametes with the different concentrations of each of the leachates for 1.5 hours before assessing the fertilization yields.  The impacts on the development of larvae were assessed by exposing embryos to leachates at 0.1, 1, and 10 g/l for 36 hours.  Abnormal D- larvae were classed as those with morphological malformations or those which had developmental arrest during embryogenesis.  The viability of oyster sperm was not significantly affected by exposure to leachates from new tires, used tires, new crumb rubber granulates, used crumb rubber granulates, and used oyster-farming rubber bands at any of the concentrations (0.1, 1 and 10g/l) tested when compared with the control treatments.  However, significant reductions in the percentage of live spermatozoa were observed at the highest tested concentrated leachate (10 g/l) from new oyster-farming rubber bands.  The viability of oyster oocytes was not significantly affected by exposure to any of the leachates at any of the tested concentrations.  The fertilization yield was not significantly affected by exposure to leachates from new tires, used tires, new crumb rubber granulates, used crumb rubber granulates, or used oyster-farming rubber bands when compared with the control treatment.  However, significant reductions in fertilization yield were observed at the highest tested concentration of leachate (10 g/l) from new oyster-farming rubber bands.  Embryo-larvae development was significantly reduced by 53% by exposure to new-tire leachate at 10 g/l.  Embryo-larval development was completely inhibited at the highest tested leachate concentration (10 g/L) of new crumb rubber granulates, used crumb rubber granulates, and used oyster-farming rubber bands.  Embryo-larval development was completely inhibited at 1g/l of new oyster-farming rubber bands leachate.  Therefore, the resistance of embryos and larvae to rubber leachates would be assessed as ‘None’, resilience as ‘Very low, and sensitivity as ‘High’. 

Overall sensitivity assessment for 'other substances'.  Most of the chemicals examined in the studies reviewed resulted in 'severe' or 'significant' mortality, especially in early life stages. Therefore, the worst-case resistance of native oyster (Ostrea edulis) beds to 'other substances' contamination is assessed as 'Low', resilience as 'Low' and sensitivity as 'High'. However, Ostrea spp. was not examined directly in the studies reviewed and the effects varied depending on the chemical examined so confidence in the assessment is 'Low'. 

De-oxygenation

Benchmark. Exposure to dissolved oxygen concentration of less than or equal to 2 mg/l for one week (a change from WFD poor status to bad status). Further detail

Evidence

Oysters are considered to be tolerant of periods of hypoxia due to their ability to survive out of water during transportation for long periods of time, and many weeks at low temperatures (Korringa, 1952; Yonge, 1960).

Lenihan (1999) reported that Crassostrea virginica could withstand hypoxic conditions (< 2mg O² /l ) for 7-10 days at 18 °C but last for several weeks at <5 °C.  However, Lenihan (1999) also suggested that many days (26) of hypoxia, contributed to the high rate of mortality observed at the base of reefs at 6 m depth together with poor condition, parasitism and reduced food availability.  In addition, a prolonged period of hypoxia in the River Neuse (North Carolina) resulted in the mass mortality of oysters (Lenihan, 1999).

Sensitivity assessment.  Ostrea edulis is probably not affected by de-oxygenation at the level of the benchmark.  Therefore, resistance and resilience are assessed as ‘High’, and Ostrea edulis is assessed as ‘Not sensitive’ at the benchmark level.

Nutrient enrichment

Benchmark. Compliance with WFD criteria for good status. Further detail

Evidence

This pressure relates to increased levels of nitrogen, phosphorus and silicon in the marine environment compared to background concentrations.  The nutrient enrichment of a marine environment leads to organisms no longer being limited by the availability of certain nutrients.  The consequent changes in ecosystem functions can lead to the progression of eutrophic symptoms (Bricker et al., 2008), changes in species diversity and evenness (Johnston & Roberts, 2009) decreases in dissolved oxygen and uncharacteristic microalgal blooms (Bricker et al., 1999, 2008).

