Status Assessment 2025 - Modiolus modiolus beds
The status of M. modiolus beds with regard to distribution, extent and condition is generally poor throughout the OSPAR Maritime Area. In the Greater North Sea (Region II), there is evidence of clear decreases in distribution and extent. In the Celtic Seas (Region III), distribution appears stable, but the extent has declined. Condition is poor in both regions, however, the trend in condition is mixed, with condition improving in certain beds. No assessments were available for the other OSPAR Regions.
The main pressures to M. modiolus beds are ‘Penetration and/or disturbance of the substrate below the surface of the seabed, including abrasion’ and ‘Physical change (to another seabed type)’. Climate change is also projected to have a significant impact on M. modiolus beds, with M. modiolus a boreal species.
There are significant knowledge gaps about the distribution, extent, condition and pressures affecting M. modiolus beds throughout the OSPAR Maritime Area, meaning that the confidence in this assessment is poor.
| Assessment of status | Distribution | Extent | Condition | Previous OSPAR status assessment | Status (overall assessment) | |
|---|---|---|---|---|---|---|
Region | I | ● | NA | |||
| II | ↓1,5 | ↓1,5 | ↓1,5 | ● | Poor | |
| III | ←→1,5 | ↓1,5 | ?1,5 | ● | Poor | |
| IV | ● | NA | ||||
| V | ● | NA | ||||
| Assessment of key pressures | Penetration and/or disturbance of the substrate below the surface of the seabed, including abrasion | Physical change (to another seabed type) | Climate change (temperature increases, marine heatwaves, ocean acidification) | Overall threat of impact | |
|---|---|---|---|---|---|
Region | I | ||||
| II | ↓1,5 | ↑1,5 | ↑3,5 | Significant threat2,3,5 | |
| III | ↓1,5 | ↑1,5 | ↑3,5 | Significant threat2,3,5 | |
| IV | |||||
| V | |||||
Explanation to table:
Distribution, extent, condition
Trends in status (since the assessment in the background document)
↓ decreasing trend or deterioration of the criterion assessed
↑ increasing trend or improvement in the criterion assessed
←→ decline or deterioration of the criterion has been halted
? trend unknown in the criterion assessed
red – poor
green – good
blue – status unknown, insufficient information available
NA – Not Applicable
Key pressure
↓ key pressures and human activities reducing
↑ key pressures and human activities increasing
←→ no change in key pressures and human activities
? Change in pressure and human activities uncertain
Threat or impact (overall assessment)
red – significant threat or impact;
green – no evidence of a significant threat or impact
blue – ? insufficient information available
Types of assessment:
1 – direct data driven,
2 – indirect data driven,
3 – third party assessment close-geographic match,
4 - third party assessment partial-geographic match
5 – expert judgement.
Confidence
Low due to low volume of data for analysis.
Background information
- Year added to OSPAR List: 2004
- The original evaluation of M. modiolus against the Texel-Faial criteria referred to sensitivity, ecological significance and decline with information also provided on threat. At the time of the original evaluation, the literature only supported evidence of threat in some parts of the OSPAR area. It was concluded that the need for more information on this habitat is essential and under the concept of precaution, the inclusion of this habitat should be considered as sensible until more research on the status of this habitat is completed.
- Sensitivity: The habitat was evaluated as sensitive when it was added to the list due to its low resilience to adverse effects from human activity resulting in physical disturbance, with recovery likely to be slow due to its life history characteristics (long-lived with sporadic and low recruitment).
- Ecological significance: The habitat supports a biodiverse community, may act as a nursery habitat for other species, and can have an important role in sediment stabilisation and nutrient transfer from the water column to the benthos.
- Decline: A decline in the extent of the habitat was considered “strong” across the whole OSPAR maritime area at the time of designation.
- Anthropogenic pressures and biological factors: The habitat was considered threatened due to bottom fishing methods which cause physical disturbance, e.g. scallop dredging in Northern Ireland was reported as having widespread and long-lasting damage.
