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 (6-12nm)
Picture of Scallop, King, scallops

Sustainability rating four info

Sustainability overview

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

The Shetland offshore (3-12nm) fishery does not attract the same level of management as those inshore fishery (see 0-3nm rating).

The offshore fleet within Scottish waters are managed by Marine Scotland. New management has been implemented into the fishery that follows scientific advice, however, it is too soon to tell how effective these are.

There are is a lack of monitoring and measures to protect the bycatch and habitats. There remains a significant data gap to understand Endangered, Threatened and Protected species interactions, particularly for angel sharks and common skate, which are known to interact with the fishery. Bycatch represents are important part of the catch (nearly 50%) and bycatch is underestimated as only a quarter of what the dredge interacts with, is brought on board the vessel.

MCS advise that no dredging should occur in vulnerable habitats and MPAs designated to protect habitats. 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.

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: 0.75 info

The waters outside the 6nm zone of the Shetland fisheries, there is less management than their inshore components and the area is managed by Marine Scotland. New management has been implemented into the fishery that follows scientific advice, however, it is too soon to tell how effective these are and management does not include a catch limit or suitable measures to protect the habitat.

Marine Scotland manages Scottish stocks in its territorial waters. There are six Regional Inshore Fishing Groups (RIFGs) that help define management measures within their districts. Scallops are managed under a harvest strategy in Scottish waters, which is based on stock trends.

Scottish scallop fisheries are not subject to EU or national Total Allowable Catches. However, in June 2017, new management measures were enacted in Scottish waters (Regulation of Scallop Fishing (Scotland) Order 2017). Management measures included a increased minimum landing size of 105mm, licence requirements for vessels over 10m in length, some effort restrictions, some spatial closures, fishing gear restrictions which are more restrictive in inshore, shallower waters. There is insufficient management to reduce fishing effort or landings in the Shetland area, which Marine Scotland have suggested, may in future, be required to manage fishing mortality through an Maximum Sustainable Yield (MSY) approach.

There are Special Areas of Conservation (SAC) under the European Natura 2000 and proposed Marine Conservation Zones (MCZs) under the UK Marine Act. The Marine Strategy Framework Directive (MSFD) have committed to Good Environmental Statusa for the seafloor habitat by year 2020.The EU habitats directive and OSPAR defines priority habitat e.g. for biogenic reefs, maerl beds, horse mussel beds and other reef habitats) can be protected or are going through this process.

Scottish regional scallop stock assessments are conducted by Marine Scotland Science (MSS) using fishery dependent (e.g. surveys in processoras factories) and fishery-independent (e.g. research vessel surveys) survey data. Trends are deduced on fishing mortality, biomass, abundance (pre-recruits and adults) and recruitment. Vessel Monitoring Systemas data are collected only every two hours which significantly reduces the ability to monitor fishing in relation to Marine Protected Areas (MPAs). However, Electronic Monitoring System (EMS) may help advise management regarding bycatch, the stock status and will help enforce management measures in the fishery.

Capture Information

Criterion score: 0.75 info

Some inshore scallop grounds contain patches of rocky ground or reef and lack protection. To mitigate the impact on these habitats, the EU habitats directive requires that priority marine features (including biogenic reefs and maerl beds, horse mussel beds and other reef habitats) require sufficient protection. Whilst there are some closed areas, and the UK Good Practice guide for scallop fishermen requires that impacts to the seabed are minimized, there are insufficient measures to protect inshore habitats.

Bycatch changes depending on the area but generally include a mixture of invertebrates and mainly impact brown crabs. 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.

Endangered, Threatened and Protected (ETP) species
There is a lack of research on the interactions between the scallop fishery and ETP species in this area but the Shetland scallop fishery have been reported catching common skate, angel shark and porbeagle shark. More monitoring and management is required to reduce the risk to these species. Implementing more durable belly-ring materials at these larger sizes, better monitoring and real-time areas have been methods recommended to improve the selectivity of the fishery.

Out of all tested bottom trawled gears, scallop dredging having the greatest negative impact across all habitat types. Dredging reduces biodiversity, particularly in fragile or sensitive habitats and homogenise the seabed. Damage may increase with the number of dredge tows performed and causes the most depletion out of surveyed bottom gears (removing 41% of biota and penetrating the seabed by an average of 16.1cm).

To mitigate damage to habitats, there are Special Areas of Conservation (SAC) under the European Natura 2000 and proposed Marine Conservation Zones (MCZs) under the UK Marine Act. Around 23% of Scottish waters are covered by Marine Protected areas (MPA)s however, scallop dredging is only banned in 4.4% of Scottish inshore waters (within 12 nautical miles of the coast Progress has been made, but further MPAs are required to protect vulnerable habitats, particularly for deep-water coral reefs and sensitive habitats such as flame shell beds.

Flame shells (Limaria hians) are bivalve molluscs, similar to mussels, named after their flame-like tentacles. They move around the seabed and build connected nests by burying into the sand. This stabilises the local seabed and provides habitat, acting as ‘spat factories’ for fish and shellfish which sustain local, commercial fisheries including Atlantic cod. Flame shells are a designated priority marine featurea and are protected by General Policy 9 of the Scottish Governmentas National Marine Plan and the Marine Nature Conservation Strategy. Of 30 MPAs implemented in 2014, five were setup to protect flame shell beds. However, some areas were not protected, thus allowing sensitive habitats to remain as legal fishing grounds such as Loch Carron. Urgent studies are required to estimate the extent and recovery time of the damage of this activity and identify measures to restore the habitat.

Spatial management is required to be precautionary and placed in areas likely to contain fragile primary marine features and other ‘essential fish and shellfish habitat’. Spatial management must include a mixture of management measures (such as curfews, low-impact zones within 3nm from the coastline and closed areas for spawning) to reduce gear conflict and ensure that effort is not just displaced. The Scottish Government’s Inshore Fisheries Bill as part of the Inshore Fisheries Strategy will help deliver this, if implemented effectively. MCS recommends greater inshore fisheries management and that evidence is urgently required, as a requirement of Scotland’s National Marine Plan, to ensure that ‘priority marine features’ are not significantly impacted by fishing activity and inshore waters undergo low-impact fishing.

The substrates in the 6 - 12nm fishery is characterised by two main substrates. The eastern side of Shetland generally consists of “flat sandy seabed with occasional pockmarks (seabed craters)” and some sandbanks, mud flats and sandy beaches. However, western Shetland has a rockier coastline. The Marine Scotland assessment suggests that most of fishery occurs inshore region.

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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