Moderate nutrient enrichment, especially in the form of organic particulates and dissolved organic material, is likely to increase food availability for suspension feeders such as Ostrea edulis.  However, long-term or high levels of nutrient enrichment may result in eutrophication and have indirect adverse effects, such as increased turbidity, increased suspended sediment, increased risk of deoxygenation and the risk of algal blooms.

Nutrient enrichment of the water column is a potential impact arising from finfish aquaculture which can potentially lead to eutrophication and the alteration of the species composition of plankton with the possible proliferation of potentially toxic or nuisance species (OSPAR, 2009b).

Ostrea edulis has been reported to suffer mortality due to toxic algal blooms, e.g. blooms of Gonyaulax sp. and Gymnodinium sp. (Shumway, 1990).  The subsequent death of toxic and non-toxic algal blooms may result in large numbers of dead algal cells collecting on the sea bottom, resulting in local de-oxygenation as the algae decompose, especially in sheltered areas with little water movement where this Ostrea edulis is often found.

Sensitivity assessment.  A slight increase in nutrients may enhance food supply to Ostrea edulis and increase growth rates in the species.  And at the pressure benchmark that assumes compliance with WFD criteria for good status, there shouldn’t be a negative impact on this species.  Therefore the resistance and resilience have been assessed as ‘High’, resulting in an assessment of ‘Not Sensitive’.

Organic enrichment

Benchmark. A deposit of 100 gC/m²/yr. Further detail

Evidence

Organic enrichment leads to organisms no longer being limited by the availability of organic carbon.  The consequent changes in ecosystem function can lead to the progression of eutrophic symptoms (Bricker et al., 2008), changes in species diversity and evenness (Johnston & Roberts, 2009) and decreases in dissolved oxygen and uncharacteristic microalgal blooms (Bricker et al., 1999, 2008).  Indirect adverse effects associated with organic enrichment include increased turbidity, increased suspended sediment and the increased risk of deoxygenation.

Sensitivity assessment.  Little empirical evidence was found to support an assessment of this species at this benchmark.  The lack of direct evidence for Ostrea edulis has resulted in this pressure being assessed as ‘No evidence’.

Physical loss (to land or freshwater habitat)

Benchmark. A permanent loss of existing saline habitat within the site. Further detail

Evidence

All marine benthic species are considered to have a resistance of ‘None’ to this pressure and to be unable to recover from a permanent loss of habitat (resilience is ‘Very Low’).  Sensitivity within the direct spatial footprint of this pressure is, therefore ’High’.  Although no specific evidence is described confidence in this assessment is ’High’, due to the incontrovertible nature of this pressure.

Physical change (to another seabed type)

Benchmark. Permanent change from sedimentary or soft rock substrata to hard rock or artificial substrata or vice-versa. Further detail

Evidence

Ostrea edulis can be found on top of a variety of sediment types including gravels, sand and mud, small boulders and bedrock.  Therefore, if rock or an artificial substratum were to be replaced with a sedimentary substratum individual Ostrea edulis could theoretically also survive in this situation.  But large populations of Ostrea edulis as beds occur on sediment and would be lost as a result of a change in seabed type. Therefore, resistance is likely to be ‘Low’, resilience ‘Very Low’ (permanent change) and sensitivity is assessed as ’High’. 

Physical change (to another sediment type)

Benchmark. Permanent change in one Folk class (based on UK SeaMap simplified classification). Further detail

Evidence

Ostrea edulis occurs on a range of sediment types and hence are not considered sensitive to change in folk class.  Resistance and resilience are, therefore, assessed as ‘High’ resulting in this species being considered ‘Not sensitive’ at the pressure benchmark.

Habitat structure changes - removal of substratum (extraction)

Benchmark. The extraction of substratum to 30 cm (where substratum includes sediments and soft rock but excludes hard bedrock). Further detail

Evidence

Ostrea edulis cements its lower valve permanently to solid pieces of substratum, such as pebbles, cobbles, boulders etc.  The removal of this layer of the substratum would lead to the loss of the biogenic layer created by oysters and its biological community, the oyster cultch (which will remove an important chemical cue used by larvae when settling), and the substratum which provides a point of attachment for larvae.

