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

An increase in temperature may affect spawning and recruitment levels. Kennish & Lutz (1995; cited in Cargnelli et al., 1999a) attributed low recruitment to adverse environmental factors such as high temperatures. In the North Sea, Arctica islandica are restricted to north of 53° 30´ N and have never been reported south of this latitude (Witbaard & Bergman, 2003). This southern limit coincides with a 30 m depth contour, which borders the southern limit of the summer stratified water mass of the Oyster Ground, where bottom water temperatures never exceed 16°C. Larvae here can successfully develop but conditions for development and survival deteriorate along the southern margins of the North Sea (Witbaard & Bergman, 2003). Similarly, the inshore limit of Arctica islandica in the eastern USA was reported to be the 16°C bottom isotherm (Cargnelli et al., 1999a). In the mid-Atlantic Bight (from Long Island Sound to Chesapeake Bay mouth), Harding et al. (2008) reported that successful recruitment was correlated with above average bottom water temperatures between January and March.  Recruitment was good in years in which the number of months with water temperatures averaging 6°C to 10°C exceeds the number of months with water temperatures less than 6°C by at least two months (Harding et al., 2008).  

Laboratory studies have shown that larvae and juveniles can survive temperatures as high as 20°C, although larvae tend to grow optimally between 13°C - 15°C (Cargnelli et al., 1999a). Field studies off Massachusetts observed the highest concentrations of larvae between August and September at temperatures of 14 -18°C (Cargnelli et al., 1999a). Merrill et al. (1969) reported a lethal temperature of 13-16°C and that adults died in a few days at 21°C in the laboratory. It was stated that it was difficult to keep adult specimens alive long enough to transport them to market in the summer months in the USA, which suggested a low tolerance of high temperatures. It was also suggested that an intolerance of high temperatures might explain their absence from shallow waters in the southern extent of its range (Merrill et al., 1969; Cargnelli et al., 1999a).  

Hiebenthal et al. (2012) exposed Arctica islandica collected from the Kiel Bight to temperatures of 4, 10 and 16°C a salinities of 15, 20 and 25.  They noted that shell growth was highest at 4°C and salinity 35, but still high at 10 and 16°C at salinity 25. The condition index of Arctica islandica was not affected by salinity but decreased with increasing temperature, while mortality was not correlated with temperature but decreased with increasing salinity. However, at full salinity (35) shell growth increased from 4 to 10°C but reduced on increase to 16°C (Hiebenthal et al., 2012).   Begum et al. (2010) also noted that low salinity and low temperature were particularly stressful conditions for Arctica islandica, based on population in Kiel Bay, Kattegat, and the White Sea.

Basova et al. (2012) suggested that the long lifespan of Arctica islandica in the North Atlantic was explained partly by low metabolic rate and high ability to repair damage (metabolic).  However, shorter lifespans were associated with raised metabolic rate due to higher temperatures and low and fluctuating salinity so that Baltic populations exhibited the lowest life spans of the populations studied.

Sensitivity assessment. Overall the larvae and spat (juveniles) appear to be able to tolerate up to 20°C while 16°C is the upper limit of tolerance for adults, and the 16°C isotherm is considered to limit their distribution. A short-term increase in temperature, e.g. 5°C for a month, may not cause mortality and Arctica islandica can burrow into the sediment to escape short-term temperature increases at the sediment surface. However, a prolonged increase e.g. 2°C for a year is likely to result in mortality, especially in the summer months and where the population is at its southern limit (e.g. Belfast Lough; Ridgway et al., 2012 or the southern North Sea; Witbaard & Mann, 2003) or in relatively shallow inshore waters (Witbaard & Mann, 2003).  Therefore, a resistance ‘Medium’ is suggested at the benchmark level.   Hence, resilience is probably ‘Medium’ and sensitivity is assessed as ‘Medium’.

Temperature decrease (local)

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

Evidence

Arctica islandica is a temperate, boreal cold water species. Cargnelli et al. (1999a) reported that juveniles could grow at temperatures as low as 1°C, while other estimates suggested an optimal temperature range for adults of 6 -16°C.  Begum et al. (2010) suggested that low temperature is particularly stressful when combined with low salinity (see salinity below) based on the unusual growth pattern observed in the White Sea.   It is likely to be tolerant of lower temperatures than it experiences around the British Isles since it also occurs as far north as the Faeroes, Iceland and the White Sea.  However, Thorarinsdottir et al. (2009) noted that Arctica islandica burrowed to a depth of 14 cm in the winter months in Iceland. Strahl et al. (2011a) noted that Arctica islandica burrowing behaviour varied with season. In February 2004 they were found 4-12 cm deep in the sediment but only 0-10 cm in June 2003. Icelandic Arctica islandica burrowed more frequently and deeper in winter than German Bight specimens in situ, while both populations remained burrowed for one to six days. They noted that in June, when the phytoplankton concentration in Eyjafjordur, North Iceland are 30 times higher than in February, 50% of bivalves are in contact with the sediment surface.  Strahl et al. (2011a) suggested that burrowing activity was in response to food availability and temperature. Crisp (1964) noted a few dead specimens but no abnormal mortality in the North Sea after the severe winter of 1962/63. Therefore, a resistance of ‘High’ is suggested.  Hence, resilience is assessed as ‘High’ and this species is probably ‘Not sensitive’ at the benchmark level.  

