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

Removal of the substratum would result in loss of the entire population. Therefore an intolerance of high has been recorded. Recovery is potentially 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

If Pecten maximus were smothered by a 5 cm layer of fine sediment, juveniles and adults could probably lift themselves clear of the new layer since thay are capable of jumping and swimming. Newly re-laid scallops are, however, more vulnerable to predators until recessed (Minchin & Buestel, 1983) although it is likely that the predators of the scallop such as starfish and crabs will be occupied in re-establishing their position themselves. Therefore an intolerance of low has been recorded. See additional information on recoverability below.

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

Growth rates of adult Pecten maximus are adversely affected by increases in suspended sediments concentrations (Bricelj & Shumway, 1991) and excessive particle bombardment may threaten the viability of the feeding apparatus (Gibson, 1956), thereby potentially decreasing ingestion rates. The great scallop has the ability to swim and as such some individuals may be able to escape although this ability is primarily reserved for escape reactions given the high energy expenditure involved. However, the distances covered by swimming or jumping are very limited and newly re-laid scallops are more vulnerable to predators until recessed (Minchin & Buestel, 1983). Therefore an intolerance of low has been recorded and, at the benchmark level, recoverability is likely to be high.

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

Evidence suggests that scallops can compensate for short-term changes in the availability of food by adjusting the clearance rate of food particles (Bricelj & Shumway, 1991). In addition, Pecten maximus feed on a wide variety of food sources including phytoplankton, especially single celled algae, particulate organic matter (POM), bacteria and other micro-organisms (Fegley et al., 1992; Reitan et al., 2002). Consequently a short term decrease in the suspended sediment (see benchmark) is not likely to have an adverse effect on the scallops and a sensitivity of tolerant has been recorded.

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

Scallops are incapable of sustaining prolonged valve closure and are relatively intolerant of aerial exposure. Given their depth range Pecten maximus are unlikely to be subjected to aerial exposure unless, for example, they are taken to the surface during dredging and subsequently discarded (see section on selective extraction). Mikolajunas (1996) looked at the effect of aerial exposure on Pecten maximus and found that at first they respond to respiratory stress with rapid adductions (valve closures) of the shell but, due to the fact that they cannot completely seal their valves, the soft tissue quickly becomes dehydrated and the animal eventually becomes fatigued (Mikolajunas, 1996). Following this the scallop will require a substantial recovery period following reimmersion although many will not recover if exposure has been prolonged. After just one hour of aerial exposure, as set in the benchmark, percentage mortality leveled off at about 60% after three weeks (Mikolajunas, 1996). Stressed scallops that are exhausted following periods of aerial exposure may not have the energy required to escape predation and other dangers. In addition they are less likely to recess and close their valves to avoid attack and the accumulation of waste and other stress related substances may attract predators (Mikolajunas, 1996). Jenkins & Brand (2001) state that exposure to the air for a little as twenty minutes could result in a significant reduction in the ability to swim.
In contrast, Brand & Roberts (1973) found that following short periods of aerial exposure most individuals responded well to reimmersion and showed a gradual and short term recovery on return to seawater. It is also notable that scallops remain alive far in excess of 24 hours in a refrigerator (K. Hiscock, pers. comm.)

Considering the benchmark for desiccation is set at one hour of continual aerial exposure for subtidal species it is likely that at least some of the population will be adversely affectedly and an intolerance of intermediate has been recorded. In light of the work by Brand & Roberts (1973), a recoverability of very high has been recorded.

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

Pecten maximus is not an intertidal species and emergence is not considered relevant.

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

Pecten maximus is not an intertidal species and emergence is not considered relevant.

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

Pecten maximus lives embedded in recesses in the seabed usually with the upper valve flush with the sediment surface. This position can facilitate feeding by bringing the inhalant current near to the seabed therefore increasing the intake of detritus (Mason, 1983). It can also reduce the vulnerability of the scallop to dislodgment through increased water flow rate and wave action.
Growth rates of scallops are generally faster in areas of relatively strong currents and reduced growth rates can occur in areas of low current speeds due to food limitation. However, excessive particle bombardment, commonly associated with areas of high water flow rate, may reduce the effectiveness of the feeding apparatus and reduce ingestion rates (Gibson, 1956).
An increase of two categories in water flow rate (see benchmark) might repeatedly dislodge the scallop. This is unlikely to adversely affect the scallop and is likely that it will eventually re-settle although feeding may be disrupted which will subsequently reduce growth rates. Therefore, the viability of the population may be reduced, although feeding would most likely resume once conditions became suitable again. Therefore, an intolerance of low has been recorded.

