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

Abra alba lives infaunally in muddy sediments. Removal of the substratum would also remove the entire population of the species and so intolerance has been assessed to be high. Recoverability has been assessed to be high (see additional information below).

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

Abra alba is a shallow burrower in muddy sediments. It requires its inhalant siphon to be above the sediment surface for feeding and respiration. Sudden smothering with 5 cm of sediment would temporarily halt feeding and respiration and require the species to relocate to its preferred depth. As an active burrower Abra alba would be expected to relocate with no mortality. However, growth and reproduction may be compromised owing to energetic expenditure and so intolerance has been assessed to be low. Growth and reproduction would return to normal following relocation so recoverability is recorded as immediate.

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

Levels of suspended sediment are likely to be most relevant to feeding. Abra alba practices two alternative modes of feeding. It either holds its feeding organ, the inhalant siphon, at a fixed position just above the sediment surface to filter out food particles suspended in the overlying water or else extends and moves its siphon around on the sediment above it to vacuum up deposited food particles. The alternative feeding methods are likely to make the species insensitive to relatively small changes in suspended sediment. If the level of suspended sediment becomes so high as to risk clogging the feeding structures, Abra alba could presumably switch to deposit feeding. Furthermore, an increase in suspended sediment is likely to increase the rate of siltation and therefore the food available to deposit feeders. Abra alba has been assessed to be tolerant at the benchmark level increase of 100 mg/ l for one month, with the potential for growth and reproduction to be enhanced by the increased food supply. However, a more substantial increase in suspended sediment levels would be expected to have a detrimental effect. For instance, the abundance of Abra alba declined over two years in the 1 km vicinity of an outfall pipe discharging fine-grained mineral waste from the china clay industry at a rate of 450, 000 tons per year to Mevagissey Bay, Cornwall. However, it was argued that persistent sediment instability was the more significant source of stress to the predominantly deposit-feeding community than the suspended sediment concentration (Probert, 1981).

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

Abra alba practices two alternative modes of feeding. It either holds its feeding organ, the inhalant siphon, at a fixed position just above the sediment surface to filter out food particles suspended in the overlying water or else extends and moves its siphon around on the sediment above it to vacuum up deposited food particles. A decrease in suspended sediment is likely to decrease the availability of food for both suspension and deposit feeding. The reduction in food availability may result in less energy available for growth and reproduction by Abra alba. However, the benchmark change period is one month, during which time it is not expected that mortality would occur and so intolerance is assessed as low. When suspended sediment returns to original levels, growth and reproduction should quickly return to normal so recoverability has been assessed to be immediate.

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

Abra alba lives infaunally in muddy sediments (Tebble, 1976) and is therefore likely to be largely protected from desiccation stress by its environmental position. Additionally, bivalves are able to respond to desiccation stress by valve adduction during periods of emersion. It is likely that Abra alba would be able to retain enough water within its shell to avoid mortality during the benchmark emersion period of one hour. However, during the period of emersion, the species would not be able to feed and respiration would be compromised, so there is likely to be some energetic cost. Intolerance has been assessed to be low. On immersion, metabolic activity should quickly return to normal and recoverability has been assessed to be immediate.

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

Although a largely subtidal species, a proportion of the population of Abra alba occurs in the proximity of the sublittoral fringe and so may be vulnerable to an increase in the emergence regime. The species does not colonize further up the shore and therefore must be limited by one or more factors including desiccation, temperature and wave exposure. An increase in the emergence regime may suppress the upper distribution of the population, some individuals may die and intolerance has been assessed to be intermediate. On return to prior conditions recolonization is likely and recoverability has been assessed to be very high (see additional information below).

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

Abra alba occurs in the subtidal zone and could therefore benefit from a decrease in the emergence regime.

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

Abra alba lives in low energy environments (Tebble, 1976) where the substratum has a high proportion of fine sediment. Increased water flow rate will change the sediment characteristics in which the species lives, primarily by winnowing away the surface layers and preventing deposition of finer particles (Hiscock, 1983). Furthermore, increased water flow rate may prevent settlement of larvae and therefore reduce recruitment. Mature adults buried at depth are likely to be unaffected as muddy substrata is particularly cohesive. An intolerance assessment of low has been made owing to reduced viability that may result from poor larval recruitment. Recoverability has been assessed to be very high as the adult population is likely to have survived (see additional information below).

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

Abra alba occurs in areas of weak water flow rate, so the benchmark decrease in water flow rate will expose the species to conditions of almost negligible flow. Decreased water flow may reduce the availability of food that may be obtained from suspension feeding and the species would have to switch to deposit feeding. A decreased water flow rate may favour the deposition of material upon which Abra alba could feed. However, a decreased water flow rate may mean that dispersion of planktonic larvae is minimal, and that recruitment to the benthos occurs in the vicinity of the parent population which may result in parent induced mortality (via feeding). Intolerance has therefore been assessed to be low and recoverability assessed to be very high (see additional information below).