Salinity increase (local)

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

Evidence

An increase in salinity at the benchmark level would result in a salinity of >40 psu, and as hypersaline water is likely to sink to the seabed, shallow water populations of Arctica islandica species may be affected by hypersaline effluents. Ruso et al. (2007) reported that changes in the community structure of soft sediment communities due to desalinization plant effluent in Alicante, Spain. In particular, in close vicinity to the effluent, where the salinity reached 39 psu, the community of polychaetes, crustaceans and molluscs was lost and replaced by one dominated by nematodes. Roberts et al. (2010b) suggested that hypersaline effluent dispersed quickly (within 10s of metres of the outfall) but was more of a concern at the seabed and in areas of low energy where widespread alternations in the community of soft sediments were observed. In several studies, echinoderms and ascidians were amongst the most sensitive groups examined (Roberts et al., 2010b).

Sensitivity assessment. An increase in salinity from full to >40 psu is probably detrimental to the Arctica islandica over a period of a year (the benchmark). However, no direct no direct evidence of the effects of hypersaline conditions or effluent on the species was found.  Therefore, ‘No evidence’ was recorded.

Salinity decrease (local)

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

Evidence

Larval Arctica islandica were collected at mean salinities of 32.4 ppt in the USA, while juveniles were grown successfully at salinities ranging from 32 - 34 ppt in the laboratory. Adults are usually found at full salinities but were kept in the laboratory at 22 ppt for several weeks (Oeschager & Storey, 1993; Cargnelli et al., 1999a). Zettler et al. (2001) reported that Arctica islandica inhabited an area in Mecklenburg Bight where the bottom water salinity ranged from 12.5 to 22 psu but also that the population was dominated by small adults with a maximum recorded age of 70 years, and a slower growth rate than in the Kattegat. Zettler et al. (2001) reported a large population of Arctica islandica in the Mecklenburg Bight with a maximum density of 571/m² and an area of 5200 km². 

Hiebenthal et al. (2012) exposed Arctica islandica collected from the Kiel Bight to temperatures of 4, 10 and 16°C at salinities of 15, 20 and 25.  They noted that shell growth was highest at 4°C and salinity 35, but still high at 10 and 16°C at salinity 25. The condition index of Arctica islandica was not affected by salinity but decreased with increasing temperature, while mortality was not correlated with temperature but decreased with increasing salinity. However, at full salinity (35) shell growth increased from 4 to 10°C but reduced on increase to 16°C (Hiebenthal et al., 2012).  

Begum et al. (2010) noted that low salinity and low temperature were particularly stressful conditions for Arctica islandica, based on population in Kiel Bay, Kattegat, and the White Sea. They also noted a reduction in overall lifespan with decreasing salinity. For example, the maximum age they recorded in the German Bight was 125 years but only 58 years in the Kattegat. But the maximum age in low salinities sites of the White Sea and Kiel Bay was less than 50 years (Begum et al., 2010).

Sensitivity assessment.  Arctica islandica is generally found in full salinity but can occur in lower salinity, although its growth rate and life expectancy are reduced.  Hiebenthal et al. (2012) noted that mortality increased with decreasing salinity in the laboratory and suggested that Baltic populations of Arctica islandica may experience severe stress under a climate change scenario of increased temperature and desalination in the Baltic region. However, Zettler et al. (2001) reported a large population. Therefore, a change in salinity from ’Full’ to ‘Reduced’ (18-30) is likely to result in additional stress to the population but may not result directly in an increase in mortality of the population in UK waters. Therefore, a resistance of ‘High’ is suggested to reflect a decrease in lifespan.  Hence, resilience is ‘High’ and sensitivity is recorded as ‘Not sensitive’ at the benchmark level. However, populations in reduced salinity exposed to low salinities, especially in combination with low temperatures (Begum et al., 2010; Hiebenthal et al., 2012) may suffer mortality due to a further reduction in salinity.

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

Arctica islandica is recorded from sandy muds to coarse sands and hence a range of water flow regimes. Recruitment and growth are influenced by the local hydrography (e.g. thermal inversions and gyres in the North Sea, Witbaard & Mann, 2003) and regional currents (e.g the North Atlantic Oscillation, and Atlantic Multidecadal Oscillation (AMO) (see Schone, 2013) that entrain larvae or alter food supply as plankton or detritus. However, little information on the water flow preferences was found.

In shallow waters, Arctica islandica populations may be vulnerable to severe storm events. For example, small numbers were washed ashore in Red Wharf Bay after a severe storm (Rees et al., 1977; Rees & Dare, 1993).  In Iceland, a heavy storm (winds >20 m/s) in April 2006 caused large waves and currents in Lonafjordur, where a dense population of Arctica islandica occurred between 9 and 30 m. As a result, large numbers of Arctica islandica were washed to shallow waters (7-9m) on hard substrata where they rapidly fell prey to wolffish, starfish and other invertebrate predators. Nevertheless, extensive deep water populations are unlikely to be vulnerable to storms (Rees & dare, 1993; OSPAR, 2009c)

However, a change in water flow of 0.1-0.2 m/s is probably of limited effect given the species preference for a range of sediment types. Therefore, a resistance of 'High' is suggested, with a resilience of 'High' so that the biotope is probably 'Not sensitive' at the benchmark level.  

