The Marine Life Information Network

Substratum loss

Benchmark. All of the substratum occupied by the species or biotope under consideration is removed. A single event is assumed for sensitivity assessment. Once the activity or event has stopped (or between regular events) suitable substratum remains or is deposited. Species or community recovery assumes that the substratum within the habitat preferences of the original species or community is present. Further details

Evidence

Loss of the substratum will also remove the resident population of Cerastoderma edule. Hall & Harding (1997) found that Cerastoderma edule abundance had returned to control levels within about 56 days after significant mortality due to suction dredging, and Moore (1991) also suggested that recovery was rapid. Recovery is dependent on the recruitment of spat or migration (active or passive) from the surrounding substratum. For example, Coffen-Smout & Rees (1999) reported that cockles could be distributed by flood and ebb tides, but especially flood tides (by rolling around the surface) up to 0.45 m on neap tides or between 94 m and 164 m on spring tides and could colonize cleared areas at a rate of 2.2 -12 individuals/m² /14 days. It seems likely therefore that the population could recover within a year, however, given the sporadic nature of recruitment in Cerastoderma edule, recovery may be more protracted.

Smothering

Benchmark. All of the population of a species or an area of a biotope is smothered by sediment to a depth of 5 cm above the substratum for one month. Impermeable materials, such as concrete, oil, or tar, are likely to have a greater effect. Further details.

Evidence

Cerastoderma edule has short siphons and needs to keep in contact with the surface of the sediment. Richardson et al. (1993(b)) reported that they burrow quickly to the surface if covered by 2 cm of sediment (under laboratory or field conditions) when emmersed (45% of cockles emerged onto the surface in light and 60% in darkness). In light the cockles quickly re-burrow, however, in darkness they move across the substratum, partly to increase the distance between neighbours. Richardson et al. (1993(b)) suggested that surface movement in darkness, perhaps accompanied by passive movement if rolled by flood and ebb tides might be a response to avoiding areas of disturbed sediment. Jackson & James reported that few Cerastoderma edule buried to 10 cm in sediment were able to burrow to the surface whereas most buried to a depth of 5 cm could reach the surface. In another experiment, Cerastoderma edule buried 10 cm in sandy substrate was able to burrow near to the surface, but still suffered 83% mortality in 6 days, whereas in muddy substrates all cockles died between 3 and 6 days. Experimental baiting digging resulted in significant mortality in dug areas rather than undug areas (48% mortality in 9 days to a maximum of 85% after 11 days) probably due to smothering (Jackson & James, 1979). Smaller individuals were more likely to die than larger ones. Fowler (1999) cites reports of 90% mortality of cockles in areas affected by bait digging, with recolonization occurring three months after bait digging, although the cockle population structure was still different from undisturbed areas. Therefore, cockles are probably of intermediate intolerance to smothering by 5 cm of sediment although smaller individuals may be more intolerant. No information on smothering and spat was found. In years of good recruitment recovery may occur within a year, however, recruitment is sporadic (see reproduction) and may take longer in 'bad' years.

Increase in suspended sediment

Benchmark. An arbitrary short-term, acute change in background suspended sediment concentration e.g., a change of 100 mg/l for one month. The resultant light attenuation effects are addressed under turbidity, and the effects of rapid settling out of suspended sediment are addressed under smothering. Further details

Evidence

Cerastoderma edule is adapted to life in a sedimentary environment and quite capable of burrowing. Increasing total particulate concentrations have been shown to decrease clearance rates and increase pseudofaeces production (Navarro et al. 1992; Navarro & Widdows, 1997). Filtration rates increased with particulate concentration until 300 mg/l at which concentration filtration rates abruptly declined. Pseudofaeces production was triggered by concentrations of total particulate matter of 1.5 mg/l (Navarro et al. 1992) or 4.8 mg/l (Navarro & Widdows, 1997), however, the absorption efficiency remained independent of particulate concentration over a large range but reduced at concentrations above 250 mg/l (Navarro & Widdows, 1997). Navarro & Widdows (1997) suggested that Cerastoderma edule was able to compensate for the decrease in particulate quality (i.e. the proportion of organic to inorganic seston) between 1.6 to 300 mg/l. This was accomplished by an effective preingestive selection of organic particulates, together with increasing filtration and rejection rates. Navarro & Widdows (1997) concluded that Cerastoderma edule was well adapted to living in turbid environments such as intertidal mudflats. Increased siltation and suspended sediment concentration result in increased pseudofaeces production and concomitant loss of energy and carbon as mucus. Therefore, Cerastoderma edule probably has a low intolerance to increased suspended sediment.

