|Researched by||Charlotte Marshall & Emily Wilson||Refereed by||Andy Beaumont|
|Other common names||-||Synonyms||-|
Both shell valves are fan shaped with an 'ear' on either side of the apex of the valve. The right valve is strongly convex and tends to be off-white, yellowish, or light brown in colour, often with bands or spots of darker pigment. The left valve is flat and is light pink to reddish brown in colour. Pecten maximus grows up to 15 cm long and both valves each have 15-17 radiating ribs.
Also known as the King scallop, Giant scallop, escallop and Coquille St. Jacques.
- none -
|Phylum||Mollusca||Snails, slugs, mussels, cockles, clams & squid|
|Class||Bivalvia||Clams, cockles, mussels, oysters, and scallops|
|Typical abundance||Moderate density|
|Male size range|
|Male size at maturity|
|Female size range||Medium(11-20 cm)|
|Female size at maturity|
|Growth rate||See additional text|
|Body flexibility||None (less than 10 degrees)|
|Characteristic feeding method||Active suspension feeder|
|Typically feeds on||Seston including phytoplankton, especially single celled algae, particulate organic matter (POM), bacteria and other micro-organisms (Fegley et al., 1992; Reitan et al., 2002).|
|Dependency||No text entered.|
|Supports||See additional information|
|Is the species harmful?||No|
Pecten maximus are hermaphrodite and, therefore, there is no separate male and female size range or size at maturity. Pecten maximus grows up to 15 cm and will be at least 6 cm when sexually mature.
Pecten maximus normally lies recessed into slight hollows (recesses) in the seabed (Mason, 1983). Recessing is achieved through a series of powerful adductions (valve closures) where water is ejected from the mantle cavity and lifts the shell at an angle to the seabed so that subsequent water jets blow a hollow into the sediment (Brand, 1991).
Swimming is generally limited to escape reactions. Experimental contact with different starfish species elicited distinct, energy adaptive types of response from Pecten maximus. Full swimming response was initiated only by extracts of Asterias rubens and Astropecten irregularis which prey on molluscs, while limited jumping or valve-closing responses were induced by non-predatory starfish (Thomas & Gruffydd, 1971).
Pecten maximus is capable of swimming by rapidly clapping the valves and expelling the water on either side of the dorsal hinge so that the scallop moves with the curved edge of the shell foremost (Thomas & Gruffydd, 1971). Jumping is achieved through the gradual relaxation of the adductor muscle followed by the rapid opening and closing of valves, which jump the scallop hinge forward (Thomas & Gruffydd, 1971).
Size and growth
Specimens of up to 21 cm have been recorded, although this is exceptional and the size range of scallops caught commercially is usually between 10 and 16 cm (Mason, 1983).
Scallop shells bear distinct and concentric annual growth rings. The shells also bear numerous regularly occurring concentric striae 0.1-0.3 mm apart which are also used to age the scallops (Mason, 1957). Minchin (2003) stated that it took between three and six years to attain 11 cm in shell length. The Minimum Landing Size (MLS) for this species in Britain and Ireland is 10-11 cm (depending on area) and growth to this size is usually achieved within four years (Brand et al., 1991).
Growth rate can be affected by several factors including salinity, temperature, competition, water depth and food supply. For example, Laing (2002) found that the growth rate of spat grown at 13-21 °C was significantly lower at 26 psu than at 28-30 psu. Mason (1957) found that specimens from inshore, shallower waters typically displayed higher growth rates and maximum sizes than those from deeper waters. Even differences in growth rate between different grounds have been reported (Mason, 1983). Growth in Pecten maximus slows down or stops altogether in the winter, starts again in spring and continues through summer when it is most active. Growth also becomes slower in older individuals and consequently the growth rings become closer together and difficult to distinguish (Mason, 1957). In contrast to many other studies on bivalves, Beaumont et al. (1985) found no association between heterozygosity and size in this species, i.e. genetic factors are relatively unimportant compared to environmental controls on growth. However, they also suggested that genetic factors may be more important during the larval stage.