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

Changes in emergence are Not Relevant to this species, which are restricted to fully subtidal conditions. The pressure benchmark is relevant only to species found in the littoral and shallow sublittoral fringe.

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

Arctica islandica is recorded from open coasts and exposed, extremely exposed, moderately exposed, and very exposed wave conditions but in sandy muds to coarse sands, sheltered from wave action, primarily due to depth. In shallow waters, Arctica islandica populations may be vulnerable to severe storm events. For example, small numbers were washed ashore in Red Wharf Bay after a severe storm (Rees et al., 1977; Rees & Dare, 1993).  In Iceland, a heavy storm (winds >20 m/s) in April 2006 caused large waves and currents in Lonafjordur, where a dense population of Arctica islandica occurred between 9 and 30 m. As a result, large numbers of Arctica islandica were washed to shallow waters (7-9m) on hard substrata where they rapidly fell prey to wolffish, starfish and other invertebrate predators. Nevertheless, extensive deep water populations are unlikely to be vulnerable to storms (Rees & Dare, 1993; OSPAR, 2009c).  However, a 3-5% change in significant wave height is unlikely to any significant effect on the population even in shallow waters. Therefore, a resistance of 'High' is suggested, with a resilience of 'High' so that the species is probably 'Not sensitive' at the benchmark level.  

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

This pressure is Not assessed.

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

This pressure is Not assessed.

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

This pressure is Not assessed.

Radionuclide contamination

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

Evidence

No evidence was found

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

This pressure is Not assessed.

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

Arctica islandica was reported to be resistant to severe hypoxia (Theede et al., 1969, Diaz & Rosenberg, 1995). Kiel Bay (Baltic Sea) has seen significant declining trends in deep water oxygen concentration since the 1950's. In 1981 salinities of 20 - 26 psu and temperatures of 10-14°C were recorded. Anoxia and hydrogen sulphide were widespread below the halocline at a depth of >20 m (Rosenberg & Loo, 1988). The anoxic event lasted several weeks and during that time, 30,000 t of macrofauna died over 750 km². However, Arctica islandica was amongst the few surviving species. Another area that has recorded severe hypoxic events was in the Kattegat (Sweden). The worst year recorded was 1988, when approximately 30,000 km² of the bottom water was hypoxic. Oxygen concentrations recorded were 3.1 ml/l (4.34 mg/l) in June, 1.0 ml/l in August, 0.9 ml/l (1.26 mg/l) in September, and Arctica islandica was amongst the surviving species. However, in an anoxic episode off New Jersey (USA), up to 13.3% of the Arctica islandica population died in shallow waters, while in deeper water the population was not affected (Ropes et al., 1979; Cargnelli et al., 1999a). Zettler et al. (2001) also reported that a long period of oxygen depletion in the Mecklenburg Bight (Baltic) resulted in the loss of the Arctica islandica population, although the density had increased in some areas by the early 1990s.

Arctica islandica can respire anaerobically when burrowed below the surface for ca seven days (but exceptionally for up to 25 days) (Taylor, 1976; Strahl et al., 2011a) and when exposed to anoxia with or without hydrogen sulphide (Oeschger, 1990; Oeschger & Storey, 1993; Cargnelli et al., 1999a). Oeschger & Storey (1993) also noted that Arctica islandica can still burrow under anoxic conditions and in the presence of hydrogen sulphide. However, Strahl et al. (2011b) noted that anaerobiosis was switched on only at anoxia and that respiration remained aerobic down to oxygen levels of 2 kPa (ca 0.88 mg/l).  

Ropes et al. (1979) reported a critical oxygen tension for Arctica islandica of 5-7 kPa (2.2-3.1 mg/l). Theede et al. (1969) and Theede (1973) investigated the tolerance of Arctica islandica to hypoxia and hydrogen sulphide. The LT50 (50% mortality) occurred in Arctica islandica around 55 days into the experiment at an oxygen concentration of <0.15 ml/l (0.21 mg/l) (at 10°C and pH 8.2 - 8.45) and the LT50 occurred after around 33-42 days with the addition of hydrogen sulphide (Theede et al., 1969; Theede, 1973). Environmental factors such as temperature can affect a species resistance to hypoxic conditions. With decreasing temperature below 10°C, the cellular resistance of Arctica islandica increases more than in species such as Mytilus edulis (Theede, 1973).

While the adults are reported to be highly resistant of low oxygen concentration, juveniles and larvae may be more sensitive (Zettler et al., 2001).  Zettler et al. (2001) suggested that oxygen depletion in the deeper parts of the Lubeck Bay, Mecklenburg Bight (due to the pycnocline) prevented the successful recruitment of juveniles and the high biomass in that area was restricted to low numbers of adults. Successful recruitment was restricted to favourable years.