- Last status assessment: 2009. OSPAR (2009) concluded that the original evaluation was accurate and that M. modiolus beds should remain on the OSPAR List of Threatened and/or Declining (T&D) Species and Habitats. Along with the threats identified in the original assessment, additional threats were identified including marine pollution through eutrophication and, at the southern limits, increased bottom water temperature due to climate change.
Geographical range and distribution
The geographical range of M. modiolus beds extends from southern parts of the Barents Sea and the White Sea to the southern parts of the North Sea and the Irish Sea (Figure 1). Records of M. modiolus beds further south than the North Sea, individual M. modiolus have been recorded as far south as the Bay of Biscay (Poppe and Goto, 1993). Records of M. modiolus beds off Ireland (in the Irish Sea) are poorly recorded and it is very likely that the habitat is under recorded with high numbers of M. modiolus individuals (often clumped) found in several grabs and whelk pots (Farinas-Franco, per comms). The depth range of M. modiolus beds can vary from 4-227 m; the deepest recorded beds are within the Sound of Canna (off the west coast of Scotland; Greathead et al., 2023). Distinguishing between records of individual specimens and beds can be difficult and open to interpretation. Furthermore, the criteria of what constitutes as a M. modiolus bed differs between OSPAR (>30% cover) and The Marine Habitat Classification for Britain and Ireland (refers specifically to a bed of M. modious with no criteria for coverage; JNCC 2022).
The OSPAR List of T&D Species and Habitats currently states that M. Modiolus beds occur in all OSPAR Regions. However, whilst current evidence supports the occurrence of M. modiolus beds in the Arctic (Region I), Greater North Sea (Region II) and Celtic Seas (Region III); records in the Bay of Biscay and Iberian Coast (Region IV) still need confirmation. There is no evidence to suggest the feature occurs in the Wider Atlantic (Region V).
The distribution of M. modiolus beds is decreasing in Region II (based on observations in the Kattegat region of the North Sea), and stable in Region III (based on beds in Northern Ireland and Wales). These assessments are based on a small number of beds surveyed. In both regions, most beds have not been assessed in the last 6 years (i.e., within Scotland, parts of Northern Ireland, Ireland and Denmark). Additionally, some records of M. modiolus in the OSPAR T&D database require review by Contracting Parties to ensure they meet the definition of a bed, rather than a few individual M. modiolus, following potential uncertainties raised for data points in Ireland and the UK (Wales and England specifically). There are reports of M. modiolus at several sites in the Irish Sea (Farinas-Franco per comms), however these are not included in in OSPAR Habitats 2022 dataset and are not mapped in Figure 1. Therefore, assessments in Regions II and III have low confidence.
The status of M. modiolus beds has not been assessed in Region I in the last 6 years, and no information was provided from Contracting Parties for Region IV. Therefore, no assessment was conducted on changes in distribution in these regions.
Method: D
Figure 1: The current distribution of M. modiolus bed records. Note that records in region IV have not been confirmed as M. modiolus beds and may be individuals. Some records of M. modiolus in the OSPAR T&D database require review by Contracting Parties to ensure they meet the definition of a bed, rather than a few individual M. modiolus, following potential uncertainties raised for data points in Ireland and the UK (Wales and England specifically)
Extent (Habitats)
The total extent of M. modiolus beds in the OSPAR maritime area has been estimated as 60.58 km2. This is likely an underestimate as polygon data is incomplete and the extent of many M. modiolus beds remain unmapped. Some of the largest beds are found in the northeast of Scotland (such as the Moray Firth, Cromaty Firth and Noss Head), Strangford Lough (east of Northern Ireland), Wales (off the Llŷn peninsula and North Anglesey) and the Kattegat.
M. modiolus beds are often fragmented in nature and, therefore, estimating bed extent can be difficult and open to interpretation. Sometimes beds can exist as obvious reefs with distinct borders; however, more often beds exist as a mosaic of clumps of M. modiolus individuals with bare patches of seabed in between.
