King scallop

Stock Status Score: 0.25

The inshore Shetland king scallop stock appears to be increasing, with no concern for biomass or fishing pressure.

Route 2 (data limited) scoring has been applied due to the lack of reliable reference points for fishing pressure and stock biomass. King scallops are considered to have high resilience to fishing pressure.

Stock assessments for Shetland scallops within inshore waters (0- 6nm) are conducted by UHI Shetland on behalf of the Shetland Shellfish Management Organisation (SSMO), with the most recent assessment published in 2025, using data up to 2024. This inshore fishery has been Marine Stewardship Council (MSC) certified since 2012.

Landings have increased substantially from around 1 million individuals in the early 2000s to a peak of 5.3 million in 2022, before declining slightly to 5.1 million in 2023 and 4.8 million in 2024. Fishing effort has been relatively stable since 2010, while LPUE (landings per dredge hour) has risen steadily from around 15 in 2000 to 35 in 2024. This combination of stable effort and increasing LPUE indicates improving catch rates, suggesting the stock is supporting current fishing pressure with no evidence of overexploitation. As such, there is no current concern regarding fishing pressure.

Although spawning stock biomass (SSB) reference points are defined, substantial uncertainty in the Virtual Population Analysis (VPA) model means these estimates are not considered reliable for management. Model outputs are sensitive to new data and rely on non-standardised inputs and proxy reference points. Consequently, SSB is not currently used within harvest control rules.

LPUE is therefore used as the primary stock indicator and has remained consistently above the target reference point (TRP = 25.91), including across all regions. It has also remained above the limit reference point (LRP = 16.48) throughout the time series (since 2000) and above the TRP since 2014, indicating a stock in good condition with no evidence of localised depletion. Management action is triggered if LPUE falls below these reference points through the harvest control rules.

Preliminary assessments suggest SSB may have increased and fluctuated around the SSB TRP (9.99 million tonnes) in recent years, but alignment with MSY remains uncertain. Overall, we consider that the increasing LPUE alongside stable effort indicates no current concern for biomass, however confidence relative to MSY is limited.

Management of the Target Stock Score: 0

Alignment with Scientific Advice Score: 0.5

Monitoring, Control and Surveillance Score: 0.25

This MSC-certified king scallop fishery is managed through a framework of effort controls, technical measures and harvest control rules, with stock indicators suggesting that the stock remains in a healthy state. While no catch limits are in place, management is responsive to changes in LPUE and can be strengthened if stock performance declines. However, reliance on fishery-dependent data introduces some uncertainty, and the incorporation of fishery-independent data would improve confidence in assessments and management decisions. The fishery also benefits from a well-developed and effective Monitoring, Control and Surveillance (MCS) system, supported by reliable enforcement and low levels of illegal, unreported and unregulated (IUU) fishing. Nevertheless, some recent instances of non-compliance involving undersized scallops demonstrate the importance of continued monitoring and enforcement.

Scallop dredging within six nautical miles of the Shetland coast is managed by the Shetland Shellfish Management Organisation (SSMO). The fishery has been Marine Stewardship Council (MSC) certified since 2012 and is prosecuted by 23 active vessels, of which approximately 14 operate as full-time scallop dredgers. There is no quota or Total Allowable Catch (TAC) in place. Instead, the fishery is managed through a combination of effort controls, spatial measures and a harvest strategy informed by ongoing monitoring of stock status through landings per unit effort (LPUE). Although catches are not directly constrained by a TAC, the harvest strategy incorporates pre-defined Harvest Control Rules (HCRs) that allow management measures to be adjusted in response to changes in stock indicators, helping to ensure that management remains aligned with stock status and available scientific evidence.

Management measures in the fishery aim to control fishing effort and maintain stock productivity. Key measures include restrictions on vessel participation through limited licensing, gear controls including a prohibition on the use of French dredges, a minimum landing size (MLS), a fishing curfew prohibiting dredging between 21:00 and 06:00, and spatial management through closed areas. These closed areas prohibit dredge fishing in locations containing confirmed or suspected sensitive seabed habitats, including maerl beds, horse mussel beds and seagrass habitats.