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

A reduction in water flow rate as set in the benchmark may reduce the availability of food particles but it is not likely that this reduction would adversely affect the growth and general condition of the scallop. Bricelj & Shumway (1991) suggested that scallops can compensate for short-term changes in the availability of food by adjusting the clearance rate of food particles. However, highly reduced water flow rates are often associated with increased siltation and growth rates of adult Pecten maximus have been found to be adversely affected by increases in suspended sediments concentrations (Bricelj & Shumway, 1991). Therefore, the viability of the population may be reduced however feeding would most likely resume once conditions became suitable again. Therefore an intolerance of low has been recorded.

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

Sexual maturation and spawning are governed by temperature which are obviously imperative for recruitment and contributing to the development of the population. Temperature is considered by many to be the primary trigger in spawning among Pectinidae and there is some evidence to suggest that there may be a critical range (Barber & Blake, 1991). In the Bay of Brest and the Bay of St Brieuc in France, for instance, the critical temperature range for spawning is thought to be between 15.5-16 °C (Paulet et al., 1988). Scallop spat reared at 17°C in the laboratory had the highest condition index, that is, the ratio of dry meat weight to dry shell weight (Laing, 2000). An increase in temperature similar to those of the benchmark may also stimulate phytoplankton production which would increase the amount of available food for both the adults and newly spawned larvae. No information was available on an upper threshold of temperature tolerance for adult Pecten maximus although Gruffydd & Beaumont (1972) observed high larval mortality above 20°C. Therefore a short term, acute increase in temperature of 5 °C may lead to the death of some individuals at the upper extreme of their temperature range, for example, West Africa, but it is not thought to affect the majority of Pecten maximus in the long term. Adults are likely to be more tolerant to changes in temperature than juveniles however and an intolerance of intermediate has been recorded accordingly.

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

Sexual maturation and spawning are governed by temperature which are obviously imperative for recruitment and contributing to the development of the population. Temperature is considered by many to be the primary trigger in spawning among Pectinidae and there is some evidence to suggest that there may be a critical range (Barber & Blake, 1991). In the Bay of Brest and the Bay of St Brieuc in France, for instance, the critical temperature range for spawning is thought to be between 15.5-16 °C (Paulet et al, 1988). Decreases in temperature have been associated with decreases in feeding activity and spawning. In the laboratory, Pecten maximus has even been kept in water conditioned at 7-8°C to prevent spawning. Colder temperatures can depress development rate and, in extreme cases, lead to death. Evidence suggests that a reduction of temperature by 2°C is unlikely to adversely affect the population provided that other factors which act synergistically with temperature, for example salinity, remain the same.
However Crisp (1964b) reported mortalities approaching 100 % of Pecten maximus from several areas around the British Coast in the severe winter of 1962-1963 where the average sea temperature fell by approximately 4°C. Therefore a short term, acute reduction in temperature of 5°C may lead to the death of many individuals and therefore an intolerance of high has been recorded.

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

An increase in turbidity due to suspended particulate matter, plankton and dissolved substances will decrease light penetration through the water and subsequently decrease phytoplankton productivity. However Pecten maximus use a variety of food sources and this factor will probably have a limited effect. Therefore this species is probably tolerant.

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

A decrease in turbidity may increase phytoplankton production which could potentially enhance the supply of food available to the scallops. However Pecten maximus use a variety of food sources and this factor will probably have a limited effect. Therefore this species is probably tolerant.

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

The water above seabeds at depths of up to 60 m can experience some oscillatory water movement in a strong swell or force 8 gale (Hiscock, 1983). Given their ability to recess it is unlikely that scallops at this depth would be dislodged by the water movement. However, scallops living in water depths of 10-20 m at the shallower extreme of their depth range may close their valves to reduce the scouring effect of the sand and gravel in high velocity water. In combination with the possible effect of sustained displacement, feeding is likely to be reduced and possibly prevented which will ultimately reduce growth rates. The action of waves has been considered a main source of mortality of Pecten maximus in some areas in the Bay of St Brieuc (Thouzeau & Lehay, 1988, cited in Orensanz et al., 1991) and, therefore, an intolerance of intermediate has been recorded.

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

A decrease in wave exposure similar to that of the benchmark is unlikely to have an adverse effect on the population therefore tolerant has been recorded.

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

This species probably has very limited ability for noise detection and therefore is thought to be tolerant.