Sensitivity assessment. Arctica islandica is regarded as highly tolerant of severe hypoxia, and anoxia even in the presence of hydrogen sulphide. Populations of the species have been reported to survive hypoxic and anoxic events lasting several weeks but succumb to events lasting for long periods (e.g. in the Baltic).  However, adults are likely to survive exposure to <2mg/l oxygen for a week.  Juveniles may be more sensitive but recruitment is only likely to be interrupted if hypoxia occurred at the same time as peak recruitment, and the long-term population dynamics is only likely to be affected if hypoxia occurs on a regular basis; as in the Mecklenburg Bight example (Zettler et al., 2001).  Therefore, a resistance of ‘High’ is suggested.  Hence, resilience is recorded as ‘High’ and the species is considered ‘Not sensitive’ at the benchmark level.

Nutrient enrichment

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

Evidence

No specific information regarding the effects of nutrients on Arctica islandica was found. Increased nutrients are likely to enhance algal and phytoplankton growth, increase organic material deposits, enhance bacterial growth and, hence, food supply. However, increased levels of nutrients may also result in eutrophication, algal blooms and a reduction in oxygen concentrations.  Off the west coast of Kattegat (Sweden), Rosenberg & Loo (1988) reported mass mortalities of the bivalves Mya arenaria and Cerastoderma edule following a eutrophication event, although no direct causal link was established. However, the abundance of Arctica islandica remained very high despite falls in other bivalve populations. Also, low synchronicity in growth records from the German Bight was attributed to tidal and wind movements, changes in salinity due to riverine input, temperature shifts and turbidity (Schone, 2013).

Borja et al. (2000) suggested that Arctica islandica was “tolerant of excess organic matter enrichment” (AMBI, Group III) while Gittenberger & van Loon (2011) suggested it was “indifferent to enrichment, always present in low densities with non-significant variations with time” (AMBI, Group II).

Sensitivity assessment. Arctica islandica is a suspension feeder and/or surface deposit feeder, found in a range of sediment types and resistant to severe hypoxia. Therefore, it may well be resistant to the adverse effects of eutrophication. In addition, as the benchmark levels comply with WFD criteria for good status, resistance is ‘High’, resilience is ‘High’ and the species is 'Not sensitive' at the benchmark level.

Organic enrichment

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

Evidence

No specific information regarding the effects organic enrichment on Arctica islandica was found. Organic enrichment will increase organic material deposits, enhance bacterial growth and, hence, food supply but also result in eutrophication and a reduction in oxygen concentrations.  Off the west coast of Kattegat (Sweden), Rosenberg & Loo (1988) reported mass mortalities of the bivalves Mya arenaria and Cerastoderma edule following a eutrophication event, although no direct causal link was established. However, the abundance of Arctica islandica remained very high despite falls in other bivalve populations. Also, low synchronicity in growth records from the German Bight was attributed to tidal and wind movements, changes in salinity due to riverine input, temperature shifts and turbidity (Schone, 2013).

Borja et al. (2000) suggested that Arctica islandica was “tolerant of excess organic matter enrichment” (AMBI, Group III) while Gittenberger & van Loon (2011) suggested it was “indifferent to enrichment, always present in low densities with non-significant variations with time” (AMBI, Group II).

Sensitivity assessment. Arctica islandica is a suspension feeder and/or surface deposit feeder, found in a range of sediment types and resistant to severe hypoxia. Therefore, it may well be resistant to the adverse effects of organic enrichment and probably has a resistance of ‘High’.  Hence, resilience is ‘High’ and the species is 'Not sensitive' at the benchmark level, albeit at low confidence.

Physical loss (to land or freshwater habitat)

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

Evidence

All marine habitats and benthic species are considered to have ‘No resistance’ to this pressure and to be unable to recover from a permanent loss of habitat.  Sensitivity within the direct spatial footprint of this pressure is, therefore ‘High’.  Although no specific evidence is described confidence in the resistance 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

A change to natural or artificial hard substratum would remove the sedimentary habitat required by the species. Based on the loss of suitable habitat,  resistance to this pressure is assessed as ‘None’. Resilience is assessed as ‘Very low’ as the pressure benchmark refers to a permanent change. Biotope sensitivity is, therefore ‘High’.

Physical change (to another sediment type)

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

Evidence

Arctica islandica is recorded from sandy muds, muddy sands, and fine to coarse sands (Rees & Dare, 1993; Cargnelli et al., 1999a).  A change in one Folk class (based on Long, 2006) to fine muds or to mixed sediments dominated by gravel would place the species outside its preferred habitat. Muds and gravels may impair burrowing, and muds may impair filter feeding. As a result, the population is likely to suffer mortality. Therefore, a precautionary resistance of 'Low' is suggested but with low confidence.  As the pressure represents a permanent change, resilience is assessed as 'Very low' and sensitivity recorded as 'High'.

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

The removal of substratum to a depth of 30 cm will remove the substratum occupied by the species together with the individuals present and any other species in the assemblage, in the impact footprint.  Resistance is, therefore, assessed as ‘None’. Where suitable substratum remains, resilience is probably 'Very low'.  Sensitivity is, therefore, assessed as 'High'.  Although no specific evidence is described confidence in this assessment is ‘High’, due to the incontrovertible nature of this pressure.

