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Croker Carbonate Slabs MPA

Status: Special Area of Conservation (SAC)

The Croker Carbonate Slabs MPA is an area in the mid-Irish Sea, approximately 30 km west of Anglesey. The site lies in 70 m water depth in the north, descending down to approximately 100 m at the south-west corner of the site.




Croker Carbonate Slabs lies in 70 m water depth in the north descending down to approximately 100 m at the south-west corner of the site.  The seabed surface is composed of extensive areas of exposed methane-derived authigenic carbonate (MDAC). These carbonate blocks and pavement slabs form when methane is released from the seabed and reacts with water and are known as 'submarine structures made by leaking gases' – a listed habitat under Annex I of the EC Habitats Directive. The seabed habitats created by these MDAC structures are distinctive, supporting a diverse range of marine species that are absent from the surrounding seabed, which is characterised by coarse sediment. Areas of 'high relief' MDAC support a diverse range of soft corals, erect filter feeders, sponges, tube worms and anemones whilst the 'low relief' MDAC is colonised with scour-resistant hydroids and bryozoans.

The Croker Carbonate Slabs MPA overlaps with a candidate Special Area of Conservation/Site of Community Importance that has been identified for the protection of Harbour porpoise – the North Anglesey Marine SAC. For more information on this MPA, please see the North Anglesey Marine MPA Site Information Centre.

More detailed site information can be found in the Summary section below.

Map displaying the Croker Carbonate Slabs MPA boundary and associated protected feature data. Visit JNCC's MPA Mapper to further view and explore data for this MPA.

Map showing Croker Carbonate Slabs Marine Protected Area and linking to the MPA mapper


Legislation behind the designation: EU Habitats Directive 1992 transposed into UK law by The Conservation of Offshore Marine Habitats and Species Regulations 2017 (as amended).


Protected Features

Feature Feature Type
1180 Submarine structures made by leaking gases Annex I Habitat

Specific information on the conservation objectives relating to this site is provided in the Conservation Advice section.

The acquisition of new data may result in updates to our knowledge on feature presence and extent within this site. The most up-to-date information is reflected on the map on this page and in JNCC’s MPA Mapper, and the evidence underpinning this can be viewed in the Monitoring and Evidence section below.


Site Timeline

The diagram below is a summary of the key milestones involved in the selection and designation of Croker Carbonate Slabs. More detail can be found within the Relevant Documentation section.

March 2011
Site formally recommended to the UK Government as a draft Special Area of Conservation (SAC).
June 2011
Formal public consultation. Site becomes a possible SAC.
August 2012
Site submitted to the European Commission. The Habitats Regulations now formally apply to this MPA.
November 2013
Site approved by the European Commission as a Site of Community Importance (SCI).
November 2017
Site boundary amendment submitted to the European Commission.
September 2019
Site is formally designated as a Special Area of Conservation (SAC) by UK Government.


Relevant Documentation

The documents referred to below and any other historical documents relating to Croker Carbonate Slabs were produced during the selection and designation process and therefore may be out of date. This Site Information Centre is the most up-to-date source of information for this MPA, and will reflect any additional information gathered since these documents were produced. Information about the SAC site selection process is available on JNCC's SAC webpages.


Amended boundary 2017:

These documents are available on JNCC's Resource Hub


Original boundary: An amendment to the site boundary for Croker Carbonate Slabs was consulted on in 2017 and approved and submitted to the European Commission in November 2017. More information can be found in the consultation archive. The original site documents from the original site designated in 2008 are available to download from The National Archives website. However, these have since been superseded by the above documents.



Last updated: August 2019

Information for this site summary was adapted from the SAC Selection Assessment Document and incorporates any further information gathered since these documents were produced. Please refer to this document in the Relevant Documentation section for further details and information sources.


Site overview

The Croker Carbonate Slabs is an area in the mid-Irish Sea, approximately 30 km west of Anglesey, where a total area of over 55 km2  of the Annex I feature "submarine structures made by leaking gases" have been identified. The seabed surface is composed of extensive areas of exposed methane-derived authigenic carbonate (MDAC). The seabed habitats created by these MDAC structures are distinctive, supporting a diverse range of marine species that are absent from the surrounding seabed characterised by coarse sediment.