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

Abra alba has a wide geographic range, occurring in waters to the south of the British Isles. It is likely therefore to be tolerant of higher temperatures than it experiences in Britain and Ireland. No specific information concerning temperature tolerances of Abra alba was found, but inferences of the effects may be made from other species of bivalve. For instance, Wilson (1978) reported the 24 hour LT50 for Fabulina fabula (studied as Tellina fabula) from Millport in Scotland to be 26.5 °C and noted that acclimation to higher temperatures enhanced the species' ability to withstand higher experimental temperatures. Similarly, Ansell et al. (1980) reported the 24 hour LT50 for Fabulina fabula (studied as Tellina fabula and acclimated at 10 °C) from Millport to be 27 °C. The 96 hour LT50 was 24-27 °C depending on acclimation temperature. Growth experiments by Salzwedel (1979) revealed that growth of Fabulina fabula (studied as Tellina fabula) correlated positively with temperature up to about 16 °C after which temperature increase inhibited growth. Considering that maximum sea surface temperatures around the British Isles rarely exceed 20 °C (Hiscock, 1998), it is unlikely that Abra alba would suffer mortality due to the benchmark increase in temperature. However, elevated temperatures would probably result in inhibition of growth and hence intolerance has been assessed to be low. A normal growth rate would resume on return to prior conditions so recoverability has been assessed to be very 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

Abra alba has a wide geographic range, occurring in waters to the north of the British Isles. It is likely therefore to be tolerant of lower temperatures than it experiences in Britain and Ireland and would probably tolerate a long term chronic decrease of 2 °C. However, a short term acute periods of extreme cold and icing conditions are likely to cause stress and some mortality in bivalve populations (Dame, 1996). For instance, during the 1978/79 winter which was very cold with severe ice conditions, water temperature in the outer Weser estuary, Germany, remained below 0 °C on 45 successive days. Characteristic species of the benthos, including Abra alba were considerably damaged (Buhr, 1981). The intolerance of Abra alba to a short-term acute decrease in temperature has been assessed to be intermediate as a substantial proportion of the population maybe killed. The species is likely to re-populate and recovery has been assessed to be very high (see additional information below).

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

Abra alba does not require light and therefore the effects of increased turbidity on light attenuation are not directly relevant. An increase in turbidity may affect primary production in the water column and therefore reduce the availability of phytoplankton food. However, phytoplankton will also be transported from distant areas and so the effect of increased turbidity may be mitigated to some extent. The increased turbidity only persists for a year, so decreased food availability would probably only affect growth and fecundity and an intolerance of low has been recorded. As soon as light levels return to normal, primary production will increase, the species would resume optimal feeding, so recoverability has been assessed to be very high (see additional information below).

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

Abra alba does not require light and therefore the effects of decreased turbidity on light attenuation are not directly relevant. It is possible that decreased turbidity would increase primary production in the water column by phytoplankton and by microphytobenthos. The resultant increase in food availability may enhance growth and reproduction in Abra alba, but only if food was previously limiting.

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

An increase in wave exposure of two categories for one year would place the majority of the population in areas frequently subject to strong wave action and the species may be affected in several ways. Strong wave action may cause damage or withdrawal of the siphons, resulting in loss of feeding opportunities and compromised growth. Furthermore, individuals may be dislodged by scouring from sand and gravel mobilised by increased wave action. During winter gales along the North Wales coast, large numbers of Abra alba were cast ashore and over winter survival rate was as low as 7% in the more exposed locations (Rees et al., 1977). Olivier et al. (1996) reported that the post-larvae and juveniles of Abra alba were most abundant in the near-bottom water stratum at flood tides. Therefore, increased wave action could result in enhanced resuspension and dispersal of early life stages. Overall, some mortality would be likely to occur and intolerance is recorded as intermediate. Recoverability has been assessed to be very high (see additional information below).

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

Olivier et al. (1996) reported that the post-larvae and juveniles of Abra alba were most abundant in the near-bottom water stratum at flood tide velocities. Therefore, a reduction in wave exposure may lead to a decrease in dispersal of Abra alba. Changes in wave exposure are likely to have marked effects on the sediment dynamics of the shore. Decreased exposure will probably lead to increased siltation and reduced grain size (muddy sediment). However, as Abra alba favours muddy substrata such a change in the substratum in unlikely to affect the species and, overall, it has been assessed tolerant.

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

No information was found concerning the intolerance of Abra alba to noise. The siphons are likely to detect vibrations and are probably withdrawn as a predator avoidance mechanism, but the species is not expected to be sensitive at the level of the benchmark.

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

Abra alba does not have visual acuity and an intolerance assessment for this factor was not considered to be relevant.