Decrease in suspended sediment

Benchmark. An arbitrary short-term, acute change in background suspended sediment concentration e.g., a change of 100 mg/l for one month. The resultant light attenuation effects are addressed under turbidity, and the effects of rapid settling out of suspended sediment are addressed under smothering. Further details

Evidence

Desiccation

1. A normally subtidal, demersal or pelagic species including intertidal migratory or under-boulder species is continuously exposed to air and sunshine for one hour.
2. A normally intertidal species or community is exposed to a change in desiccation equivalent to a change in position of one vertical biological zone on the shore, e.g., from upper eulittoral to the mid eulittoral or from sublittoral fringe to lower eulittoral for a period of one year. Further details.

Evidence

When emersed Cerastoderma edule gapes, actively expels water from the mantle cavity and takes in a bubble of air. In the laboratory, emersed Cerastoderma edule absorbed oxygen from the enclosed air bubble, probably through the mantle and mantle edges, and took in another bubble within 2 hrs (Boyden, 1971). However, when immersed Cerastoderma edule exhibited an oxygen debt, suggesting that some anaerobic respiration had occurred (Boyden, 1971; Widdows & Shick, 1985). Boyden (1971) reported that Cerastoderma edule survived 42.9% water loss. Cerastoderma edule is likely to be protected from significant desiccation due to its infaunal habit and the humidity of the surrounding sediment (Boyden, 1971; Widdows & Shick, 1985). However, increased desiccation, equivalent to raising the population from mid to high water is likely to reduce the species abundance and the extent of the population. The population could probably recover within a year, however, given the sporadic nature of recruitment in Cerastoderma edule, recovery may be more protracted.

Increase in emergence regime

Benchmark. A one hour change in the time covered or not covered by the sea for a period of one year. Further details

Evidence

Cerastoderma edule is found from high water to the sublittoral fringe although its highest abundance occurs at mid tidal level. Cerastoderma edule is capable of aerial respiration when emmersed at low tide, by gaping, and like most bivalve molluscs is capable of anaerobic respiration (Boyden, 1972; Widdows & Shick, 1985). Boyden (1972) noted that on emersion the cockle actively takes air into the mantle cavity, which may be replaced within two hours if still emersed. The bubble is ejected upon immersion. Increased emergence is likely to depress this species' height on the shore due to increased exposure to temperature extremes (see temperature) and desiccation (see above), however, this may be countered by its extent at the bottom of the shore. Decreased emergence, perhaps as a result of sea level rise may result in a reduction in intertidal area and, therefore, a decrease in available habitat (Wilson, 1993).

Decrease in emergence regime

Benchmark. A one hour change in the time covered or not covered by the sea for a period of one year. Further details

Evidence

Increase in water flow rate

A change of two categories in water flow rate (view glossary) for 1 year, for example, from moderately strong (1-3 knots) to very weak (negligible). Further details

Evidence

The hydrodynamic regime strongly influences the sediment structure, oxygenation, food supply and recruitment. Increasing water flow may remove adult cockles from the sediment surface and carry them to unfavourable substratum or deep water, where they may be lost from the population. Coffen-Smout & Rees (1999) reported that cockles could be distributed by flood and ebb tides, but especially flood tides (by rolling around the surface) up to 0.45 m on neap tides or between 94 m and 164 m on spring tides. Newly settled spat and juveniles (<4.8 mm) are capable of bysso-pelagic dispersal. Therefore, water flow rates probably affect the distribution and dispersal of juveniles and adults. More importantly, increased water flow rate is likely to increase the grain size of the sediment. Cerastoderma edule prefers muddy sand to sandy-mud substrata. Decreasing water flow rate may increase siltation and favour muddy substrates that are unsuitable for Cerastoderma edule. Boyden & Russell (1972) suggested that lack of tidal flow may exclude Cerastoderma edule possibly due to reduced food availability as suggested by Brock, (1979). Therefore, decreased water flow rates may exclude Cerastoderma edule from the affected area.