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

Arctica islandica lives at the surface of the sediment while feeding but burrows to depths of 14 cm periodically and,  in Iceland, over the winter months  (Taylor, 1976; Thorarinsdottir et al. 2009; Strahl et al., 2011a). It is harvested by hydraulic dredging but is also damaged by bottom trawl fisheries. Therefore, it is exposed to both surface abrasion and penetration of the sediment.

Arctica islandica is fished commercially in the U.S.A. The fishery began in 1943 and has operated continuously since 1967 (Hennen, 2015). Simulations suggest that the fishery operation requires very low fishing mortality rates as rates greater than 0.03 tended to result in fishery closures (Hennen, 2015). He also noted that the length of the commercial fishery was less than the generation time for Arctica islandica (ca 83 years). Jacobson & Weinberg (2006) noted that total ocean quahog biomass had decreased since the 1970s but that fishing mortality had remained stable since 1985.  The target fishing mortality of 2.8% was not exceeded and hence the stock was not overfished.

It was fished commercially in Iceland between 1995 and 2008 (Ragnarsson et al., 2015). Thorarinsdottir & Jacobson (2005) and Thorarinsdottir et al. (2010) noted that Arctica islandica was vulnerable to overfishing due to its long lifespan, slow growth, uncertain recruitment, low productivity, and poor estimates of stock biomass and capture efficiency. Ragnarsson et al. (2015) examined the effects of hydraulic dredging on the benthos of Þistilfjörður Bay, NE Iceland over a five year period.  The benthic community was significantly affected by dredging with up to 50% reductions or increases in abundance immediately after dredging. Most taxa recovered their prior abundances within a year.  However, Arctica islandica experienced a 93% decrease in abundance after dredging and recovered within the dredge tracks to only 7% to 26% of controls within five years.  Ragnarsson et al. (2015) noted that in north-west Iceland it took ca 50-60 years for Arctica islandica to attain a fishable size. They speculated that 50-100 years was required to for full recovery in abundance and population size distributions. Gilkinson et al. (2005, 2015) examined recovery of commercial bivalve populations, including Arctica islandica after experimental dredging over a ten-year period (1998-2008) on the Scotia Shelf, Canada. Dredging in 1998 resulted in a 67% reduction in the commercial bivalve biomass (Gilkinson et al., 2005). Gilkinson et al. (2015) suggested that recovery of the commercial species, including Arctica islandica, would take more than 10 years.

Arctica islandica has a thick, solid and heavy shell but is known to be vulnerable to physical abrasion. Klein & Witbaard (1993) and Witbaard & Klein (1994) examined damaged shells of Arctica islandica is the southern North Sea, an area subject to beam trawling. The damage to this species was related to their body size, and larger specimens were more affected than smaller ones (Klein & Witbaard, 1993; Witbaard & Klein, 1994). The number of damaged shells was highest in the southern North Sea, impacted by the beam trawl fishery, compared to mid-west and northern North Sea areas (Witbaard & Klein, 1994). Only 10% of empty shells collected in the southern North Sea were undamaged, compared to 40% in the mid-west North Sea (Witbaard & Klein, 1994). In addition, 80-90% of shell scars were found on the posterior side due to the animal's position in the upper sediment and the transit of the tickler chains along the seabed. Up to 90% of Arctica islandica caught by a commercial trawler were severely damaged with an estimated mortality rate ranging from 74% - 90% (Fronds, 1991; cited in Witbaard & Klein, 1994). However, shells were also damaged on board as well as during the fishing process. The number of damaged shells and the number caught increased when tickler chains were used. For example, 74% were damaged with the use of tickler chains whereas only 27% were damaged without their use (Fronds, 1991; cited in Witbaard & Klein, 1994).  Witbaard & Klein (1994) also suggested that the age-frequency distribution was skewed by trawling, as juveniles were uncommon and the population was dominated by large adults. OSPAR (2009c) also noted that trends in shell scar frequencies corresponded to fluctuations in the total engine capacity of the Dutch beam trawl fleet. Thorarinsdottir et al. (2008) noted that 30-50% of ocean quahog caught in a hydraulic dredge were damaged, as were 11-29% of those left in the dredge track, and concluded that mortality of clams remaining on the track was due to shell damage or predation.

Declines in Arctica islandica populations were reported in the North Sea (OSPAR, 2009c). Examination of historic surveys with surveys in 1986 noted that Arctica islandica was present in 20-30% of stations in 1986 compared to 45% in 1902-1912 (Rumohr et al., 1998, OSPAR, 2009c) because of its absence from shallow sites (30-50 m) in 1986.  A significant decline in abundance between 1972-80 and 1990-1994 was also noted in the southern North Sea (Witbaard, 1997; Witbaard & Bergman, 2003).  Improved survey techniques in 2000 revealed higher abundances of Arctica islandica in the North Sea than previously recorded but its occurrence in the North Sea had still decreased between the 1900s and 1990s (Witbaard & Bergman, 2003; OSPAR, 2009c).  Bergman & van Santbrink (2000) reported direct mortality from a single pass of a beam trawl in Arctica islandica of ca 11% but only <0.5% for an otter trawl.  Witbaard & Bergman (2003) suggested that this level of mortality (11%) would have been enough to decimate the population of adults in the Oyster Ground (southern North Sea) within 25 years and result in the observed densities of only 10 individuals /100m², which were low compared to the Fladen Grounds.