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

Scallops have eyes around the margin of the shell (see adult general biology) and will swim, orient themselves or close their shell in response to shadows or movement (Wilkens, 1991). Buddenbrock & Möller-Racke (1953, cited in Mason, 1983) found that Pecten maximus' reaction to moving objects depended on the velocity of the moving object and fast moving objects could result in the closure of the valves. Scallops living in shallower water between 10-40 m may therefore be affected by the visual presence of divers and boats for example, however this is unlikely to cause an adverse effect. In addition. sight reaction decreases in sensitivity to repetitive stimulation (Wilkens, 1991) and a recoverability of immediate has been recorded.

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

Scallop dredging can cause damage to the scallop shells and in particular to the growing edge. Ansell et al., (1991) stated that up to 19 % of the scallops left behind by a dredge are affected to some extent. Effects might include shell damage, burial, increased stress and feeding difficulties associated with the increased suspended sediment produced by the action of the dredge. Individuals with damaged shells are more prone to predation. In addition, the energy budget would be altered so that energy previously reserved for spawning would be allocated to new shell growth and therefore reduce the viability of the population. However, Jenkins et al,, (2001) reported that, during dredging, more than 90 % of Pecten maximus that came into contact with a dredge (including those landed, discarded and left behind by the dredge) were in good condition overall and showed little or no shell damage. It is possible that some smaller individuals may be crushed and killed by a scallop dredge although for the majority of the population it is unlikely that it will have an adverse effect so an intolerance of low has been recorded.

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

Pecten maximus is capable of righting itself if disturbed by repeated ejection of water jets directed at the sediment. Individuals can also perform swimming movements and move around randomly until a suitable substrate for recessing is located (Ansell et al., 1991). The distances covered by scallop movement are very limited and newly re-laid scallops are more vulnerable to predators until recessed (Minchin & Buestel, 1983) however it is not thought that this factor will have a particularly adverse effect on the scallops and tolerant has been recorded.

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

Pecten maximus naturally accumulates metal-phosphates in concretions in the renal organs (George et al., 1980). TBT-based antifouling paint was shown to be detrimental to growth and survival of juvenile scallops (Paul & Davies, 1986) and there is some recent evidence that recruitment to inshore scallop beds may have been affected by TBT used in anti-fouling paints (Minchin et al., 1987). Declining populations of P. maximus in Mulroy Bay, Northern Ireland correspond well with the introduction of organotin net dips which had been used in the local salmon farms (Minchin et al., 1987). The first year after the use of the dips had ceased saw a good settlement of scallops compared to, for example, 1984 and 1985 when no settlement was observed. No information concerning the effects of other synthetic compounds on Pecten maximus was found but it is likely that at least some of the population may be killed and therefore an intolerance of intermediate has been recorded.

Heavy metal contamination

Evidence

Scallops concentrate metals in their tissues with an efficiency greater than that of other bivalves (Gould & Fowler, 1991). When Pecten maximus is grown in close proximity to copper-oxide based antifouling paint, high levels of copper may be accumulated in the tissues although much of the copper is gradually lost from the scallop even when still in the presence of the copper oxide (Davies & Paul, 1986). Further loss of copper was seen after the scallops had been transferred to untreated enclosures (Davies & Paul, 1986). Further experiments looked at growing adult and juvenile Pecten maximus in enclosures treated with various anti-fouling compounds and found that trays treated with copper-nickel compounds induced high mortality in juveniles and prevented growth in adults (Paul & Davies, 1986). In contrast, the copper-oxide based paint actually increased spat growth to some extent and had no effect on the adult specimens. It is likely that different heavy metals and their compounds will have various effects on adult Pecten maximus although the majority of research on this subject focuses on the partitioning of metals within the scallop tissues and little information on the effects was found. Nevertheless, the mortality of juveniles indicated by the work by Paul & Davies (1986) has led to an intolerance of intermediate being reported.