MDAC is formed when calcite precipitates and infills the pore spaces between the sand grains, creating a layer or crust that can form carbonate 'pavements' and 'chimneys'; significant hard ground compared to the surrounding sediment. When exposed at the seabed surface, MDAC appears to be broken down and eroded rapidly both through biological activity (boring by bivalve molluscs) and by water currents into sand and gravel sized fragments.

Earlier surveys in 2005 confirmed the existence of MDAC in the site. Acoustic data indicated a generally flat seabed with large depressions up to 500 m in diameter, with steep sides, alongside small mounds and sediment waves. In addition, a cliff structure 6–8 m high and up to 500 m long was recorded. Survey of this area found cemented rocks providing a firm substrate for a diverse range of fauna. Chemical analysis of carbonate samples collected during this survey indicated they were methane-derived and thermogenic in origin.

Additional survey work undertaken in 2008 further established the presence of MDAC over a wider area. The feature was mapped using high resolution acoustics (multibeam echo-sounder and sidescan sonar) and validated using seabed imagery and grab samples. Within the site, the MDAC structures took two key forms, extensive MDAC 'pavement' or 'slabs' up to 20 mm thick (termed 'low relief' MDAC) and larger structures over 20 mm thick and up to 2 m high (termed 'high relief' MDAC). The exposed MDAC was observed forming two longitudinal features with a SSW–NNE orientation.

Further survey work in 2012 and 2013 provided evidence to support an amendment to the site boundary. Full coverage acoustic data, combined with video imagery suggested the extent of Annex I Submarine structures made my leaking gases is more extensive than previously thought. In 2015, a monitoring survey of the SAC was undertaken to contribute to the development of a monitoring time-series for the site.

The hard substratum provided by the MDAC provides an ideal physical habitat for a range of marine life, in stark contrast to the surrounding coarse sediment. Information on the biological communities was gained through analysis of the seabed imagery; over 79 species were identified. Further detail on the evidence for this SAC can be found in the Monitoring and Evidence section.

Site location:  Co-ordinates for SAC can be found in the Standard Data Form listed in the Relevant Documentation.

Site area:  116 km

Site depth range: 65 m below sea-level on top of the slabs feature, down to 109 m below sea-level at their base.

Charting Progress 2 biogeographic region: Irish Sea. 


Site boundary description: 

The original boundary was delineated following the SAC boundary guidelines resulting in a simple polygon enclosing the minimum area necessary to ensure protection of the Annex I habitat, following the extent of the habitat feature as closely as possible. It included a margin to allow for mobile gear on the seabed being at some distance from the location of a vessel at the sea surface.

The amended boundary, which came into place in 2017, extends the site boundary to the north-east, north-west and south-west of the existing boundary, following a contiguous area of Annex I habitat known as MDAC (approximately 55 km2 (5,500 ha)) identified through surveys undertaken in 2012 and 2013. The revision to the boundary encompasses a large area of MDAC to the east of the previously mapped extent, together with smaller patches of MDAC to the south-west. The western boundary of the site follows the median line between the offshore waters of the UK and the Republic of Ireland. Following JNCC’s UK SAC boundary guidance, the boundary includes a buffer zone of ~240 m around the known extent of MDAC, based on a fishing warp ratio of 3:1 using the average depth of the seabed (80 m) (JNCC 2008). This buffer was manually adjusted to give a 300 m buffer in the southern part of the site where the feature occurs in deeper waters (~100 m on average).


Monitoring and Evidence

Last updated: November 2023

There are a range of data that underpin this SAC. The full overview of the data used to support site identification, along with information on confidence in feature presence and extent is available in the Croker Carbonate Slabs SAC Selection Assessment Document. JNCC will be adding relevant survey data for this MPA to its MPA mapper in due course.

Some of the data for this SAC have been collected through JNCC-funded or collaborative surveys and some through other means. Data from these surveys provides direct evidence confirming the presence of the protected features within the site.


Survey and data gathering

  • Croker Carbonate Slabs Monitoring Survey (2015) – A dedicated monitoring survey of Croker Carbonate Slabs SAC was conducted in October/November 2015. The principal aim of the survey was to collect information. Reporting is underway and will be made available in due course.
  • North St George’s Channel rMCZ Verification Survey (2012/2013) – JNCC commissioned a survey to North St George’s rMCZ which spatially overlaps with Croker Carbonate Slabs SAC. The survey collated a range of data including grab samples, images and multibeam. 
  • The distribution and extent of methane-derived authigenic carbonates (2005) – (A.G. Judd) DTI Strategic Environmental Assessment, Area 6 (SEA6). Department of Trade and Industry, UK
  • Shallow Gas Accumulation and Migration in the Western Irish Sea (1995) – IN: Croker, P.F. and Shannon, P.M. (ed.), The Petroleum Geology of Ireland's Offshore Basins, Geological Society of London, London, Special Publication 93, 41–58. Jorgensen, 1992.