Sensitivity assessment.  The resistance to the removal of the substratum is ‘None’.  The resilience is probably ‘Very low’.

Abrasion / disturbance of the surface of the substratum or seabed

Benchmark. Damage to surface features (e.g. species and physical structures within the habitat). Further detail

Evidence

Abrasion may cause damage to the shell of Ostrea edulis, particularly to the growing edge.  Regeneration and repair abilities of the oyster are quite good.  Power washing of cultivated oysters routinely causes chips to the edge of the shell increasing the risk of desiccation.  This damage is soon repaired by the mantle.  Oysters were often harvested by dredging in the past, which their shells survived relatively intact.  On mixed sediments, the dredge may remove the underlying sediment and cobbles and shell material with effects similar to substratum loss above.

In a review of anthropogenic threats to restored Ostrea edulis broodstock areas, Woolmer et al. (2011) reported that fishing mortality from commercial fisheries (for oysters and other mobile gear fisheries) is a key pressure on native oyster populations and habitats.  Impacts include stock removal, disturbance of spat (juvenile oysters) and habitat disturbances (to oyster banks and reefs).  Woolmer et al. (2011) stated that dredging over oyster beds removes both cultch material and target oysters.  Over time, with sufficient effort, the net effect is a flattening of the bed.  The flatter bed is more susceptible to siltation and hypoxia in some water bodies (Woolmer et al., 2011).  However, they also stated that although dredges have the negative effects stated above, the use of dredges on managed Ostrea edulis beds in some areas is often seen as necessary if siltation and smothering by algae and Crepidula fornicata are to be controlled.

Sensitivity assessment.  Ostrea edulis is somewhat resistant to some abrasion and is able to recover from some damage to shells e.g. chipping caused by pressure washers.  However, damage caused to oyster beds and their habitats by commercial fishing is considered to be of importance to levels of mortality and health of oyster beds.  Therefore, resistance has been assessed as ‘Low’, and resilience is assessed as ‘Low’.  Hence, the sensitivity of the species’ is assessed as ‘High’.

Penetration or disturbance of the substratum subsurface

Benchmark. Damage to sub-surface features (e.g. species and physical structures within the habitat). Further detail

Evidence

In general, fishing activities that penetrate the substratum to a greater extent (e.g. beam trawls, scallop dredges and demersal trawls) will potentially damage Ostrea edulis to a greater degree than fishing activities using lighter gear (e.g. light demersal trawls and seines) (Hall et al., 2008).  One of the major reasons for the decline of the oyster population at Chesapeake Bay was mechanical destruction (Rothschild et al., 1994).

Sensitivity assessment.  The effect of subsurface disturbance will be to displace, damage and remove individuals.  Therefore, resistance is assessed as ‘Low’.  Resilience is assessed as ‘Low’ and sensitivity is, therefore, assessed as ‘High’.

Changes in suspended solids (water clarity)

Benchmark. A change in one rank on the WFD (Water Framework Directive) scale e.g. from clear to intermediate for one year. Further detail

Evidence

In a field experiment in Canada, the summer growth of Ostrea edulis on coarse sandy substrata was found to be enhanced at low levels of sediment resuspension and inhibited as sediment deposition increased (Grant et al., 1990, summarised in Ray et al., 2005).  In a review of the biological effects of dredging operations, Ray et al. (2005) stated that sediment chlorophyll in suspension at low levels may act as a food supplement, enhancing growth, but at higher concentrations may dilute planktonic food resources and suppress food ingestion).