The extent of M. modiolusbeds is decreasing in both Region II (based on observations in the Kattegat), and Region III (based on beds in Northern Ireland and Wales). These assessments are based on a small number of beds surveyed. In both regions most beds have not been assessed in the last 6 years (i.e., within Scotland, parts of Northern Ireland, Ireland and Denmark). Therefore, assessments in Regions II and III have low confidence.
The status of M. modiolus beds has not been assessed in Region I in the last 6 years and no information was provided from Contracting Parties for Region IV. Therefore, no assessment was conducted on changes in extent in these regions.
Method: D
Condition
The condition of M. modiolus beds can be judged in several different ways including, spatial integrity (e.g. do fishing gear tracks cut across the bed), topographic integrity (e.g. the presence of continued ridges and mounds), size distribution (e.g. measures of recruitment) and abundance, composition and diversity of the associated biota
There is a poor understanding of M. modiolus bed condition in the OSPAR maritime area. Only a few M. modiolus beds have been surveyed over sufficient time spans to assess changes in condition. In previous assessments, examples of M. modiolus beds in poor condition were noted where fishing activity (primarily scallop dredging) had damaged beds.
In Region II, M. modiolus beds are assessed to be in poor condition in the Kattegat area. This, in part, could be due to high fishing activity in the Kattegat, damaging key larvae sources. No recruitment has been observed in this area and, therefore, condition is thought to be decreasing.
In Region III, M. modiolus beds have also been assessed to be in poor condition (based on beds in Wales and Northern Ireland). However, no clear trend in condition was observed in Region III: the condition of beds in Wales was reported to be decreasing, whilst the bed in Strangford Lough, Northern Ireland, was reported to be increasing in condition.
For Region II and III assessments are based on a small number of beds surveyed. In both regions, most beds were not assessed in the last 6 years (i.e., within Scotland, parts of Northern Ireland, Ireland and Denmark). Beds within the Scottish MPA network were assessed as having good condition in 2014, which could indicate the condition in some parts of Region II and III is good. However, as beds have not been assessed in the last 6 years, this information could not be included in the assessment. Beds in Ireland have not been assessed, although a range of size classes and evident recruitment for some sites suggests that the beds are in good condition (Farinas-Franco per comms). The confidence in the assessments for Regions II and III is poor.
The status of M. modiolus beds has not been assessed in Region I has not been assessed in the last 6 years and no information was provided from Contracting Parties for Region IV. Therefore, no assessment was conducted on changes in condition in these regions.
Method: D
Threats and Impacts
In the background document in 2009 (OSPAR, 2009), the key impacts to M. modiolus beds were identified as ‘Penetration and/or disturbance of the substrate below the surface of the seabed, including abrasion’ and ‘Physical change (to another seabed type)’. The assessment of threats and impacts conducted in this status assessment agreed with those outlined in the background document. However, the following additional impacts were identified:
- Introduction or spread of non-indigenous species
- Nutrient enrichment
- Organic enrichment
- Physical change (to another seabed types)
- Siltation rate changes, including smothering
- Penetration and/or disturbance of the substrate below the surface of the seabed, including abrasion
The biggest causes of the above threats are fishing (particularly from mobile bottom-contact fisheries, although pot fishing has been noted by Contracting Parties) and infrastructure development (e.g. coastal development or offshore renewables). Mariculture, runoff, dredge and spoil disposal, and coastal docks, ports and marinas, and anchoring and moorings also contribute.
Mobile bottom-contact fisheries are the biggest cause of ‘Penetration and/or disturbance of the substrate below the surface of the seabed, including abrasion’. The OSPAR Feeder Report for Fisheries (OSPAR, 2021a) states that demersal fishing effort (mainly beam and otter trawling) has declined since 2003. Mobile bottom-contact fisheries have also been increasingly managed at many M. modiolus beds, which will reduce this pressure in some localised areas. This suggests that this key pressure may be reducing.