Decrease in water flow rate

A change of two categories in water flow rate (view glossary) for 1 year, for example, from moderately strong (1-3 knots) to very weak (negligible). Further details

Evidence

Increase in temperature

1. A short-term, acute change in temperature; e.g., a 5°C change in the temperature range for three consecutive days. This definition includes ‘short-term’ thermal discharges.
2. A long-term, chronic change in temperature; e.g. a 2°C change in the temperature range for a year. This definition includes ‘long term’ thermal discharges.

For intertidal species or communities, the range of temperatures includes the air temperature regime for that species or community. Further details

Evidence

High mortalities of cockle populations due to severe winters have been reported by many authors (Kristensen, 1958; Hancock & Urquhart, 1964; Beukema, 1990; Ducrotoy et al., 1991). Kristensen (1958) reported that the sediment froze to a depth of 10 cm and 15 cm, resulting in the death of cockles in areas of the Wadden Sea in the severe winter of 1954. Hancock & Urquhart (1964) report almost 100% mortality of cockles in Llanrhidian Sands, Burry Inlet and high mortalities of cockles in other areas around the UK after the winter of 1962/63. Beukema (1990) considered Cerastoderma edule to be intolerant of cold winters. Kristensen (1958) reported that Cerastoderma edule from the Dutch Wadden Sea died within 24 hrs at -1.9°C with an upper-temperature tolerance of 31°C for 24 hrs, but that spat (3-6 mm) were more tolerant. All cockles died after 6 min at 36°C. Ansell et al. (1981) reported an upper median lethal temperature of 35°C after 24hrs (29°C after 96 hrs exposure), and Wilson (1981) reported a upper lethal temperature of 42.5°C. Wilson (1981) noted that Cerastoderma edule had limited ability to acclimate. Smaal et al. (1997) stated that Cerastoderma edule is unable to acclimate to low temperatures. However, Newell & Bayne (1980) stated that Cerastoderma edule was able to acclimate to a temperature change of 10°C and regulate its metabolic rate in response to rising spring temperatures. Temperature tolerance in the above studies was dependent on the environmental temperature, i.e. specimens collected in summer or areas of higher average temperature tolerated higher temperatures than specimens collected in winter and/or at lower average temperatures. Cerastoderma edule larvae showed optimal growth between 15 and 20°C, but poor or erratic growth at 10°C or 30°C (Kingston, 1974). Wilson (1993) concluded that Cerastoderma edule was probably tolerant of a long-term temperature rise of 2°C associated with climate change. Warmer temperatures during winter result in increased metabolic rate and hence depletion of energy reserves in a time of low food availability and may contribute to post-winter mortality of adult cockles (Wilson & Elkaim, 1991). Therefore, the intolerance of Cerastoderma edule to temperature change will be dependent on season, an acute short-term temperature rise in summer or decrease in winter may be detrimental. However, Cerastoderma edule may be tolerant to long-term temperature increases although decreasing temperatures are likely to result in more frequent freezing events in winter. Therefore, Cerastoderma edule is probably highly intolerant of acute decreases in temperature in winter or summer months. Rapid increases in temperature during the spawning season may initiate spawning (Ducrotoy et al. 1991). High shore populations are likely to be more vulnerable to extremes of temperatures due to their longer emergence time (see emergence). However, Wilson (1981) showed that Cerastoderma edule had the highest upper lethal temperature of the species he studied, presumably due to its close contact with the sediment surface. The observed high mortalities in winter months may be due to low food availability and predation rather than temperature alone, except in severe winters. The upper lethal temperature of 42.8°C is unlikely to occur on mudflats except in extremely hot summers. Therefore, an intolerance of intermediate rather than high has been reported. Beukema (1990) reported that the biomass and species richness of winter tidal-flat zoobenthos were restored within one to two years after severe winters in the Dutch Wadden Sea. The population could probably recover within a year, however, given the sporadic nature of recruitment in Cerastoderma edule, recovery may be more protracted and, therefore, recovery has been assessed as 'High'.