Oysters respond to an increase in suspended sediment by increasing pseudofaeces production with the occasional rapid closure of their valves to expel accumulated silt (Yonge, 1960) both of which exert an energetic cost. Korringa (1952) reported that an increase in suspended sediment decreased the filtration rate in oysters.  This study is supported by Grant et al. (1990) who found declining clearance rates in Ostrea edulis in response to an increase in suspended particulate matter.  Suspended sediment was also shown to reduce the growth rate of adult Ostrea edulis and to result in shell thickening (Moore, 1977).  Reduced growth probably results from increased shell deposition and an inability to feed efficiently.  Hutchinson & Hawkins (1992) reported that filtration was completely inhibited by 10 mg/l of particulate organic matter and significantly reduced by 5 mg/l.  Ostrea edulis larvae survived 7 days exposure to up to 4 g/l silt with little mortality.  However, their growth was impaired at 0.75 g/l or above (Moore, 1977). Yonge (1960) and Korringa (1952) considered Ostrea edulis to be intolerant of turbid (silt laden) environments.  Moore (1977) reported that variation in suspended sediment and silted substratum, and resultant scour, was an important factor restricting oyster spat fall, i.e. recruitment.  Therefore, an increase in suspended sediment may have longer term effects of the population by inhibiting recruitment, especially if the increase coincided with the peak settlement period in summer.  

Sensitivity assessment.  A short-term increase in sedimentation is likely to have an impact on Ostrea edulis.  Ostrea edulis has a coping mechanism to remove increased levels of silt from within the mantle.  This behaviour is energetically expensive, and may cause a decrease in growth rate of the organism, but is unlikely to cause mortality.  However, at the level of the benchmark, there will be mortality at as the level of sediment in the water column will exceed that of what the organism can survive.  With the change in the benchmark for a year, there will likely be complete mortality.  Therefore, resistance and resilience are assessed as ‘Low’  and a sensitivity is assessed as  ’High’.

Smothering and siltation rate changes (light)

Benchmark. ‘Light’ deposition of up to 5 cm of fine material added to the seabed in a single discrete event. Further detail

Evidence

Ostrea edulis is an active suspension feeder on phytoplankton, bacteria, particulate detritus and dissolved organic matter (DOM) (Korringa, 1952; Yonge, 1960).  The addition of fine sediment, pseudofaeces or fish food would potentially increase food availability for oysters.  But even small increases in sediment deposition have been found to reduce growth rates in Ostrea edulis (Grant et al., 1990).  Smothering by 5 cm of sediment would prevent the flow of water through the oyster that permits respiration, feeding and removal of waste.  Ostrea edulis is permanently fixed to the substratum and would not be able to burrow up through the deposited material.  Ostrea edulis can respire anaerobically, and is known to be able to survive for many weeks (Yonge, 1960) or 24 days (Korringa, 1952) out of water at low temperatures used for storage after collection.  However, it is likely that at normal environmental temperatures, the population would be killed by smothering.  Yonge (1960) reported the death of populations of Ostrea edulis due to smothering of oyster beds by sediment and debris from the land as a result of flooding (Yonge, 1960).  In a review of anthropogenic threats to restored Ostrea edulis broodstock areas, Woolmer et al. (2011) reported that the deposition of faeces and waste food from finfish aquaculture developments or deposition from shellfish culture developments (particularly mussel bottom culture) may present a smothering risk to Ostrea edulis beds directly below or close by.

Sensitivity assessment.  Ostrea edulis is unlikely to survive this pressure at the benchmark level and deposited sediment is likely to interfere with subsequent recruitment.  As filter feeders that are permanently attached to the substratum, they would be unable to borrow up to the surface.  In the low energy environments in which populations of this species develop (i.e. weak water flow, sheltered from wave action, or at greater depths in moderately wave exposed conditions), the deposited sediment is likely to remain for several tidal cycles, depending on local hydrography. Therefore, resistance to the pressure is probably ‘Low’, resilience is ‘Low’ and the species sensitivity at this pressure benchmark is given as ‘High’.

Smothering and siltation rate changes (heavy)

Benchmark. ‘Heavy’ deposition of up to 30 cm of fine material added to the seabed in a single discrete event. Further detail

Evidence

No direct evidence was found to assess this pressure at the benchmark.  A deposit at the pressure benchmark would cover Ostrea edulis with a thick layer of fine materials.  Ostrea edulis would be unable to survive this pressure at the benchmark.  As filter feeders that are permanently attached to the substratum, they would be unable to borrow up to the surface to enable basic life functions to occur.  A deposit of 30 cm of material is likely to remain for a longer period of time than 5 cm (see above), Therefore, resistance to the pressure is probably ‘None’ and resilience ‘Very low’ so that sensitivity is assessed as ‘High’.