The minimum landing size for king scallops in Shetland is 100 mm shell width, which is lower than the 105 mm MLS applied elsewhere in Scotland. This reflects the slower growth rates observed in the colder waters surrounding Shetland.

A key component of the management system is the harvest strategy, which incorporates Harvest Control Rules linked to LPUE. Management responses are determined according to the position of LPUE relative to a target reference point (TRP) of 25.91 and a limit reference point (LRP) of 16.48. The fishery is classified into three management zones:

• Green zone: LPUE at or above the TRP (≥ 25.91)
• Amber zone: LPUE between the TRP and LRP (16.48-25.91)
• Red zone: LPUE below the LRP (< 16.48)

The objective of the harvest strategy is to maintain the fishery within the green zone. If LPUE declines into the amber zone, additional measures may be implemented, including a prohibition on new licences, increases to the minimum landing size, spatial closures, temporal fishing restrictions, or limits on further increases in fishing effort. If LPUE declines below the limit reference point, management measures may be strengthened further, including extended closures, fishery-wide closures, and enhanced research and monitoring programmes. The harvest strategy therefore provides a responsive mechanism for translating changes in LPUE into management action. However, as a fishery-dependent measure, LPUE can be influenced by factors other than stock size and may mask declines in abundance. The incorporation of fishery-independent surveys or direct biomass estimates would therefore increase confidence in stock status assessments and strengthen the basis of management decisions.

At present, the stock indicator remains above the target reference point and therefore no additional management measures have been triggered through the HCR framework. LPUE has remained at or above the target reference point since monitoring began in 2008, suggesting that scallop abundance and catch rates have remained consistent with the fishery's management objectives. Nevertheless, concerns regarding the perceived lack of smaller scallops on some fishing grounds have resulted in no new licences being issued, demonstrating a precautionary approach to management despite the positive stock indicators.

Monitoring, control and surveillance of UK fisheries are delivered through licensing, vessel tracking, statutory reporting, observer coverage, inspections and other monitoring systems. All commercial fishing vessels must hold a valid UK fishing licence to operate in UK waters, although exemptions may apply to some unpowered vessels under 10 metres. Depending on the area, target species and activity, additional permits or authorisations may also be required.

Vessel tracking requirements vary by vessel size, area and administration through Vessel Monitoring Systems (VMS) and, where applicable, inshore VMS (iVMS).

• Vessels 12 m and over must report their position every two hours via VMS.
• Vessels under 12 m report their position continuously where iVMS applies, or may operate full VMS where required by licence conditions or when fishing offshore.

Reporting of fishing activity, catches, landings, discards and port movements is mandatory, with logbooks supporting traceability:

• Vessels 12 m and over submit electronic logbooks under Council Regulation 1224/2009.
• Vessels 10–12 m (and under 10 m Producer Organisation vessels) use paper logbooks, updated daily and submitted within 48 hours of landing.
• Non-PO vessels under 10 m are generally exempt unless required by licence conditions.

UK fisheries are also subject to independent monitoring through fisheries observers and, increasingly, Remote Electronic Monitoring (REM) systems, which use cameras, sensors and positional data to verify fishing activity and improve data collection. Together, these measures support compliance with fisheries regulations and help prevent illegal, unreported and unregulated (IUU) fishing. Enforcement actions may include fines, prosecutions, licence suspensions and other sanctions where breaches are identified.

The Shetland inshore scallop fishery is certified to the Marine Stewardship Council (MSC) standard, providing an additional layer of traceability, monitoring and reporting. Within the certified fishery, six vessels are over 12 m in length and are therefore required to operate VMS. Five of these vessels are active scallop dredgers and are also fitted with REM systems. In addition, under Scottish regulations, all vessels actively fishing for scallops using dredges, regardless of size, have been required to have fully operational REM systems since June 2024.

Although compliance within the fishery is generally considered good, instances of non-compliance have been recorded. In March and April 2024, Marine Directorate Operations identified quantities of undersized king scallops during inspections at two Shetland processing plants. The scallops originated from catches landed by SSMO-licensed vessels. Following these incidents, the SSMO issued a warning to all licensed scallop fishers, advising that any future breaches would be considered by the Board and dealt with according to their severity. Serious infringements may result in the suspension of an SSMO licence for up to eight weeks.