Decrease in temperature

1. A short-term, acute change in temperature; e.g., a 5°C change in the temperature range for three consecutive days. This definition includes ‘short-term’ thermal discharges.
2. A long-term, chronic change in temperature; e.g. a 2°C change in the temperature range for a year. This definition includes ‘long term’ thermal discharges.

For intertidal species or communities, the range of temperatures includes the air temperature regime for that species or community. Further details

Evidence

Increase in turbidity

1. A short-term, acute change; e.g., two categories of the water clarity scale (see glossary) for one month, such as from medium to extreme turbidity.
2. A long-term, chronic change; e.g., one category of the water clarity scale (see glossary) for one year, such as from low to medium turbidity. Further details

Evidence

Increasing turbidity may reduce phytoplankton productivity and hence decrease food availability, however, Cerastoderma edule is capable of ingesting organic seston (see siltation) and is adapted to life in sedimentary and estuarine conditions where turbidity is high (Navarro & Widdows, 1997). Therefore, Cerastoderma edule is probably tolerant to changes in turbidity.

Decrease in turbidity

1. A short-term, acute change; e.g., two categories of the water clarity scale (see glossary) for one month, such as from medium to extreme turbidity.
2. A long-term, chronic change; e.g., one category of the water clarity scale (see glossary) for one year, such as from low to medium turbidity. Further details

Evidence

Increase in wave exposure

A change of two ranks on the wave exposure scale (view glossary) e.g., from Exposed to Extremely exposed for a period of one year. Further details

Evidence

Changes in wave exposure are likely to have marked effects on the sediment dynamics of the shore. Increased exposure will result in increased grain size or erosion of the sediment, while decreased exposure will lead to increased siltation and reduced grain size (muddy sediment). In both cases, the sediment may become unsuitable for Cerastoderma edule populations resulting in a reduction of the extent or abundance of the population. Increased wave exposure is also likely to remove adult cockles from the sediment surface which may be subsequently lost from the population (see water flow rate).

Decrease in wave exposure

A change of two ranks on the wave exposure scale (view glossary) e.g., from Exposed to Extremely exposed for a period of one year. Further details

Evidence

Noise

1. Underwater noise levels e.g., the regular passing of a 30-metre trawler at 100 metres or a working cutter-suction transfer dredge at 100 metres for one month during important feeding or breeding periods.
2. Atmospheric noise levels e.g., the regular passing of a Boeing 737 passenger jet 300 metres overhead for one month during important feeding or breeding periods. Further details

Evidence

Cerastoderma edule can probably detect the vibration caused by predators and will withdraw its siphons. However, little information was found concerning the effect of noise or vibration on cockle populations and it is unlikely to be sensitive to noise or vibration.

Visual presence

Benchmark. The continuous presence for one month of moving objects not naturally found in the marine environment (e.g., boats, machinery, and humans) within the visual envelope of the species or community under consideration. Further details

Evidence

Cerastoderma edule has well developed eyes on the sensory tentacles of the inhalant and exhalent tentacles (Charles, 1966). These probably enable the cockle to response to shadowing by predators and withdraw the siphons. However its visual acuity is probably limited and it is unlikely to be sensitive to visual presence.

Abrasion & physical disturbance

Benchmark. Force equivalent to a standard scallop dredge landing on or being dragged across the organism. A single event is assumed for assessment. This factor includes mechanical interference, crushing, physical blows against, or rubbing and erosion of the organism or habitat of interest. Where trampling is relevant, the evidence and trampling intensity will be reported in the rationale. Further details.

Evidence

Coffen-Smout (1998) studied simulated fisheries impacts on Cerastoderma edule and reported that the cockle shell withstood between 12.9 and 171.4 newtons (N) of force depending on shell size and position of load (a 1 kg weight exerts about 10 N). Cockles are often damaged during mechanical harvesting, e.g. 5-15% were damaged by tractor dredging (Cotter et al., 1997) and ca 20% were too damaged to be processed after hydraulic dredging (Pickett, 1973). Physical disturbance equivalent to a passing scallop dredge is likely to cause a similar degree of damage. Therefore an intolerance of intermediate has been recorded. Recoverability is likely to be high (see additional information below).