Conversely, impacts from infrastructure development hashave been increasing in the North-Atlantic, in part due to the expansion in Offshore Renewable Energy Generation (OSAR, 2021b). Infrastructure development is the biggest cause of ‘Physical change (to another seabed type)’.
M. modiolus beds are highly sensitive to climate change. M. modiolus are a boreal species, making them highly sensitive to temperature rises and marine heatwaves. As calcifiers, the species is also sensitive to ocean acidification (OA; Leung et al., 2022). As M. modiolus are the founding species of M. modiolus beds, the negative impacts of climate change on the species threaten the future existence of M. modiolus beds. At present, it is not possible to confidently discriminate climate change declines against a background of fishing impacts. However, climate change is projected to have an increasing impact on this habitat.
Measures that address key pressures from human activities or conserve the species/habitat
Other recommendations from OSPAR Recommendation 2013/03 on furthering the protection and recovery of M. modiolus beds in the OSPAR maritime area relate to monitoring, reporting and assessment. Since the 2009 background report, 707 point and 26 polygons records have been submitted to OSPAR. Benthic monitoring programmes have been implemented in several countries including Norway (MAREANO benthic mapping programme), the UK, Denmark (BIOREEF programme) and Sweden (in the waters outside of Helsingborg in Oresund). Whilst these programmes do not explicitly focus on M. modiolus, monitoring in areas that contain suitable habitat will mean that M. modiolus will likely be recorded. Some work is underway in Ireland to better understand the population structure of M. modiolus beds in the area, with plans for a larger mapping presence, extent and condition (funding dependent).
| Each Contracting Party should consider: | DE | DK | ES | FR | IE | IS | NL | NO | SE - H | SE - VG | UK - S | UK - E* | UK - W | UK - NI |
| a. the possibility to introduce legislation | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ||||||||
| b. assess sufficiency of existing management measures | ✗ | ✓ | ✓ | ✓ | ✓ | ✓ | ||||||||
| c. investigating the distribution, quality and extent of M. modiolus beds | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ||||||||
| d. gather additional knowledge from other sources | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ||||||||
| e. reporting any existing and new data to the OSPAR Commission | ✗ | ✓ | ✓ | ✓ | ✓ | ✓ | ||||||||
| f. consider MPAs for the the conservation and recovery of M. modiolus beds | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ||||||||
| g. addressing and minimising adverse impacts on M. modiolus beds | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ||||||||
| h. improving access to bottom trawl fishing distribution, frequency and intensity data | ✓ | NR | ✓ | ✓ | ✓ | ✓ | ||||||||
| i. raising awareness of the importance of M. modiolus beds | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ||||||||
| j. acting for the fulfilment of the purpose of this Recommendation within the framework of other component organisation bodies | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ |
Conclusion (including management considerations)
The status of M. modiolus beds with regard to distribution, extent and condition is generally poor throughout the OSPAR maritime area. In Sweden and in some areas around the UK (Region II), there is evidence of clear decreases in distribution and extent. In the Celtic Sea (Region III), distribution appears stable, but extent has declined. Condition is poor in both regions, however, the trend in condition is mixed, with condition improving in certain beds around Northern Ireland and decreasing in Wales and the Kattegat. In other regions (I, IV, and V), the information ofon trends is uncertain with no assessments available.
Key pressures to M. modiolus beds were ‘Penetration and/or disturbance of the substrate below the surface of the seabed, including abrasion’ and ‘Physical change (to another seabed type)’. Fishing and infrastructure developments were the main activities to contribute to these pressures. Additional anthropogenic impacts were also noted during this assessment along with climate change. As M. modiolus are boreal calcifiers, increasing temperatures, marine heatwaves and ocean acidification are projected to have negative consequences on M. modiolus beds in the North-East Atlantic. The impact of cumulative anthropogenic pressures, along with climate change, means that pressures are increasing in the assessed Regions (II and III).