Sensitivity assessment. The above evidence suggests that Arctica islandica can be damaged by abrasion due to mobile fishing gear, e.g. beam trawls and otter trawls. Direct mortality can be attributed to mobile gear and to targeted fishing using hydraulic dredges.  In addition, the decline in the population of Arctica islandica in the southern North Sea may correspond with beam trawling (OSPAR, 2009c).  Therefore, a resistance of ‘Low’ is suggested. Resilience is probably ‘Very low’ so that a sensitivity of ‘High’ is suggested.

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

The relevant evidence on the effects of fishing activities is presented above under abrasion. Direct mortality can be attributed to mobile gear and to targeted fishing using hydraulic dredges.  In addition, the decline in the population of Arctica islandica in the southern North Sea may correspond with beam trawling (OSPAR, 2009c).  Therefore, a resistance of ‘Low’ is suggested. Resilience is probably ‘Very low’ so that a sensitivity of ‘High’ is suggested.

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

Morton (2011) examined the anatomy of Arctica islandica and concluded that it was an unusual, specialist, deposit feeder with typical large intestine but unusually large ctenidia. Its siphons sit at the sediment-water interface so that it probably feeds off the rich surface film and deposited organic material brought in on the inhalent current since it has an extremely sedentary lifestyle (Morton, 2011). It can also feed on phytoplankton if available. Winter (1969) recorded that Arctica islandica (3.3-8.3 mm in length) could filter 0.7-7 litres/hour and 30-280 mg dry weight of algae in 24h, in the laboratory.

Arctica islandica occurs in silty sediments in sheltered to wave exposed conditions, where the surface of the sediment is probably regularly mobilized, and where accretion rates are moderate to high. Therefore, increase in turbidity (suspended sediments) may not adversely affect the species, especially as it can avoid sudden changes by burrowing for several days.  Although, no specific evidence was found a resistance of 'High' is suggested but with Low confidence. Therefore, resilience is assessed as 'High' and Arctica islandica is probably 'Not sensitive' at the benchmark level.

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

Powilleit et al. (2006) examined the effects of experimental spoil disposal in Mecklenburg Bay.  They deposited ‘till’ and ‘sand/till’ mixtures up to ca 1.5 m deep on existing sediment, although the depth of deposit varies over time. The resident Arctica islandica population was ‘almost’ unaffected, and its population structure was similar two years later with no apparent change in growth rates. Powilliet et al. (2009) exposed Arctica islandica to smothering in the laboratory. They were exposed to ‘till’ (median grain size 8 µm) and ‘sand/till’ mixtures (median grain size 220 µm) to a depth of 10-20 cm and 14-40 cm. Arctica islandica was able to burrow to the surface of 32-41 cm of either sediment and regained contact with the surface. Arctica islandica reached burrowing velocities of 0.37-3.89 cm/day. Powilleit et al. (2009) stated that a considerable number of the test organism were able to burrow to the sediment surface. No clear difference between the depths of sediment tested was noted. However, they also note that the texture of the ‘till’ posed burrowing difficulties for Arctica islandica, and reported 16% and 0% reached the sediment surface in the ‘till’ and ‘sand/till’ mixtures respectively after 28 days. In comparison, they noted that in field experiments (Powilliet et al., 2006) after initial deposition, 78% and 26% reached the surface under the ‘till’ and ‘sand/till’ mixtures respectively. Side scan sonar revealed that 75% of the initial deposit varied in depth from 0 to 40 cm after two months, while another 25% was >40 cm deep.

Sensitivity assessment.  Arctica islandica may take many days to reach the surface of sediment in laboratory experiments, although no mortality was seen.  However, in field conditions, it was able to reach the surface of sediment and no effect on its growth or population structure was evident (Powilliet et al., 2006; 2009).  Therefore, a deposit of 5 cm of fine material is unlikely to have a negative effect and a resistance of ‘High’ is suggested. Hence, resilience is ‘High’ and ‘Not sensitive at the benchmark level’ is recorded.