Displacement

Benchmark. Removal of the organism from the substratum and displacement from its original position onto a suitable substratum. A single event is assumed for assessment. Further details

Evidence

Coffen-Smout & Rees (1999) noted that cleared areas of sediment could be recolonized by 2.2 -12 cockles /m² / 14 days. They also reported that cockles could be distributed by flood and ebb tides, but especially flood tides (by rolling around the surface) up to 0.45 m on neap tides or between 94 m and 164 m on spring tides. Similarly, cockles are capable of burrowing rapidly into the substratum and >50% burrowed into the substratum within 1 hour in experimental trials (Coffen-Smout & Rees, 1999), although this rate was inhibited by prior disturbance. Brock (1979) reported that 80% began to burrow within 60 min and 50% had successfully burrowed into sediment within 60 min. He also noted that young cockles could burrow quickly, and were nearly buried within 5 min. Disturbance and displacement may also reduce the growth rates (Orton, 1926) or interfere with the reproductive cycle (Hummel & Bogaards, 1989). Cockles on the surface of the sediment, are at an increased risk of predation, depending on the time of day, light, and tide. However, populations of cockles are probably moved, buried or displaced naturally by storms and once exposed can burrow relatively quickly into suitable sediment, and therefore are probably adapted to being displaced. Reduction in the local population density may enable good recruitment in following years, dependent on larval supply.

Synthetic compound contamination

Sensitivity is assessed against the available evidence for the effects of contaminants on the species (or closely related species at low confidence) or community of interest. For example:

• evidence of mass mortality of a population of the species or community of interest (either short or long term) in response to a contaminant will be ranked as high sensitivity;
• evidence of reduced abundance, or extent of a population of the species or community of interest (either short or long term) in response to a contaminant will be ranked as intermediate sensitivity;
• evidence of sub-lethal effects or reduced reproductive potential of a population of the species or community of interest will be assessed as low sensitivity.

The evidence used is stated in the rationale. Where the assessment can be based on a known activity then this is stated. The tolerance to contaminants of species of interest will be included in the rationale when available; together with relevant supporting material. Further details.

Evidence

Bryan & Gibbs (1991) reported evidence suggesting that TBT may be responsible for recruitment failure, due to either reproductive failure or larval mortality in bivalve molluscs, e.g. Pecten maximus at ca. 50 ng/l TBT. Cerastoderma edule was reported to accumulate PCBs from ingestion of contaminated particulates but no biological effects were described (Langston, 1978). However, little other information concerning the effects of synthetic chemicals was found. Recruitment failure in Cerastoderma edule may be significant due to the natural high levels of winter mortality and could result in decline of the local population, therefore an intolerance of intermediate is reported, albeit at very low confidence.

