Scallop, King, scallops

Pecten maximus

Method of production — Caught at sea
Capture method — Dredge
Capture area — North East Atlantic (FAO 27)
Stock area — Shetland
Stock detail — 4a (0-6nm)
Certification — Marine Stewardship Council (MSC)
Picture of Scallop, King, scallops

Sustainability rating three info

Sustainability overview

There are conflicting trends the stock abundance but fishing mortality has around the long-term average.

There are a variety of management measures implemented to protect the stock, bycatch and habitat. There are voluntary agreements to ensure that these sensitive sites are protected, but it is not clear how the SSMO attempts to identify these sites and ensure their protection before they are fished. Neither bycatch, Endangered, Threatened or Protected species or habitat receive effective monitoring in this fishery. This is particularly an issue given that only a quarter of what the dredges interact with, is brought on board the vessel. Scallop dredging (particularly with the Newhaven dredge) is one of the most destructive bottom-towed year methods. This is significant given that there are sensitive habitats such as horse mussel beds in the area. The MSC certification specifies that bedsa warrant protection, however, the assessment lacks clarity for situations where sensitive species (which are protected by EU law under General Policy 9 of the Scottish Governmentas National Marine Plan such as these are detected otherwise.

There are Marine Protected Areas (MPAs) to protect seabed features within the area of this assessment and some components of this fishery may be fishing in these areas. If appropriate management measures are not in place to protect these features, or, an appropriate impact or risk assessment of the activity has not been undertaken to indicate the commercial fishing activity does not damage the integrity of the site, MCS considers these fishing activities as default red rated.


King scallops are bivalve molluscs found in a range of depths from shallow waters in sea lochs to over 100m. They inhabit sandy-gravel and gravel seabeds. They have 2 shells or valves, the upper being flat, and the under or right valve, cup shaped. They are hermaphrodites (i.e. both male and female) and become fully mature at about 3 years old (80 to 90mm in length). Spawning occurs in the warmer months, from April to September. The species can grow to more than 20cm in length and live for more than 20 years, although average sizes are in the range of 10-16cm.

Stock information

Criterion score: 0.5 info

Stock Area


Stock information

There are no reference points to tell if the stock is overfished or undergoing overfishing. Therefore, data-limited indicators (recruitment and stock spawning biomass (SSB)) are used to determine the status of the stock. There are conflicting trends the stock abundance but fishing mortality is around the long-term average.

Recruitment is considered to be moderate in recent years. SSB has been declining since 2012. The older scallops (>10 years old) represent a significant component of the catch which shows that there is a healthy old spawning population but, mean weight-at-age appears to have declines more recently.

Another assessment (produced by the NAFC Marine Centre), suggest that both assessments (MSS and the NAFC Marine Centre) show a decline in stock biomass since 2012. However, these assessments are not directly comparable and there is no clear definition of the Shetland scallop stock.

Fishing mortality and landings have been increasing since 2009 and fishing mortality is around the long-term average. Therefore, Marine Scotland advise no increases in fishing effort and more measures to protect the spawning stock.


Criterion score: 0.5 info

Shellfish fisheries are managed under a Regulating Order (1999) by the Shetland Shellfish Management Organisation (SSMO). Management measures used in the fishery include maximum vessel size (17m), gear restrictions (prohibition of French dredges, tow bar restrictions, no more than 10 dredges, curfew overnight (to protect mobile benthic species)), a ban on dredging in closed areas, minimum landing sizes of 100mm (The Regulation of Scallop Fishing (Scotland) Order 2017). Scottish scallop fisheries are not subject to EU or national TAC regulations. A key measure is limiting fishing effort under the SSMO Licencing Policy.

Landings and fishing effort data are collected regularly. There have been some observer studies conducted over the years but the data are largely derived from fishery-dependent records. Vessel Monitoring Systems (VMS) is mandatory for larger vessels but not for smaller shellfish vessels (which have been voluntarily fitted since 2013). The vesselas location is reported every ten minutes.

Capture Information

Criterion score: 0.5 info

There are management measures to protect vulnerable habitats (such as horse mussel beds) but there is conflict on how effective measures are at protecting vulnerable species

Dredges that use teeth to collect scallops are among the most damaging as they can cause around 20-30% mortality among bycatch species. However, most (around 75%) of the megafauna that scallop dredges interact with remains on the seafloor and is not recorded in bycatch surveys. Shelmerdine (2010) found that bycatch represented around 47.4% of the catch: the main bycatch were sea urchin, horse mussels and brown crab. Nearly half of the urchins encountered during dredging had spine loss, minor cracks or were crushed/dead. Though urchins have high productivity and usually favour non-dredged habitats, therefore, urchins are not considered to be at-risk in scallop dredging.

Horse mussels were recorded during observer and research surveys. Horse mussels are locally common although horse mussel beds are rare. Within the observer and research surveys they were not encountered in high enough density to constitute a bed and so were not considered to form a sensitive habitat. Closures are expected to cover the majority of the horse mussel stock. It is considered highly likely that horse mussels are within biologically based limits. However, there is a lack of sufficient information to determine their stock status.

Endangered, Threatened and Protected (ETP) species

The SSMOas Code of Conduct specifies that interactions with ETP species are recorded in their database. Potential ETP species to interact with the scallop fishery include common skate, angel shark and porbeagle. No interactions have been recorded in the last three years, though a study by Shelmerdine (2010) suggested that common skate represented 0.06% of the total catch. The effectiveness of ETP monitoring system has been contested as fishery-independent studies have found different results to fishery-dependent studies.

Inshore habitat seabed types, biotopes and sensitive habitats have been mapped as part of the Shetland Marine Spatial Plan (SIMSP), which are predominately mixed sand, coarse sand, sandy kelp, muddy sand and gravel. Data from Vessel Monitoring Systems (VMS) crossed with habitat maps show that scallop dredging does not occur mud sediments. The SSMO has a Code of Conduct which depicts their proactive approach to protecting vulnerable seabed habitats. Where sensitive seabed habitats are identified the SSMO will implement appropriate measures to protect them. For example, all known maerl and horse mussel beds are included within a closure. However, the effectiveness of this policy has been challenged: the bycatch study showed that horse mussels represented 6.9% of the overall catch. Over the size of the scallop catch, horse mussel catches can represent a large amount and this does not determine the true impact on the horse mussel bed or the catch that didnat remain in the catch when brought aboard. To reduce the risk of this, the SSMOas strategy aims to close all known mearl and horse mussel beds. However, this means that a vulnerable marine ecosystem (VME) or protected area only constitutes beds and not the presence of individuals of these species.

The scallop fishery here is known to overlap with Marine Protected Areas (MPAs), but it is not clear by how much. For these specific components, MCS considers dredging in MPAs as a default red rating unless there is evidence indicating the activity does not damage the integrity of the site.


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Food Certification International. 2014. Off Site Surveillance Visit - Report for SSMO Shetland Inshore Brown and Velvet Crab and Scallop Fishery - 2nd Annual Surveillance. Inverness, UK.