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

Despite their robust body form, bivalves are vulnerable to physical abrasion. For example, as a result of dredging activity, mortality and shell damage has been reported in Mya arenaria and Cerastoderma edule (Cotter et al., 1997). Abra alba is a shallow burrower and has a fragile shell (Tebble, 1976) and may be damaged by impact Bergmann & Santbrink (2000) reported between <0.5%and 18% mortality of Abra alba due to trawling in the southern North Sea, depending on the type of trawl (12 m or 6 m beam trawl or otter trawl). They included Abra alba amongst their list of bivalve species most vulnerable to trawling. However, they noted that many bivalve species were able to maintain a population in the face of fishing effort, depending on their life history characteristics. Therefore, intolerance has been assessed to be intermediate. Recoverability has been assessed to be very 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

Abra alba probably burrows into the sediment within a few minutes when displaced to the surface of muddy substrata. The species is therefore probably relatively tolerant of displacement. However, exposure at the sediment surface would increase the risk of predation, for example by the starfish Asterias rubens and flatfish predators (Allen, 1983; Rainer, 1985; Basimi & Grove, 1985), so intolerance has been assessed to be intermediate. Recoverability for the majority of individuals has been assessed to be immediate.

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

Abra alba demonstrated alterations of its behaviour in response to exposure to marine sediments contaminated with pesticides (6000 ppm parathion, 200 ppm methyl parathion, 200 ppm malathion). No burrowing occurred in the most contaminated sediment, whilst burrowing was impaired in the moderately contaminated sediment with a median effective burrowing time (ET₍₅₀₎) of 9.0 (±3.0 - 28) minutes in comparison to a control time of 4.5 (±2.8 - 7.2) minutes (Møhlenberg & Kiørboe, 1983). Impaired burrowing would result in the species being exposed to predatory starfish and fish. However, no other information was found concerning the effects of synthetic chemicals specifically on Abra alba. However, inference can be drawn from related species. Beaumont et al. (1989) concluded that bivalves are particularly sensitive to tri-butyl tin (TBT), the toxic component of many antifouling paints. For example, when exposed to 1-3 µg TBT/l, Cerastoderma edule and Scobicularia plana suffered 100% mortality after 2 weeks and 10 weeks respectively. There is also evidence that TBT causes recruitment failure in bivalves, either due to reproductive failure or larval mortality (Bryan & Gibbs, 1991). Stirling (1975) investigated the effects of phenol, a non-persistent, semi-synthetic organic pollutant, on Tellina tenuis. Exposure to phenol produced a measurable effect on burrowing at all concentrations tested, i.e. 50 mg/l and stronger. Sub-lethal effects of exposure to phenol included delayed burrowing and valve adduction to exclude the pollutant from the mantle cavity. After exposure to 100 mg/l for 24 hours, the majority of animals were extended from their shells and unresponsive to tactile stimulation. Following replacement of the phenol solution with clean seawater, good recovery was exhibited after 2 days for animals exposed to 50 mg/l and some recovery occurred after 4 days for animals exposed to 100 mg/l. In light of the intolerance of other bivalve species, intolerance of Abra alba to synthetic chemicals has been assessed to be high. Recoverability has been assessed to be high (see additional information below) but would be partially dependent on the persistence time of synthetic contaminants in the sediments.

Heavy metal contamination

Evidence

Abra alba can live in polluted sediments (Dauvin, pers. comm.), for example, near Calais where high densities of Abra alba were found in sediment containing 8mg/g iron and 4 mg/g titanium (Dewarumez et al, 1976). The capacity of bivalves to accumulate heavy metals in their tissues, far in excess of environmental levels, is well known. Reactions to sub-lethal levels of heavy metals include siphon retraction, valve closure, inhibition of byssal thread production, disruption of burrowing behaviour, inhibition of respiration, inhibition of filtration rate, inhibition of protein synthesis and suppressed growth (see review by Aberkali & Trueman, 1985). Bryan (1984) states that Hg is the most toxic metal to bivalve molluscs while Cu, Cd and Zn seem to be most problematic in the field. In bivalve molluscs Hg was reported to have the highest toxicity, mortalities occurring above 0.1-1 g/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). Owing to evidence in the literature of sub-lethal effects and mortality of bivalves, intolerance of Abra alba to heavy metal contamination has been assessed to be intermediate. Recoverability is recorded as high (see additional information below) but would be partially dependent on the persistence time of heavy metals in the sediments.