Heavy metal contamination

Evidence

Bryan (1984) states that Hg is the most toxic metal to bivalve molluscs while Cu, Cd and Zn seem to be the most problematic in the field. In bivalve molluscs, Hg was reported to have the highest toxicity, with mortalities occurring above 0.1-1 µg/l after 4-14 days exposure (Crompton, 1997), toxicity decreasing from Hg > Cu and Cd > Zn > Pb and As > Cr ( in bivalve larvae, Hg and Cu > Zn > Cd, Pb, As, and Ni > to Cr). Although Hg concentrations in Cerastoderma edule were found to increase with proximity to industrial discharges in Limfjord, Denmark, Cd and Pb showed no such relationship (Brock, 1992), however, laboratory exposure to 100 ppb Hg (as HgCl₂) for 32 days did not affect growth. Brock (1992) isolated a metallothionein-like protein that may be involved in detoxification. Jenner & Bowmer (1990) exposed cockles to different proportions of pulverised fuel ash (PFA) from a coal-powered power station, which contained a variety of heavy metals. Exposure to 100% PFA resulted in 43.3% mortality after 90 days, whereas regular addition of PFA to the surface of uncontaminated sediment resulted in 71% mortality in 90 days. Cerastoderma edule accumulated Zn and Ni but not As, Cu, Cr or Sb in their experiments. However, the observed mortality could be due to PFA being an unsuitable sediment. Wilson (1983) demonstrated that Cerastoderma edule accumulated Ni, but suffered no mortality at 100 µg/l or associated change in body condition or respiration. Wilson (1983) suggested that Cerastoderma edule may be a suitable indicator species for Ni. Studies of Cerastoderma edule populations from polluted and un-contaminated sites in Southampton Water showed that tissue heavy metal concentrations were lower in summer than winter/spring, tissue heavy metal concentrations decreased with the size of the cockle, and that cockles in sediments contaminated with metals and hydrocarbons had lower life expectancies, growth rates and body condition index (Savari et al., 1991(a), (b)). Transplantation of Cerastoderma edule into Restronguet Creek (highly polluted by heavy metals) resulted in 10-15% mortality within 63 days but 100% within ca. 4 months (Bryan & Gibbs, 1983). Bryan & Gibbs (1983) report that Cerastoderma edule takes up heavy metals mainly from solution rather than from sediment. They concluded that the toxic body load for Cu in Cerastoderma edule was ca. 250 µg/g tissue and that it was excluded from Restronguet Creek by the high levels of Cu and Zn.

Hydrocarbon contamination

Evidence

Savari et al. (1991(a)) stated there was a concentration related reduction in scope for growth of Cerastoderma edule with increasing concentration of hydrocarbons in the water column. Timmermans et al. (1996) noted a reduction in gonad development and percentage fertilization exposed for 9 months to Rotterdam harbour sediment contaminated with PCBs, PAHs, Cd, Hg, Pb and Zn, probably due to an increased level of parasitism (and presumably susceptibility to parasites) in the polluted specimens with respect to the control. However, no significant difference in larval development was found. McLusky (1982) examined the fauna of the intertidal mudflats at Kinneil in the Forth estuary that received petroleum, chemical and domestic effluents. Spatfall of Cerastoderma edule occurred in 1976 but the abundance declined steadily between 1976 and 1980. Cerastoderma edule, together with many other species, was excluded from sediment within 1.5 km of effluent discharges. Between 1.5 - 2.25 km the abundance of fauna, including Cerastoderma edule increased markedly (McLusky, 1983). Large numbers of moribund and dead marine animals, including Cerastoderma edule, were washed ashore after the Sea Empress oil spill, however, no commercial stocks were affected (SEEEC, 1998). Therefore, an intermediate intolerance has been reported.

Radionuclide contamination

Evidence

Miramand & Germain (1985) examined the uptake of plutonium 239 and americium 241 in Cerastoderma edule. They showed that americium 241 was taken up preferentially to plutonium 239, from water (and interstitial water), rather than from sediment. Americium 241 accumulated mainly in the tissues of the digestive tract while plutonium 239 accumulated mainly in the shell. Chassaud-Bouchard (1992) demonstrated similar accumulation of americium 241, plutonium 239 and uranium 238. Chassaud-Bouchard (1992) also reported that uptake of these nuclides resulted in damage to the nucleus, increased activity of the golgi apparatus and decreased numbers of mitochondria in the cells of the bivalves studied. However, the evidence was not clearly presented and its ecological significance was not addressed.

Changes in nutrient levels

Evidence

Changes in the nutrient concentrations (e.g. nitrogen and phosphates) are likely to have indirect rather than direct affects on Cerastoderma edule. Increased levels of nutrients at low level may increase phytoplankton productivity and increase food availability for Cerastoderma edule. However, higher nutrient inputs are associated with eutrophication, resulting in increased oxygen consumption and decreased oxygen concentration. Rosenberg & Loo (1988) suggested that the mass mortalities of Cerastoderma edule observed in Laholm Bay, western Sweden during the 1980s was correlated with increased nutrient levels, and associated decrease in oxygen levels during the this period (see oxygenation below). However, no direct causal link was established. It is likely that increased nutrient levels leading to eutrophication may contribute indirectly to mass mortalities in Cerastoderma edule populations.