Data analysis reports

Further analysis of data gathered as part of the surveys listed above are available via the following reports:


Additional relevant literature

References for further supporting scientific literature consulted during the identification of this site can be found in the Selection Assessment Document.


Knowledge gaps

As part of the UK Marine Monitoring and Assessment Strategy (UKMMAS), JNCC led the development of a UK Marine Biodiversity Monitoring Strategy, working with partners across the UK monitoring community. The Strategy spans UK territorial and offshore waters, focusing on biodiversity in the wider environment and within Marine Protected Areas.  Its aim is to implement efficient, integrated monitoring of marine biodiversity to provide the evidence needed for all the UK's policy drivers.

The evidence collected during MPA monitoring surveys is used in combination with other available evidence to:

  • Enable assessment of condition of the features within sites;
  • Contribute to the assessment of the degree to which management measures are effective in achieving the conservation objectives for the protected features;
  • Support the identification of priorities for future protection and/or management; and,
  • Enable Government to fulfil its national and international assessment and reporting commitments in relation to MPAs and help identify where further action may be required.

More detail on offshore MPA monitoring can be found on the Offshore MPA monitoring webpage. A list of monitoring surveys and relevant reports can be found  on the MPA monitoring survey reports webpage.

If you are aware of any additional information not listed in the Relevant Documentation section, or above, please contact us.


Conservation Advice

Last updated: February 2018 

Updated formal conservation advice for this MPA was produced in February 2018. Further information on the approach used to develop this advice is available on the Conservation Advice webpages along with a Glossary of Terms used in JNCC conservation advice and a short video explaining how to use the conservation advice packages. 

You must refer to this advice if you:

  • undertake a Habitats Regulation Assessment (HRA) for a plan or project that could impact the site;
  • provide information for a HRA;
  • respond to specific measures to support delivery of the conservation objectives for the site; and 
  • consider the need to put new or additional management measures in place.

You may also find it useful to refer to this advice if you:

  • carry out any other activity that could impact the site.

We will engage with stakeholders to identify any lessons which JNCC can learn from customers who have used the advice, with a view to continuing to ensure it is fit-for-purpose.

The following table provides an overview of the components of the conservation advice, and provides hyperlinks to each of the products for this MPA. These elements together form JNCC’s formal conservation advice for this site and should be read in conjunction with each other. This updated advice replaces the previous Regulation 18 package for the site. This advice reflects the most up-to-date evidence held by JNCC (correct as of February 2018, before the site was designated a SAC).  

Document Overview
Background Information Explains the purpose of the advice and when it must be referred to.

Conservation Objectives

Supplementary Advice on the Conservation Objectives (SACO)

The Conservation Objectives set out the broad ecological aims for the site. JNCC provides supplementary advice in the SACO which is essential reading to support interpretation of these conservation objectives.

You can use these documents to assess the impacts of your planned activity on the important attributes of the site.

Please note our current understanding of whether the available evidence indicates that each attribute needs to be restored or maintained is not provided here. However, links to available evidence for the site are provided and should you require further site-specific information for the site, please contact us.

Conservation Advice Statements

These statements provide a summary of the Supplementary Advice on the Conservation Objectives (SACO).

  • Site condition presents our up-to-date understanding of the condition of features within the site;
  • Conservation benefits which the site can provide, these help you understand what is important about the site and why it needs protecting; and
  • Conservation measures which JNCC considers are needed to support achievement of the conservation objectives. These provide clarity around measures needed to support restoration or maintenance of the feature(s) within the site.
Advice on Operations

Provides information on the activities capable of affecting site integrity and therefore achievement of the site’s conservation objectives.

This is a starting point for determining potential management requirements. It does not take into account the intensity, frequency or cumulative impacts from activities taking place. It is simply to advise you of the possible adverse impacts that your activity can have on a MPA’s features.

Use the advice on operations to determine those pressures your activity causes that could harm the habitat and/or species features of the site.

These documents are available on JNCC's Resource Hub



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