Hydrocarbon contamination

Evidence

The effects of oil spills on scallops are considered to be relatively short lived (Gould & Fowler, 1991) and diving investigations in Bantry Bay following the release of 30,000 tons of Arabian light crude oil from a tanker explosion in 1979 found that, although there was some minor contamination, the oil did not affect spatfalls in 1979 or 1980 (Grainger et al., 1984). The amount of sunken oil was limited and Pecten maximus showed no abnormal behaviour or mortality as a result of the oil contamination. However, a taste panel test revealed that scallops living in the vicinity of the sunken oil were still tainted two years after the spillage. Oil pollution may therefore affect the viability of scallop fisheries resulting from the reduction in meat quality although it is unlikely to adversely affect the viability of the population per se.
Endosulfan, a chlorinated hydrocarbon, was found to affect the oxygen consumption in Pecten maximus. Roberts (1975) found that if the concentration of Endosulfan in the water exceeded 1mg /day, the valves of Pecten maximus remained closed for increasingly longer periods of time and oxygen consumption fell. The exact consequences of a sustained lack of oxygen for scallops are not known but it is probable that the animal will experience respiratory stress. Scallops at first respond to respiratory stress with rapid adductions of the shell and the animal eventually becomes fatigued (Mikolajunas, 1996). Stressed scallops may not have the energy required to escape predation and other dangers and the viability of the population will therefore be reduced. Therefore, an intolerance of low has been recorded.

Radionuclide contamination

Evidence

No information was found on the effects of radionuclide contamination on Pecten maximus but field collections of other scallop species have shown that radionuclides are accumulated but few adverse effects on growth and survival were seen (Gould & Fowler, 1991). For example, Baptist et al. (1976, cited in Gould & Fowler, 1991) exposed juvenile Bay scallops Argopecten irradians to a cumulative radiation dose of ca 70 Krads over three months but did not observe any deleterious effects on either the growth or survival of the scallops.

Changes in nutrient levels

Evidence

The effects of an increase in the amount of nutrients will depend on the form of enrichment and on the primary production it stimulates. A study in the Bay of Brest (Chauvaud et al., 1998) found that, regardless of the specific phytoplankton composition, high concentrations of chlorophyll-a reduced the daily growth rate of juvenile Pecten maximus. High concentrations of chlorophyll-a following diatom blooms have also been implicated in causing negative effects on the ingestion and respiration of Pecten maximus juveniles either by clogging their gills or by depleting the oxygen at the water-sediment interface during the degradation of organic matter (Lorrain et al., 2000). The high levels of nutrient enrichment as set in the benchmark may lead to (depending on other environmental conditions) eutrophication and the possibility of subsequent increases in turbidity and suspended material and decreases in the amount of available oxygen. A decrease in Pecten maximus growth rate and reproduction has been observed in the presence of certain toxic algal blooms (Chauvaud et al., 1998). For instance Gymnodinium cf. nagasakiense can lead to the death of post-larval and juvenile Pecten maximus in the wild (Erard-Le Denn et al., 1990, cited in Chauvaud et al., 1998) and in 1995, three major blooms of Gymnodinium cf. nagasakiense in the Bay of Brest inhibited the settlement of spat although a rapid return to normal shell growth rates was reported once the numbers of Gymnodinium sp. had decreased (Chauvaud et al., 1998).
In contrast, Reitan et al. (2002) experimentally enhanced the nutrient supply in a landlocked bay in Norway and found that the resulting increase in the phytoplankton biomass had a significant positive effect on growth rates of Pecten maximus .
It is likely that some of the population will be adversely affected by such a large increase in nutrient levels and an intolerance of intermediate has been recorded. However, it is likely that the recovery period will be relatively rapid.

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

Pecten maximus invariably live in areas associated with full salinity water and as such, an increase in salinity is not thought to be relevant.

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

The inability of Pecten maximus to close its valves makes them highly vulnerable to low salinity stress (Bricelj & Shumway, 1991). Christophersen & Strand (2003) found that in the laboratory, the shells of spat held in water with a low salinity (20 ppt) became thin and easily damaged which ultimately led to a negative shell growth rate. They found that, in general, behaviour was also affected and the scallops made fewer foot movements and retracted the mantle from the shell margin. This could presumably decrease the effectiveness of the feeding apparatus. Several authors have reported the synergistic effects of salinity and temperature on various aspects of Pecten maximus physiology. For instance, Laing (2002) found that between 13-21 °C the growth rate was significantly lower at 26 psu than at 28-30 psu and Christophersen & Strand (2003) found that at 25 ppt and 30 ppt, higher growth rates were seen at 18 °C than at 15 °C. Laing (2002) also found that the food cell clearance rate decreased with salinity. However, the reductions in growth rate were temporary and growth rates returned to that of those spat held in ambient salinities within 10 days of exposure (Laing, 2002). For short term acute changes (see benchmark), the viability of the population is likely to be reduced however recovery will be fairly rapid. Long term chronic changes are likely to have an adverse affect on the population affecting both juvenile and adult scallops. The juveniles are likely to suffer to the extent that they may not survive into adulthood by becoming more vulnerable to, for example, predation and general wear and tear. This could have severe implications for recruitment to the population and it is likely that the population will take several years to recover therefore an intolerance of intermediate has been recorded.