Increase in salinity

1. A short-term, acute change; e.g., a change of two categories from the MNCR salinity scale for one week (view glossary) such as from full to reduced.
2. A long-term, chronic change; e.g., a change of one category from the MNCR salinity scale for one year (view glossary) such as from reduced to low. Further details.

Evidence

Boyden & Russell (1972) stated that Cerastoderma edule prefers salinities between 15 and 35 psu. Russell & Peterson (1973) reported lower median salinity limits of 12.5 psu and upper median salinity limits of 38.5 psu. Rygg (1970) noted that Cerastoderma edule did not survive 23 days exposure to >10 psu or at 60 psu, although they did survive at 46 psu. Rygg (1970) also demonstrated that salinity tolerance was temperature dependant (after 3 days, 100% survival at 33 psu and 35-38°C, but 50% mortality occurred at 20 psu and 37°C and 100% mortality at 13 psu and 37°C). Wilson (1984) noted that Cerastoderma edule remained open during 1 hour exposure to salinities between 13.3 and 59.3 psu. It should be noted that the tolerances reported above depend on the duration of the experiment. Field observations by Russell & Petersen (1973) demonstrated that Cerastoderma edule died when transplanted to Widewater lagoon. Kristensen (1958) reported the death of young spat (1 to 2 mm) in the Dutch Wadden Sea due to heavy rain, whereas the adults were able to dig deeper into the sediment. Kingston (1974) found that Cerastoderma edule larvae grew optimally at 30 and 35 psu, but were frequently deformed at 20 psu. and did not metamorphose at 45 psu. He noted that Cerastoderma edule larvae survived between 20 -50 psu, but died after 11 days at 55 psu or 10 days at 10 psu. Cerastoderma edule is primarily intertidal and exposed to heavy rainfall, freshwater runoff, evaporation and potential hypo-saline conditions. The studies reported above show a wide tolerance range of salinity for both adults and larvae.

Decrease in salinity

1. A short-term, acute change; e.g., a change of two categories from the MNCR salinity scale for one week (view glossary) such as from full to reduced.
2. A long-term, chronic change; e.g., a change of one category from the MNCR salinity scale for one year (view glossary) such as from reduced to low. Further details.

Evidence

Changes in oxygenation

Benchmark.  Exposure to a dissolved oxygen concentration of 2 mg/l for one week. Further details.

Evidence

McMahon & Wilson (1981) pointed out that the surface water film on sand flats have oxygen concentration at or above atmospheric saturation, which was reduced to 9.7 -25.2% saturation at 2 cm depth and 4.6-7.2% saturation at 5cm. Therefore, Cerastoderma edule rarely encounters hypoxic or anoxic conditions, although Rosenberg & Loo (1988) document decreased oxygen levels of surface waters in the Kattegat. Boyden (1972) reported that emersed, air breathingCerastoderma edule had a median lethal survival time of 129 hrs, whereas specimens unable to 'breathe' air (clamped) or in an oxygen-free environment had median lethal times of 69 and 75 hrs respectively, indicating that Cerastoderma edule was capable of anaerobic respiration. Rosenberg et al. (1991) reported 100% mortality of Cerastoderma edule exposed to 0.5 - 1.0 ml/l oxygen for 43 days and 98% mortality after 32 days. Cerastoderma edule migrated to the surface of the sediment in response to decreased oxygen concentrations. Theede et al. (1969) reported 50% mortality after 4.25 days at 1.5 ml/l oxygen. Theede et al. (1969) also noted that Cerastoderma edule only survived 4 days exposure to 0.0-6.1 cm³/l of hydrogen sulphide, which is associated with anoxic conditions. This suggests that Cerastoderma edule could survive several days anoxia but it is likely that continued exposure to 2 mg/l oxygen for a week would be lethal.

Introduction of microbial pathogens/parasites

Benchmark. Sensitivity can only be assessed relative to a known, named disease, likely to cause partial loss of a species population or community. Further details.