Litter

Benchmark. The introduction of man-made objects able to cause physical harm (surface, water column, seafloor or strandline). Further detail

Evidence

‘Not assessed’.

Electromagnetic changes

Benchmark. A local electric field of 1 V/m or a local magnetic field of 10 µT. Further detail

Evidence

No evidence was found.

Underwater noise changes

Benchmark. MSFD indicator levels (SEL or peak SPL) exceeded for 20% of days in a calendar year. Further detail

Evidence

Ostrea edulis does not have hearing perception but vibrations may cause a reaction, e.g. valve closure. But it is unlikely to be affected by underwater noise from passing vessels etc.

Introduction of light or shading

Benchmark. A change in incident light via anthropogenic means. Further detail

Evidence

The native oyster has no dependence on light availability, so changes in turbidity and thus light reaching the seabed, for example, would have no direct effect on this species.  However, prevention of light reaching the seabed may affect Ostrea edulis indirectly through changes in phytoplankton abundance and primary production.  

Sensitivity assessment.  Resistance and resilience are assessed as ‘High’, resulting in an assessment of ‘Not sensitive’.

Barrier to species movement

Benchmark. A permanent or temporary barrier to species movement over ≥50% of water body width or a 10% change in tidal excursion. Further detail

Evidence

Not relevant – this pressure is considered applicable to mobile species, e.g. fish and marine mammals rather than seabed habitats. Physical and hydrographic barriers may limit propagule dispersal.  But propagule dispersal is not considered under the pressure definition and benchmark.

Death or injury by collision

Benchmark. Injury or mortality from collisions of biota with both static or moving structures due to 0.1% of tidal volume on an average tide, passing through an artificial structure. Further detail

Evidence

‘Not relevant’ - to Ostrea edulis. NB. Collision by grounding vessels is addressed under ‘surface abrasion’.

Visual disturbance

Benchmark. The daily duration of transient visual cues exceeds 10% of the period of site occupancy by the feature. Further detail

Evidence

‘Not relevant’.

Genetic modification & translocation of indigenous species

Benchmark. Translocation of indigenous species or the introduction of genetically modified or genetically different populations of indigenous species that may result in changes in the genetic structure of local populations, hybridization, or change in community structure. Further detail

Evidence

Organisms are frequently transplanted from one location to another in marine aquaculture and these transplanted species may pose potentially serious impacts to native populations through interbreeding and thus alteration of the gene pool.

The Pacific oyster (Magallana gigas) has been intentionally imported from Japan into Ireland because they are larger and faster-growing than the native oyster (Ostrea edulis).  Pacific oysters cannot hybridize with the native oyster but indirect effects may occur through alterations in gene frequencies as a result of ecological interactions with the Pacific oyster (Heffernan, 1999).

Sensitivity assessment.  Very little information is available on the effect of this pressure on Ostrea edulis.  Although Ostrea edulis may be translocated, ‘No evidence’ was found on which to base an assessment.

Introduction or spread of invasive non-indigenous species

Benchmark. The introduction of one or more invasive non-indigenous species (INIS). Further detail

Evidence

The American slipper limpet Crepidula fornicata was introduced to the UK and Europe in the 1870s from the Atlantic coasts of North America with imports of the eastern oyster Crassostrea virginica. It was recorded in Liverpool in 1870 and the Essex coast in 1887-1890. It has spread through expansion and introductions along the full extent of the English Channel and into the European mainland, including native and cultured oyster beds (Blanchard, 1997, 2009; Bohn et al., 2012, 2013a, 2013b, 2015; De Montaudouin et al., 1999, 2018; Helmer et al., 2019; Hinz et al., 2011; McNeill et al., 2010; Powell-Jennings & Calloway, 2018; Preston et al., 2020; Stiger-Pouvreau & Thouzeau, 2015). It occurs in large numbers in most of the oyster producing areas of England and Wales (Blanchard, 1997; Thieltges, 2005; Powell-Jennings & Calloway, 2018). 