The Marine Conservation Society views Remote Electronic Monitoring (REM) with cameras as one of the most cost-effective tools for providing reliable fisheries data and aiding informed management decisions. Fully monitored fisheries enhance collaboration, data accuracy, stock recovery, and reduce impacts on marine wildlife and habitats. However, the full potential of REM may only be achieved when it tracks fishing location and documents catch and bycatch, particularly where vulnerable species and habitats are at risk. 

The Fisheries Act (2020) requires the development of Fisheries Management Plans (FMPs) (replacing EU Multi-Annual Plans) in the UK. 43 FMPs have been proposed and are at various stages of development and implementation, these should all be published by the end of 2028. FMPs have the potential to be very important tools for managing UK fisheries, although data limitations may delay them for some stocks. It is also essential the UK governments define and adopt a standardised approach or model across the four nations to a universally defined FMP design, to ensure the consistence, quality and coherence of all the proposal FMPs. 

The Marine Conservation Society is keen to see publicly available Fishery Management Plans for all commercially exploited stocks, especially where stocks are depleted, that include: 

• An overview of the fishery including current stock status, spatial coverage, current fishing methods and impacts 

• Targets for fishing pressure and biomass, and additional management when those targets are not being met, based on the best scientific evidence 

• Timeframes for stock recovery 

• Improved data collection, transparency, and accountability, supported by technologies such as Remote Electronic Monitoring (REM) 

• Consideration of wider environmental impacts of the fishery, including habitat impacts and minimising bycatch 

• Stakeholder engagement

Capture Method Score: 0.75

Habitat Impacts: 0.75

Bycatch Impacts: 0.25

Vulnerable and ETP Species: 0.5

Ecosystems and Trophic Webs: 0.5

King scallops are predominantly caught using dredges that physically penetrate and disturb the seabed. Although available evidence suggests that impacts on the main bycatch species are minimal, uncertainty remains regarding unquantified mortality of some non-target species which remain on the seabed. In addition, a condition has been raised for improved understanding of interactions with ETP species, particularly the critically endangered common skate complex, as it cannot be concluded that the fishery is not hindering the recovery of the species. While the direct ecosystem effects of scallop removal are considered limited, dredging-related impacts, including bycatch and habitat disturbance, may contribute to cumulative pressures on already altered ecosystems and food webs in the region.

Dredges dominate the king scallop fishery because they are highly efficient, but they also pose significant environmental risks, particularly to sensitive seabed habitats. A standard king scallop dredge (Newhaven dredge) consists of a heavy steel frame and chainmail net, with a toothed bar that flips scallops into a net. Dredges are attached to tow bars dragged along the seabed, with up to two dredges per bar. The number of dredges used is limited by vessel engine power and regional legislation.

As they move, dredges penetrate the substrate by around six centimetres and can reduce benthic species abundance and richness by 8–12% with each pass. In naturally dynamic habitats, such as sandy sediments exposed to strong wave action, benthic communities often recover relatively quickly. However, recovery can be much slower in more stable or structurally complex habitats, including gravel beds, maerl beds and seagrass meadows, where disturbance may persist for many years or even decades. Long-lived and slow-growing species are particularly vulnerable to repeated dredging activity.

Around Shetland, king scallops are primarily found on sand, mud and gravel substrates. The region also supports a range of sensitive benthic habitats, the distribution and condition of which have been assessed through seabed mapping undertaken as part of the Shetland Islands Marine Spatial Plan. This information has informed the fishery's spatial management framework, resulting in a number of highly vulnerable habitats, including horse mussel beds, seagrass meadows and maerl beds, being closed to scallop dredging. Furthermore, the management framework adopts a precautionary approach, whereby any newly identified sensitive seabed habitats are investigated, and may be closed where necessary.  