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

Powilleit et al. (2006) examined the effects of experimental spoil disposal in Mecklenburg Bay.  They deposited ‘till’ and ‘sand/till’ mixtures up to ca 1.5 m deep on existing sediment, although the depth of deposit varies over time. The resident Arctica islandica population was ‘almost’ unaffected, and its population structure was similar two years later with no apparent change in growth rates. Powilliet et al. (2009) exposed Arctica islandica to smothering in the laboratory. They were exposed to ‘till’ (median grain size 8 µm) and ‘sand/till’ mixtures (median grain size 220 µm) to a depth of 10-20 cm and 14-40 cm. Arctica islandica was able to burrow to the surface of 32-41 cm of either sediment and regained contact with the surface. Arctica islandica, reached burrowing velocities of 0.37-3.89 cm/day. Powilleit et al. (2009) stated that a considerable number of the test organism were able to burrow to the sediment surface. No clear difference between the depths of sediment tested was noted. However, they also note that the texture of the ‘till’ posed burrowing difficulties for Arctica islandica and reported that 16% and 0% reached the sediment surface in the ‘till’ and ‘sand/till’ mixtures respectively after 28 days. In comparison, they noted that in field experiments (Powilliet et al., 2006) after initial deposition, 78% and 26% reached the surface under the ‘till’ and ‘sand/till’ mixtures respectively.  side scan sonar revealed that 75% of the initial deposit varied in depth from 0 to 40 cm after two months, while another 25% was >40 cm deep.

Sensitivity assessment.  Arctica islandica may take many days to reach the surface of sediment in laboratory experiments, although no mortality was seen.  However, in field conditions, it was able to reach the surface of sediment and no effect on its growth or population structure was evident (Powilliet et al., 2006; 2009).  Therefore, a deposit of 30 cm of fine material is unlikely to have a negative effect and a resistance of ‘High’ is suggested. Hence, resilience is ‘High’ and ‘Not sensitive at the benchmark level’ is recorded.

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

Morton (2011) noted that Arctica islandica has a shadow reflex to light, that is, to its interference (e.g. by an approaching predator) and is also very sensitive to vibration. Once retracted it takes a long time for the siphons to be extended again (Morton, 2011).  However, no evidence was found to suggest that underwater sound and vibrations caused by passing vessels, construction etc have any effect. Therefore 'Not relevant' is recorded.

Introduction of light or shading

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

Evidence

Morton (2011) noted that Arctica islandica has a shadow reflex to light, that is, to its interference (e.g. by an approaching predator) and is also very sensitive to vibration. Once retracted it takes a long time for the siphons to be extended again (Morton, 2011).  However, artificial light or shading at the sea surface is unlikely to be detected, especially at the depths that most populations live. Therefore 'Not relevant' is recorded.

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. Adult Arctica islandica are extremely sedentary and unlikely to move across the sediment (Morton, 2011).  However, the dispersal of larvae and, hence recruitment, has been shown to be controlled in part by local and regional hydrography, localised gyres and water stratification (thermoclines and pycnoclines) (Holmes et al. 2003; Zettler et al., 2001; Witbaard & Mann, 2003; Schone, 2013). Barriers that interfere with larval transport could reduce recruitment or entrain larvae depending on local conditions, although no examples were found. But 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 seabed habitats.  NB. Collision by interaction with bottom towed fishing gears and moorings are 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

Morton (2011) noted that Arctica islandica has a shadow reflex to light, that is, to its interference (e.g. by an approaching predator) and is also very sensitive to vibration. Once retracted it takes a long time for the siphons to be extended again (Morton, 2011). However, the shadow reflex is probably very localised so that visual disturbance by passing vessels or humans at the sea surface is probably '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

No evidence was found to suggest that Arctica islandica was subject to translocation or genetic modification.

Introduction or spread of invasive non-indigenous species

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

Evidence

No evidence was found to suggest that Arctica islandica populations were adversely affected by invasive non-indigenous species.

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

The nemertean worm Malacobdella grossa was recorded living commensally in the mantle cavity of Arctica islandica. Records show that Malacobdella grossa was always present in large numbers (Lauckner, 1983). No information on the effects of Malacobdella grossa on Arctica islandica could be found, however, it could lower the viability of the population. Therefore, a resistance of 'High' of low has been recorded. Hence, resilience is 'High' and a sensitivity of 'Not sensitive' is recorded. 

Removal of target species

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

Evidence

Arctica islandica is not targeted by a fishery in the North East Atlantic but it is fished commercially in the U.S.A. The fishery began in 1943 and has operated continuously since 1967 (Hennen, 2015). Simulations suggest that the fishery operation required very low fishing mortality rates as rates greater than 0.03 tended to result in fishery closures (Hennen, 2015). He also noted the length of the commercial fishery, was less than the generation time for Arctica islandica (ca 83 years). Jacobson & Weinberg (2006) noted that total ocean quahog biomass had decreased since the 1970s but that fishing mortality had remained stable since 1985.  The target fishing mortality of 2.8% was not exceeded and hence the stock was not overfished.