High densities of Crepidula fornicata cause ecological impacts on sedimentary habitats. The species can smother the seabed in shallow bays, changing and modifying the habitat structure (Blanchard, 1997, 2009; Helmer et al., 2019; Tillin et al., 2020). At high densities, the species physically smothers the sediment, and the resultant build-up of silt, pseudofaeces, and faeces is deposited and trapped within the bed (Fitzgerald, 2007, Blanchard, 2009, Stiger-Pouvreau & Thouzeau, 2015; Tillin et al. 2020). The biodeposition rates of Crepidula are extremely high and once deposited, form an anoxic mud, making the environment suitable for other species, including most infauna (Blanchard, 2009; Stiger-Pouvreau & Thouzea, 2015). The resultant modification of the substratum can render it unsuitable for native oysters (Blanchard, 1997, 2009).

For example, the Bay of Mont Saint-Michel supports shellfisheries for the Pacific oyster (Magallana gigas), flat oyster (Ostrea edulis) and blue mussels (Mytilus edulis). The flat oyster fishery was impacted by Crepidula which spread rapidly into the oyster beds and became attached to the oyster shells (Blanchard, 2009). After forty years, Crepidula occupied most of the subtidal area of the Bay, its spread facilitated by oyster farming and shellfish dredging (Blanchard, 2009). However, it occurred at lower densities and co-existed with other shellfish in Arcachon Bay (De Montaudouin et al. 1999, 2018) and Bourgnerf Bay (Decottignies, 2006, 2007 cited in Blanchard, 2009). 

Similarly, Helmer et al. (2019) reported that Ostrea edulis numbers in the Chichester Harbour had decreased by 96% in a 19-year period since 1998, while Crepidula numbers had increased by 441% in the same period. Low densities of Ostrea edulis and high densities of Crepidula were also reported in Portsmouth and Langstone Harbours, which are within one of the few remaining oyster fisheries in the UK. Helmer et al. (2019) suggested that the lack of recovery of Ostrea edulis populations was probably due to a lack of habitat heterogeneity and suitable settlement substratum, together with ongoing fishing activity and disease.  Helmer et al. (2019) suggested that the native oyster population in the Solent was "on the brink of ecological collapse" without active management to mitigate the dominance of Crepidula.  Preston et al. (2020) reported that Crepidula larvae probably competed with Ostrea larvae for food in the plankton, with resultant negative effects on recruitment, and that the Solent is probably substratum-limited for Ostrea due to larval preference for native oyster shell and mixed sediment rather than the muddy sediments created by dense populations of Crepidula. Preston et al. (2020) also noted that the biogenic habitat created by Crepidula was less diverse and species-rich than that provided by native oysters. 

Crepidula fornicata has been implicated in the decline of Ostrea edulis beds across the North Sea. However, Hayer et al. (2019) concluded that the decline of native oyster beds was almost complete before Crepidula began to invade the North Sea. The decline in native oyster beds by the 1940s was most probably due to overfishing combined with reductions in water quality, cold winters (hence poor spat fall), flooding, the introduction of non-native competitors and pests, and outbreaks of disease (Korringa, 1952; Yonge, 1960; Edwards, 1997). The population dynamics of oyster populations are dependent on positive feedback between adult abundance and recruitment via the provision of reef habitat for the settlement of larvae (e.g. adult shell), and the growth of the height of the reef about the sediment and the supply of food (facilitated by current flow) (Bayne, 2017). Nevertheless, the presence of high densities of Crepidula, modification of the substratum, competition for food as larvae, and exclusion from suitable settlement substratum, probably prevents the recovery of Ostrea edulis beds (Blanchard, 2009; Helmer et al., 2018; Preston et al., 2020). 