Embedded among the bases of the sensory tentacles around the edge of the mantle are numerous tiny eyes (Mason, 1983). The eyes are a blue green colour no more than ca 1.5 mm in diameter. The eyes bear a superficial resemblance to the camera eyes of vertebrates and have a highly specialized retina (Wilkens, 1991). Light has both inhibitory and excitatory effects and scallops will swim, orient themselves or close their shell in response to shadows or movement (Wilkens, 1991).
Campbell et al. (2001) reported that, in July 1999, the Amnesic Shellfish Poisoning toxin, Domoic Acid (DA), was found in Pecten maximus at levels exceeding the regulatory limit of 20 µg DA / gram across large areas of northern and western Scotland. The risk of human illness resulting from consuming toxic scallops is, according to Shumway & Cembella (1993, cited in Campbell et al., 2001), a significant threat to both public health and the shellfish industry.
|Physiographic preferences||Open coast, Offshore seabed, Sea loch / Sea lough, Enclosed coast / Embayment|
|Biological zone preferences||Lower circalittoral, Lower infralittoral, Upper circalittoral|
|Substratum / habitat preferences||Coarse clean sand, Fine clean sand, Gravel / shingle, Muddy sand, Sandy mud|
|Tidal strength preferences||Moderately Strong 1 to 3 knots (0.5-1.5 m/sec.), Weak < 1 knot (<0.5 m/sec.)|
|Wave exposure preferences||Exposed, Extremely sheltered, Sheltered, Very sheltered|
|Salinity preferences||Full (30-40 psu)|
|Depth range||10-110 m|
|Migration Pattern||Non-migratory / resident|
Water flow and wave exposure
Pecten maximus tend to be most abundant just inside or just away from areas of strong currents (Mason, 1983). Gibson (1956) found that scallops living in sheltered areas grew faster than those on wave exposed beds and suggested that this was because the feeding apparatus become overwhelmed by particulate matter in the highly wave exposed areas. It is also possible that the delicate processes of larval settlement and byssal attachment would be disturbed in strong currents (Brand, 1991).
Aggregation and population subdivision
Adult scallops have a limited mobility and rely on the dispersal of larvae in terms of geographic distribution (Brand, 1991). The extent of this distribution will in turn be affected by factors including local hydrographic regimes and the survival of larvae. Consequently, all scallops have an aggregated distribution within their geographic range and the major fishing grounds are generally widely separated so much so that respective environmental conditions produce marked differences in population parameters (Brand, 1991).
|Reproductive type||Permanent (synchronous) hermaphrodite|
|Reproductive frequency||Annual protracted|
|Fecundity (number of eggs)||>1,000,000|
|Generation time||2-5 years|
|Age at maturity||Reach first maturity at 2 years and full maturity at 3-5 years.|
|Season||April - September|
|Life span||11-20 years|
|Duration of larval stage||11-30 days|
|Larval dispersal potential||Greater than 10 km|
|Larval settlement period||Insufficient information|
The MarLIN sensitivity assessment approach used below has been superseded by the MarESA (Marine Evidence-based Sensitivity Assessment) approach (see menu). The MarLIN approach was used for assessments from 1999-2010. The MarESA approach reflects the recent conservation imperatives and terminology and is used for sensitivity assessments from 2014 onwards.
|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).|
|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.|
|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.|
|Tolerant||Not relevant||Not sensitive||Not relevant|
|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.|
|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.
|Not relevant||Not relevant||Not relevant||Not relevant|
|Pecten maximus is not an intertidal species and emergence is not considered relevant.|
|Not relevant||Not relevant||Not relevant||Not relevant|
|Pecten maximus is not an intertidal species and emergence is not considered relevant.|
|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.
|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.|
|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.|
|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.