It was fished commercially in Iceland between 1995 and 2008 (Ragnarsson et al., 2015). Thorarinsdottir & Jacobson (2005) and Thorarinsdottir et al. (2010) noted that Arctica islandica was vulnerable to overfishing due to its long lifespan, slow growth, uncertain recruitment, low productivity, and poor estimates of stock biomass and capture efficiency. Ragnarsson et al. (2015) examined the effects of hydraulic dredging on the benthos of Þistilfjörður Bay, NE Iceland over a five-year period.  The benthic community was significantly affected by dredging with up to 50% reductions or increases in abundance immediately after dredging. Most taxa recovered their prior abundances within a year.  However, Arctica islandica experienced a 93% decrease in abundance after dredging and recovered within the dredge tracks to only 7% to 26% of controls within five years.  Ragnarsson et al. (2015) concluded that hydraulic dredging could have a drastic effect on Arctica islandica populations. Ragnarsson et al. (2015) noted that in north-west Iceland it took ca 50-60 years for Arctica islandica to attain a fishable size. They speculated that 50-100 years was required to for full recovery in abundance and population size distributions and that fishery closure was required to rebuild stocks. However, they also noted that the small Icelandic fishery probably did not affect the ocean quahog stock size in Icelandic waters. Gilkinson et al. (2005, 2015) examined recovery of commercial bivalve populations, including Arctica islandica after experimental dredging over a ten-year period (1998-2008) on the Scotia Shelf, Canada. Dredging in 1998 resulted in a 67% reduction in commercial bivalve biomass (Gilkinson et al., 2005). Gilkinson et al. (2015) suggested that recovery of the commercial species, including Arctica islandica, would take more than 10 years. Thorarinsdottir et al. (2008) also noted that 30-50% of ocean quahog caught in a hydraulic dredge were damaged, as were 11-29% of those left in the dredge track, and concluded that the mortality of clams remaining on the track was due to shell damage or predation.

Sensitivity assessment. The life history of Arctica islandica, coupled with its longevity means that it is vulnerable to overfishing (Thorarinsdottir & Jacobson, 2005; Thorarinsdottir et al., 2010; Hennen, 2015; Ragnarsson et al., 2015). The evidence suggests that hydraulic dredging can have drastic, if localised, effects on the population and that recovery is prolonged. The U.S.A. fishery may be sustainable due to the low fishing mortality. Therefore, the resistance of Artica to targeted fishing activity is assessed as 'Low'.  Resilience is probably 'Very low' and 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

Arctica islandica has a thick, solid and heavy shell but despite this is known to be vulnerable to physical abrasion. Klein & Witbaard (1993) and Witbaard & Klein (1994) examined damaged shells of Arctica islandica in the southern North Sea, an area subject to beam trawling. The damage to this species was related to their body size, and larger specimens were more affected than smaller ones (Klein & Witbaard, 1993; Witbaard & Klein, 1994). The number of damaged shells was highest in the southern North Sea, impacted by the beam trawl fishery, compared to the mid-west and northern North Sea areas (Witbaard & Klein, 1994). Only 10% of empty shells collected in the southern North Sea were undamaged, compared to 40% in the mid-west North Sea (Witbaard & Klein, 1994). In addition, 80-90% of shell scars were found on the posterior side due to the animal's position in the upper sediment and the transit of the tickler chains along the seabed. Up to 90% of Arctica islandica caught by a commercial trawler were severely damaged with an estimated mortality rate ranging from 74% - 90% (Fronds, 1991; cited in Witbaard & Klein, 1994). However, shells were also damaged on board as well as during the fishing process. The number of damaged shells and the number caught increased when tickler chains were used. For example, 74% were damaged with the use of tickler chains whereas only 27% were damaged without their use (Fronds, 1991; cited in Witbaard & Klein, 1994).  Witbaard & Klein (1994) also suggested that the age-frequency distribution was skewed by trawling, as juveniles were uncommon and the population was dominated by large adults. OSPAR (2009) also noted that trends in shell scar frequencies corresponded to fluctuations in the total engine capacity of the Dutch beam trawl fleet. Thorarinsdottir et al. (2008) also noted that 30-50% of ocean quahog caught in a hydraulic dredge were damaged, as were 11-29% of those left in the dredge track, and concluded that the mortality of clams remaining on the track was due to shell damage or predation.

Declines in Arctica islandica populations were reported in the North Sea (OSPAR, 2009). Examination of historic surveys with surveys in 1986 noted that Arctica islandica was present in 20-30% of stations in 1986 compared to 45% in 1902-1912 (Rumohr et al., 1998, OSPAR, 2009) because of its absence from shallow sites (30-50 m) in 1986.  A significant decline in abundance between 1972-80 and 1990-1994 was also noted in the southern North Sea (Witbaard, 1997; Witbaard & Bergman, 2003).  Improved survey techniques in 2000 revealed higher abundances of Arctica islandica in the North Sea than previously recorded but that its occurrence in the North Sea had decreased between the 1900s and 1990s (Witbaard & Bergman, 2003; OSPAR, 2009).  Bergman & van Santbrink (2000) reported direct mortality from a single pass of a beam trawl in Arctica islandica of ca 11% but only <0.5% for an otter trawl.  Witbaard & Bergman (2003) suggested that this level of mortality (11%) would have been enough to decimate the population of adults in the Oyster Ground (southern North Sea) within 25 years and result in the observed densities of only 10 individuals /100m², which were low compared to the Fladen Grounds.

Sensitivity assessment. The above evidence suggests that Arctica islandica can be damaged and removed by non-targeted fishing, e.g. beam trawls and otter trawls. In addition, the decline in the population of Arctica islandica in the southern North Sea may correspond with beam trawling (OSPAR, 2009).  Therefore, a resistance of ‘Low’ is suggested. Resilience is probably ‘Very low’ so sensitivity is assessed as ‘High’.