Management considerations for M. modiolus beds include, amongst others: minimising or avoidance of damage, establishment of marine protected areas, controlling inputs of pollutants, increasing awareness of the importance of M. modiolus beds through assessments and reports. Re-assessment of the status of and threats to M. modiolus beds should preferably be done during six yearly cycles.
Knowledge gaps
M. modiolus beds are difficult to assess. There are difficulties in distinguishing records of individual specimens from aggregations which are sufficient to be reliably classified as beds. Furthermore, the fragmented nature of beds means that total extent can be difficult to map. Very few countries have monitored M. modiolus beds periodically over sufficient time scales to detect changes in extent and condition. Furthermore, often surveys focus on a couple of beds and many beds have not been assessed for a decade or more. New records of M. modiolus are emerging in some areas (i.e. Northern Ireland, Ireland (Farinas-Franco per comms) and Norway). Work is needed to survey these areas and determine if the records constitute as M. modiolus beds. If they do, records should be added to OSPAR's habitat dataset, and the extent and condition of the beds assessed.
Uncertainties around how to define M. modiolus beds, and the limited information available about the distribution, extent, and condition of M. modiolus beds in Regions II and III (and no information in Regions I, IV and V) resulted in a low confidence associated with this status assessment. Some records are currently uncertain, further lowering the confidence in the geographical distribution shown in Figure 1. Work is needed to better clarify some of these records and remove them from the OSPAR habitat dataset if there is evidence that they do not represent beds.
Knowledge gaps also exist for some threats. For example, there is limited understanding about the exposure of M. modiolus beds to pressures from static fishing gear. Furthermore, scientific research suggests that M. modiolus are sensitive to climate change. However, there is a paucity of species-specific research, particularly regarding ocean acidification.
We recommend that a working group be created, made up of Contracting Parties that have records of M. modiolus beds. The working group could facilitate discussions regarding best practice for surveying M. modiolus beds and discuss shared trends across within and between OSPAR regions. The group could also agree on a Standard Operating Procedure for monitoring of M. modiolus beds across Contracting Parties, enabling improved comparison between beds across the OSPAR maritime area. A more detailed assessment should be carried out alongside this T&D status assessment to record information specific to certain countries and beds.
Main source of information:
- Assessment derived from a mix of OSPAR data assessment and assessments from third parties.
Assessment is based upon:
- mainly on expert opinion with very limited data.
Greathead, C., Boschen-Rose, R.E., Langton, R., Clarke, J., Wright, P.J., and Boulcott, P. 2023. Priority marine feature surveys within the Small Isles MPA and surrounding waters. Main Report. Scottish marine and Freshwater Science, 14(1): 52p
JNCC. 2022. The Marine Habitat Classification for Britain and Ireland Version 22.04. [26/09/2024]. Available from: https://mhc.jncc.gov.uk/
Leung, J., Zhang, S., and Connell, S. 2022. Is ocean acidification really a threat to marine calcifiers? A systematic review and meta-analysis of 980+ studies spanning two decades. Small, 18: 2107407.
OSPAR 2021a. Feeder Report 2021 – Fisheries. Version 1.0.0. Assessed [31/08/2024] at: https://oap.ospar.org/en/ospar-assessments/quality-status-reports/qsr-2023/other-assessments/fisheries/
OSPAR 2021b. Feeder Report 2021 - Offshore Renewable Energy Generation. Version 1.0.0. Assessed [31/08/2024] at: https://oap.ospar.org/en/ospar-assessments/quality-status-reports/qsr-2023/other-assessments/renewable-energy/
Poppe, G.E., and Gotto, Y. 1993. European Seashells Vol II. Scaphopods, Bivalves, Cephalopods. Conch Books, Haekenheim. 221p.
Strong, J.A., Service, M., & Moore, H. 2016. Estimating the historical distribution, abundance and ecological contribution of Modiolus modiolus in Strangford Lough, Northern Ireland. Biology and Environment: Proceedings of the Royal Irish Academy 116B(1), 1-16. https://doi.org/10.1353/bae.2016.0024
Sheet reference:
BDC2025/Modiolus_modiolus