The American oyster drill Urosalpinx cinerea was first recorded in 1927 and occurs in the south-east and south-west of the UK.  Urosalpinx cinerea is a major predator of oyster spat and was considered to be a major pest on native and cultured oyster beds (Korringa, 1952; Yonge, 1960) and contributed to the decline in oyster populations in the first half of the 20th century. For example, in the Oosterschelde, Korringa (1952) reported 90% mortality in oyster spat by their first winter, with up to 75% being taken by Urosalpinx cinerea, while Hancock (1955) noted that 73% of spat settling in the summer of 1953 died by December, 55 -58% being taken by Urosalpinx cinerea. 

Didemnum vexillum (leathery sea squirt) was first recorded in the UK in Holyhead Marina in 2008 (Laing et al., 2010).  This species can colonize a range of substrata, and has been found on commercial oyster lays on the south coast of England.  There are no studies on the effects of Didemnum vexillum on Ostrea edulis.  However, it is likely that if Ostrea edulis were to be smothered by this species there could be negative impacts.

Sensitivity assessment.  There is a chance that an INIS might invade the same habitats as those within which Ostrea edulis is found.  In particular, Crepidula is reported to damage oyster culture and is thought to prevent oyster bed recovery and compete for habitat, while oyster drills affect oyster culture and native oyster beds. Where abundant, Crepidula fornicata is likely to change the entire habitat. Therefore, resistance is assessed as ‘Low’. Resilience is assessed as 'Very low' because the successful removal of an INIS is extremely rare.  Hence, sensitivity is assessed as 'High'. Due to the constant risk of new invasive species, the literature on this pressure should be revisited. 

Introduction of microbial pathogens

Benchmark. The introduction of relevant microbial pathogens or metazoan disease vectors to an area where they are currently not present (e.g. Martelia refringens and Bonamia, Avian influenza virus, viral Haemorrhagic Septicaemia virus). Further detail

Evidence

Numerous diseases and parasites have been identified in oysters, partly due to their commercial importance and partly because of incidences of disease related mass mortalities in oyster beds.  Diseases in oysters and other commercial bivalve species may be caused by bacteria (especially in larvae), protists, fungi, coccidians, gregarines, and trematodes, while annelids and copepods may be parasites.  The reader should refer to reviews by Lauckner (1983) and Bower & McGladdery (1996) for further detail.  The following species have caused mortalities in Ostrea edulis populations in the UK. 

Polydora ciliata burrows into the shell, weakening the shell and increasing the oyster’s vulnerability to predation and physical damage, whereas Polydora hoplura causes shell blisters.  Boring sponges of the genus Cliona may bore the shell of oysters and cause shell weakening, especially in older specimens.  The flagellate protozoan Heximata sp. resulted in mass mortalities on natural and cultivated beds of oysters in Europe in 1920-21, from which many populations did not recover (Yonge, 1960).  Another protozoan parasite Marteilia refingens, present in France has not yet affected stocks in the British Isles.  The copepod parasite of mussels, Mytilicola intestinalis, has also been found to infect Ostrea edulis and has the potential to cause considerable loss of condition, although in most infections there is no evidence of pathology.

The transportation of Pacific oysters from Japan to the west coast of North America is thought to have resulted in the introduction of the bacterium Nocardia crassostreae leading to nocardiosis (a bacterial infection that can invade every tissue) in Pacific oysters (Magallana gigas) and Ostrea edulis (Forrest et al., 2009).

The protistan parasite Bonamia ostrea is a serious threat to Ostrea edulis in the UK (Laing et al., 2005, cited in Woomer et al. 2011).  Bonamia ostrea has caused mortality of Ostrea edulis throughout northern Europe (France, the Netherlands and Spain) and Iceland and England after its accidental introduction in 1980's and resulted in a further reduction in oyster production (Edwards, 1997). Disease events reduced populations by 80% or higher Heffernan (1999).  Disease transmission can occur from oyster to oyster.  However, Bonamia ostrea is also found in other marine invertebrates, including zooplankton (indicating the possibility of interspecies transmission; Lynch et al., 2007 cited in Woolmer et al., 2011).  Ostrea edulis larvae may also be vectors for disease between populations (Arzul et al., 2011 cited in Woolmer et al., 2011).