|Tolerant||Not relevant||Not sensitive||Not relevant|
|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.|
|Tolerant||Not relevant||Not sensitive||Not relevant|
|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.|
|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.|
|Tolerant||Not relevant||Not sensitive|
|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.|
|Tolerant||Not relevant||Not sensitive||Low|
|This species probably has very limited ability for noise detection and therefore is thought to be tolerant.|
|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.|
|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.|
|Tolerant||Not relevant||Not sensitive||Not relevant|
|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.|
|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.|
|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.|
|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.
|No information||Not relevant||No information||Not relevant|
|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.|
|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.
|Not relevant||Not relevant||Not relevant||Not relevant|
|Pecten maximus invariably live in areas associated with full salinity water and as such, an increase in salinity is not thought to be relevant.|
|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.|
|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.|
|No information||No information||No information||Not relevant|
|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.
|No information||No information||No information||Not relevant|
|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.).|
|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
There is significant concern regarding the effects of scallop dredging on the benthic environment and especially reef communities - see importance.
|Not relevant||Not relevant||Not relevant||Not relevant|
|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.|
- no data -
|National (GB) importance||-||Global red list (IUCN) category||-|
Legislation and management
The most common management tool in the scallop fishery in the Minimum Landing Size (MLS) which, for Pecten maximus, ranges from 10-11 cm, depending on area. Protective measures in the Isle of Man fishery also include restriction on vessel size and a seasonal closure from June to November inclusive (Ansell et al., 1991). Irish Sea stocks are also subject to a seasonal closure from June - October inclusive. On the south coast of Devon, an Inshore Potting Agreement (IPA) exists whereby the use of towed gears is restricted in inshore areas that have traditionally been the focus for static gear e.g. pots (Blyth et al., 2004).
Scallop fisheries have received attention due to the impact that scallop dredging can have on the benthic communities involved and the damage that can be caused to adjacent non-target habitats. Electronic navigation equipment now allow scallop dredgers to work close to reef habitats and those reef habitats are frequently impacted.
Species diversity and the abundance of scallops and maerl have been compared between areas that have and have not been dredged for scallops (e.g. Hall-Spencer, 1998; Hall-Spencer & Moore, 2000c; and Hall-Spencer et al., 2003). Negative effects on the benthic communities include reductions in biodiversity and structural complexity of the habitat (Hall-Spencer et al.,2003), significant reductions in the number of nodules of maerl forming species (Hall-Spencer, 1998) and little evidence of recovery, in terms of amount of live maerl, within four or five years (Hall-Spencer & Moore, 2000c; Hall-Spencer et al.., 2003). Furthermore, maerl are often buried under several centimetres of sediment during the process of scallop dredging.Blyth et al. (2004) compared the benthic communities between sites subject to varying levels of towed fishing gear use in and around the IPA at Salcombe. Sites ranged from those that were used exclusively by towed gear fishers to sites where no towed gear had been used. They found that total species richness and biomass of the benthic communities was significantly higher in sites that had only temporary towed gear (although not in the 18-24 months prior to the study) or no towed gear use than in sited with exclusive or annual / seasonal towed gear use. Bradshaw et al. (2001) compared two areas off the Isle of Man in the Irish Sea: a closed area that had previously been heavily fished for the species and an area that continues to be heavily dredged. They reported that the dredged area had a less diverse community structure and fewer upright species. Hall-Spencer et al. (2003) noted that, whilst smaller organisms, deep burrowers and heavily armoured species may survive scallop dredging, larger and more fragile species are often killed. Not only is scallop dredging catastrophic for maerl beds and reef species but is may also result in the detriment of the targeted species. Hall-Spencer & Moore (2003), for example, found that compared with an unfished site where mature scallops dominated the population, no Pecten maximus over the age of seven were found in a commercially fished site in the Firth of Clyde.
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This review can be cited as:
Last Updated: 24/04/2008