Changes in oxygenation

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

Evidence

Scallops, as sublittoral, epifaunal bivalves which are incapable of sustaining prolonged valve closure, are relatively intolerant of anoxia (Bricelj & Shumway, 1991). Brand & Roberts (1973) found that scallops transferred to de-oxygenated water (13 mm Hg) for three hours experienced rapid bradychardia (reduced heart rate). However, the length of exposure time set in the benchmark is one week which is significantly longer than the length of Brand & Roberts (1973) experimental work. It is likely that scallops will experience some respiratory stress at the level set in the benchmark. It is possible that feeding will be reduced and the animal may become lethargic thus making it more susceptible to predation due to a weakened escape response. This will reduce the viability of the population and therefore a sensitivity of low has been recorded. However, Brand & Roberts (1973) found that the scallops that had been exposed to the de-oxygenated water recovered well upon return to well-oxygenated water (135 mm Hg) therefore a recoverability of high has been recorded.

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

Minchin (2003) lists some examples of parasites and diseases affecting scallops including polychaete, copepod and gastropod infestations.Pseudoklossia pectinis, a protistan parasite, causes hypertrophy in the kidney cells of Pecten maximus from Roscoff, France although the overall damage to the kidney appears to be light (Léger & Duboscq, 1917, cited in Kinne, 1983).
A rickettsial disease has been known to cause mortality in French stocks (Le Gall et al., 1988; Ansell et al., 1991).
Mortensen et al. (2000) reported the loss of over one million spat, approximately one third of Norway's scallop production, due to the heavy infestation of the scallops with Polydora species (worms that bore into calcium carbonate shells). Overall, the information on microbial pathogens affecting Pecten maximus is comparatively little in relation to other commercial bivalves.

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

The leathery tunicate Styela clava is occasionally found attached to the upper valve of Pecten maximus but is unlikely to cause displacement (K. Hiscock, pers. comm.).

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

Various reasons including overexploitation have resulted in declines in wild populations of Pecten maximus and due to the highly variability nature of recruitment in this species, much emphasis is now being placed on its aquaculture. In Shetland, for instance, scallop landings in 1969 neared 600 t but then fell to 96 t only four years later (Mason, 1983). By 1977 the catches had increased again but only to 224 t. Many scallops are now artificially cultured in farms and management measures have been enforced in many areas. Due to the nature of scallop dredging the extraction of this species is likely to adversely affect the species by removing a proportion of the population, although estimates for the efficiency of the dredge are usually below 20% (Mason, 1983).
There are two main scenarios concerning the extraction of this species: the effects resulting from scallop dredges on those scallops that are not caught and also the effects on the population when scallops are caught.
For those that manage to escape being caught there will be a considerable amount of stress involved if the scallop has come in close contact with the dredge. Frequently, the scallop may be exhausted from trying to escape capture by the dredge and may therefore be more vulnerable to predation. Jenkins & Brand (2001) reported that dredging caused a significant increase in the response time of scallops to predators.
For those scallops that are caught but subsequently discarded due to their size, namely scallops below the Minimum Landing Size (MLS), aerial exposure will also cause a certain amount of stress, especially if the period of exposure is prolonged (see section on desiccation). Jenkins & Brand (2001) state that exposure to the air for even a twenty minute period cause a significant reduction in the ability to swim.
As far as the landed scallops are concerned, Brand (1991) reported that the age structure of scallops from a population off the Isle of Man had shifted from almost half the population being older than ten years to over two thirds of the population being under the MLS. The MLS for this species is 110 mm and this size is usually achieved within four years (Brand et al., 1991). Therefore the average age of the population will be reduced and recruitment may then be dependant on a younger portion of the population.

The extraction of this species will cause part of the population to be removed therefore an intolerance of intermediate has been recorded. Blyth et al. (2004), when comparing sites that were trawled for scallops to those that were untrawled or previously trawled (but not in the 18-24 months prior to the study), found that significantly fewer scallops were caught in the trawled sites. They suggested that at least a two year period was necessary for the benthic community to recover to state that was indistinguishable from non-trawled areas. See additional information on recoverability below.

There is significant concern regarding the effects of scallop dredging on the benthic environment and especially reef communities - see importance.

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

Pecten maximus has no known obligate relationships with other species that are extracted. However, in some areas the edible crab Cancer pagurus is one of the predominant predators of Pecten maximus (Brand et al., 1991) and the removal of this commercially important species may consequently reduce the risk of being eaten by a predator.