Sensitivity assessment.  Although the impact of individual species of microbial pathogen on Ostrea edulis varies, pathogens known to affect this species in the UK can cause significant mortality.  Bonamia ostrea is known to cause in excess of 80% mortality of oyster beds within the UK.  Therefore, resistance and resilience have been assessed as ‘Low’ and sensitivity as ‘High’.

Removal of target species

Benchmark. Removal of species targeted by fishery, shellfishery or harvesting at a commercial or recreational scale. Further detail

Evidence

Ostrea edulis is long lived, has notably unreliable reproduction and low levels of recruitment, which makes it vulnerable to over fishing (Orton, 1927; Spärck, 1951; Laing et al., 1951; taken from Gravestock et al., 2014).  British native oyster beds were exploited in Roman times.  The introduction of oyster dredging in the mid-19th century developed the oyster beds into one of Britain's largest fisheries, employing about 120,000 men around the coast in the 1880's.  However, by the late 19th century stocks were beginning to be depleted so that by the 1950s the native oyster beds were regarded as scarce (Korringa, 1952; Yonge, 1960; Edwards, 1997).  This species is still regarded as scarce today.  Over-fishing, combined with reductions in water quality, cold winters (hence poor spat fall), flooding, the introduction of non-native competitors and pests, and outbreaks of disease were blamed for the decline (Korringa, 1952; Yonge, 1960; Edwards, 1997).  As a result, although 700 million oysters were consumed in London alone in 1864, the catch fell from 40 million in 1920 to 3 million in the 1960s; from which the catch has not recovered (Edwards, 1997). 

Most populations are now artificially laid for culture and protected by Protection Orders (Fowler, 1999; Edwards, 1997). For example, the Ostrea edulis fishery in The Solent was once considered to be the largest self-sustaining fishery in Europe (Gravestock et al., 2014).  However, since the turn of the 20^th century the population has collapsed significantly three times.  The first collapse occurred between 1919 and 1921 due to a disease epidemic caused by the flagellate protozoan Hexamita (Tubbs, 1999).  The second collapse was caused by the 1962 - 1963 winter, during which temperatures were significantly below average (Kamphausen, 2012).  And finally, in 2006, when poor recruitment led to sharp drop in the population (Gravestock et al., 2014).  Although a number of potential causes of recruitment failure have been suggested (see Gravestock et al., 2014), it is suggested that overfishing exacerbated the effect of poor recruitment.

Sensitivity assessment.  The current scarcity of oyster beds in the UK is due to the pressure the populations were put under due to commercial fishing.  Stock from beds can remain sustainable under commercial fishing pressure.  However, if these populations have a period of bad recruitment or are affected by another negative pressure, then fishing can compound this effect.  Ostrea edulis have no ability to remove themselves from fishing pressure as they are permanently attached to the substratum once they have settled from larvae.  Therefore, resistance is assessed as ‘None’.  A number of native oyster beds in the UK have been destroyed by fishing and have had to undergo human intervention to return the oyster population.  In some areas oysters have not returned.  Resilience is assessed as ‘Very low’ so that sensitivity is assessed as ‘High’.

Removal of non-target species

Benchmark. Removal of features or incidental non-targeted catch (by-catch) through targeted fishery, shellfishery or harvesting at a commercial or recreational scale. Further detail

Evidence

Direct, physical impacts from harvesting are assessed through the abrasion and penetration of the seabed pressures.  Ostrea edulis could easily be incidentally removed from its habitat as by-catch when other species are being targeted.

Sensitivity assessment.  The resistance to removal is ‘None’ due to the inability of Ostrea edulis to evade collection. The resilience is ‘Very low’, with recovery only being able to begin when the harvesting pressure is removed altogether. Therefore sensitivity is assessed as ‘High’.