Evidence

Jonsson & Andre (1992) suggested that mass mortality of Cerastoderma edule occurring on the west coast of Sweden in the summer of 1991 was due to infestation by the larvae of the digenean trematode Cercaria cerastodermae I. However, Cercaria cerastodermae I was considered rare in the British Isles. Cerastoderma edule may be infected by numerous larval digenean trematodes, and the parasitic copepod Paranthessius rostatus (see General Biology). No evidence of mass mortality of cockles in the British Isles due to parasites was found but Boyden (1972) reported castration of a population of cockles in the river Couch. Parasites undoubtedly reduce the condition and reproductive potential of infected Cerastoderma edule. Therefore, parasites are likely to directly or indirectly result in a reduction in the abundance and extent of a population of Cerastoderma edule.

Introduction of non-native species

Sensitivity assessed against the likely effect of the introduction of alien or non-native species in Britain or Ireland. Further details.

Evidence

No information regarding alien or non-native species likely to complete with or exclude Cerastoderma edule was found.

Extraction of this species

Benchmark. Extraction removes 50% of the species or community from the area under consideration. Sensitivity will be assessed as 'intermediate'. The habitat remains intact or recovers rapidly. Any effects of the extraction process on the habitat itself are addressed under other factors, e.g. displacement, abrasion and physical disturbance, and substratum loss. Further details.

Evidence

Hall & Harding (1997) examined the effects of hydraulic and tractor dredging of Cerastoderma edule on macrobenthic communities. They concluded that although significant mortality of Cerastoderma edule and other infauna occurred, recovery was rapid and the overall effects on populations was low. Hall & Harding (1997) found that Cerastoderma edule abundance had returned to control levels within about 56 days and Moore (1991) also suggested that recovery was rapid. Cotter et al. (1997) noted that tractor dredging reduced the Cerastoderma edule stock by 31-49% depending on initial density, while Pickett (1973) reported that hydraulic dredging removed about one third of the cockle fishery. Tractor dredging leaves visible tracks in the sediment, which can act as lines for erosion and accelerate erosion of the sediment (Moore, 1991; Gubbay & Knapman, 1999). In most cases subsequent settlement was good especially in areas of previously high population density, however, Franklin & Pickett (1978) noted that subsequent spat survival was markedly reduced. Cotter et al. (1997) reported appreciable loss of spat and juveniles, partly due increased predation of exposed juveniles. Pickett (1973) also noted reduced survivability of 1-2 year old cockles after hydraulic dredging. However, most studies concluded that the impact of mechanised dredging on cockle populations and macrofauna in the long term was low (Pickett, 1973; Franklin & Pickett, 1978; Cook, 1990; Moore, 1991; Cotter et al., 1997; Hall & Harding, 1997; Ferns et al., 2000). Time of year of exploitation will influence recovery and avoiding seasonal spawning or larval settlement periods is likely to reduce the time taken for recovery (Gubbay & Knapman, 1999). Cockle beds have been mechanically fished for decades but several beds are closed from time to time depending on settlement and recruitment to the population, which is sporadic (see importance). Recovery may take less than a year in years of good recruitment but longer in bad years, therefore a recoverability of high has been used.

Extraction of other species

Benchmark. A species that is a required host or prey for the species under consideration (and assuming that no alternative host exists) or a keystone species in a biotope is removed. Any effects of the extraction process on the habitat itself are addressed under other factors, e.g. displacement, abrasion and physical disturbance, and substratum loss. Further details.

Evidence

Experimental baiting digging resulted in significant mortality in areas dug areas rather than undug areas (48% mortality in 9 days to a maximum of 85% after 11 days) probably due to smothering (Jackson & James, 1979). Smaller individuals were more likely to die than larger ones. Fowler (1999) cites reported 90% mortality of cockles in areas affected by bait digging, with recolonization occurring three months after bait digging, although the cockle population was still different from undisturbed areas. Jackson & James (1979) pointed out that bait digging disturbs sediment to a depth of 30-40 cm and probably buries many cockles below 10 cm and surface exposure of others that are then taken by predators. They suggested that bait digging was involved in the decline in the cockle fishery on the north Norfolk Coast in the 1950s and 60s. Therefore, cockles are probably of high intolerance to bait digging although smaller individuals may be more intolerant. In years of good recruitment recovery may occur within a year, however, recruitment is sporadic (see reproduction) and may take longer in 'bad' years.