Despite these protections, some overlap remains between the fishery and burrowed mud habitats, which are listed by OSPAR as threatened and declining and are widely distributed throughout the region, making them a commonly encountered habitat. These habitats typically occur in deeper, low-energy environments and support diverse communities of burrowing crustaceans, sea pens, anemones and polychaete worms. Two burrowed mud biotopes are present within the area, one of which is classified as a Vulnerable Marine Ecosystem (VME). Although the spatial overlap between dredging activity and these habitats is limited, affecting less than 5% of their known distribution, some uncertainty remains regarding the extent of fishing impacts. In particular, studies do not always distinguish between the two burrowed mud biotopes, making it difficult to determine whether dredging is causing significant damage to the structure or functioning of VME habitats. Nevertheless, the limited extent of overlap relative to the widespread distribution of burrowed mud habitats suggests that any habitat impacts are likely to be proportionate to the area affected and therefore unlikely to result in serious or irreversible harm at the habitat scale.

Bycatch is another important consideration within the king scallop fishery. Common bycatch species include queen scallops, brown crab, whelk, sea urchins and horse mussels. However, the design of scallop dredges incorporates features that help reduce the retention of some non-target species. For example, the belly bags are constructed from interlocking rings that allow smaller organisms to escape. In addition, the 65 mm ring size used in king scallop dredges is relatively inefficient at retaining queen scallops, which are typically targeted using smaller dredges fitted with 50 mm rings and more closely spaced teeth.

Several of the principal bycatch species are also subject to their own management and assessment frameworks. Queen scallop abundance and catches have fluctuated in recent years but show no clear long-term trend, while brown crab stocks around Shetland have generally exhibited stable or increasing LPUE and are themselves MSC certified. Whelk fisheries in the region are similarly managed by the SSMO and supported by regular stock assessments, which indicate stable catch rates. As these species generally comprise only a small proportion of the catch, are regularly monitored, and are managed through established regulatory frameworks, there is little evidence that the king scallop fishery is having a significant impact on their populations.

Nevertheless, uncertainty remains regarding the full extent of bycatch mortality. Organisms that are damaged during dredging but not retained within the catch are unlikely to be recorded, and mortality rates among these individuals can be high. Consequently, although there is limited evidence of population-level impacts on the principal bycatch species, the wider effects of dredging on non-target benthic organisms and ecosystem functioning are less certain. Risks therefore remain for species and communities that are not routinely monitored or accounted for within catch data.

Scallop dredges may incidentally catch endangered, threatened, or protected (ETP) species. In this area, the species most likely to interact with the fishery are demersal elasmobranchs, including the critically endangered common skate complex and vulnerable porbeagle shark. These species are protected and cannot be retained, so any individuals caught must be returned to the sea as quickly as possible.

The SSMO Code of Conduct and logsheet requirements require fishers to record and report all ETP species interactions, as well as habitat interactions, to the SSMO. Since 2013, logsheet reporting has been based on daily records, with fishers submitting completed logsheets to the SSMO on a regular basis. Information on landings, ETP interactions, and habitat interactions is incorporated into the SSMO database.

Despite these reporting mechanisms, information on ETP interactions within the fishery remains limited. Consequently, a condition has been raised as part of the certification process, as there is currently insufficient evidence to conclude with a high degree of confidence that the scallop fishery is highly unlikely to hinder the recovery of these protected species.

King scallops are low-trophic-level suspension feeders that filter plankton from the water column and form part of the diet of a range of predators, including fish, crustaceans, cephalopods and elasmobranchs. Scallops are not keystone species and current levels of biomass removal by the fishery are unlikely to result in trophic cascades or other substantial food-web effects.

Nevertheless, the wider North Sea ecosystem has undergone significant change in recent decades as a result of climate change and fishing pressure. Rising sea temperatures have altered trophic pathways through reductions in zooplankton abundance, while declines in key forage species, such as sandeels, have affected food-web structure and energy transfer. In Shetland, reductions in sandeel abundance have also been linked to declines in seabird populations that depend on them for food.

Although the removal of scallops is unlikely to drive ecosystem change directly, scallop dredging can result in bycatch mortality, seabed disturbance and the smothering of benthic habitats. These impacts may act cumulatively alongside other ecosystem pressures, meaning that the overall ecosystem impact of the fishery cannot be considered negligible.