Hydrocarbon contamination

Evidence

Suchanek (1993) reviewed the effects of oil on bivalves. Sublethal concentrations may produce substantially reduced feeding rates and/or food detection ability, probably due to ciliary inhibition. Respiration rates may increase at low concentrations and decrease at high concentrations. Generally, contact with oil causes an increase in energy expenditure and a decrease in feeding rate, resulting in less energy available for growth and reproduction. However, the Abra alba population affected by the 1978 Amoco Cadiz benefited from the nutrient enrichment caused by the oil pollution (see nutrient enrichment, below). The biomass of the fine-sand community remained low in 1979, a year after the spill, owing to the decimation of the Ampelisca amphipod population, but the biomass then doubled as a result of an increase in Abra alba abundance in 1980 and Abra alba remained a dominant species over the 20 year duration over which recovery of the community was monitored (Dauvin, 1998). Intolerance has been assessed to be low as the Abra alba population was apparently resilient to the presence of hydrocarbons in the subtidal sediments just two weeks after the wreck. The fact that Abra alba occurs subtidally may mitigate the effects of oil pollution on the species, as it avoids a direct oiling. Recoverability has been assessed to be very high as the species is able to adapt its demographic strategy in order to benefit from the resulting nutrient enrichment (Dauvin & Gentil, 1989).

Radionuclide contamination

Evidence

No information was found concerning the effects of radionuclide contamination on Abra alba.

Changes in nutrient levels

Evidence

In a sewage dumping region of the North Sea, a great increase in the abundance of Abra alba occurred in much of the dumping area because of the ecological adaptations of the species enabled it to exploit the greatly increased supply of nutrients (Caspers, 1981). For example, the Amoco Cadiz oil spill in March 1978 caused vast disturbance to the fine-sand communities of the Bay of Morlaix, France (Dauvin, 1982). Drastic qualitative and quantitative changes in species abundance, diversity and biomass were recorded after the spill. However, the Abra alba population persisted in the disturbed environment under eutrophic conditions and as an 'opportunistic species' (Hily & Le Bris, 1984), it rapidly adapted its reproductive strategy by increasing its reproductive output to three spawnings per year. Increased growth and abundance was attributable to increased food availability and vacant ecological niches (Dauvin & Gentil, 1989). As Abra alba was able to adapt its demographic strategy in order to benefit from the nutrient enrichment, it has been assessed to be tolerant*.

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

Abra alba is typically found in full salinity conditions. No information was found concerning the effects of hypersaline conditions on Abra alba.

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

Abra alba is typically found in full salinity conditions and is therefore likely to be intolerant of reductions in salinity in some way. The benchmark decrease in salinity would place the population in areas of variable salinity for one year or reduced salinity for one week. The change would be likely to cause inhibition of growth and reproduction and exposure to low salinity may result in some mortality. Intolerance has been assessed to be intermediate. Recoverability has been assessed to be very high as a proportion of the adult population are likely to remain.

Changes in oxygenation

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

Evidence

Abra alba is typically found in organically enriched sediments where it may be present in high densities (Dauvin & Gentil, 1989). Such areas can be prone to periodic oxygen deficiency and individual growth and survival is dependent upon the maintenance of a continuous balance between high energy input (food availability) and high metabolic costs which result from periodic anaerobic metabolism and regulation of oxygen uptake (Hylland et al., 1996). Experimental examination of the interactions between eutrophication and oxygen deficiency (2.4 - 3.5 mg O₂/ l over a 93 day experimental period) revealed that Abra alba became inefficient in its use of the available organic matter under prolonged conditions of hypoxia, as evidenced by a decreased growth rate (Hylland et al., 1996). However, the benchmark assesses species' intolerance to hypoxia for one week and as Abra alba is able to shift from aerobic to anaerobic respiration, a short period of hypoxia is unlikely to have a significant effect upon the species and Abra alba has been assessed to be not sensitive at the benchmark level. However, prolonged exposure to oxygen concentrations below 3 mg O₂/ l may severely decrease growth and survival (Hylland et al., 1996) and intolerance would be expected to be higher.

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

More than 20 viruses have been described for marine bivalves (Sinderman, 1990). Bacterial diseases are more significant in the larval stages and protozoans are the most common cause of epizootic outbreaks that may result in mass mortalities of bivalve populations. Parasitic worms, trematodes, cestodes and nematodes can reduce growth and fecundity within bivalves and may in some instances cause death (Dame, 1996). However, no information specifically concerning the effects of microbial pathogens and parasites on the viability of Abra alba was found.

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 evidence was found concerning the effect of non-native species on the viability of Abra alba.

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

Abra alba is not a species targeted for extraction.

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

Dredging for scallops and use of other mobile fishing gear may cause abrasion and displacement of Abra alba. The delicate shell are vulnerable to physical damage (e.g. by otter boards; Rumohr & Krost, 1991), but the small size of Abra alba relative to meshes of commercial trawls may ensure survival of at least a moderate proportion of disturbed individuals which pass through (Rees & Dare, 1993). Intolerance has been assessed to be intermediate and recoverability very high (see additional information below).