4f4c4942-4343-5764-6473-313130393236 English NRW_DS114501 Natural Resources Wales (NRW) Maes-y-Ffynnon, Penrhosgarnedd, Bangor, Gwynedd, LL57 2DW, Wales enquiries@naturalresourceswales.gov.uk http://naturalresources.wales/?lang=en 2026-03-03T16:09:55.643Z NRW 1.0 wel Latitude and Longitude (EPSG::4326) Monitoring of Carbonate Reef Extent and Fish Population at Holden's Reef (2003 - 2010) Monitoring of Carbonate Reef Extent and Fish Population at Holden's Reef (2003 - 2010) Monitro poblogaeth y pysgod a maint y riff carbonad yn Riff Holden 2003 CCW Holden's Reef initial survey MRCCW30000000030 2005 CCW/Pelagial/Sea-Scope Holden's Reef, Pen Llyn a'r Sarnau sublittoral survey MRCCW3010000000D Carbonate reef extent and rugosity, Holden's Reef Fish abundance and diversity at Holden's Reef 2010-12-31 NRW_DS110926 Holden's Reef is a small (approximately 40 m diameter), roughly circular reef situated in the northern sector of Cardigan Bay. It is unusual in being formed as a result of a chemical reaction between seawater and the escape of natural gas from the seabed, a phenomenon generally found in much deeper waters. Holden's Reef is one of an assemblage of methanogen-precipitated calcium carbonate reefs and was the first of the group to be discovered. This dataset contains the results of a programme of monitoring established to study fish populations and reef extent. It also includes data from an in-house survey carried out at Holden's Reef in 2003, initiated following the reef's discovery, and Marine Nature Conservation Review (MNCR) phase 2 surveys carried out in 2005 of \Little\" and \"Big\" Reef. Holden's Reef is monitored for fish abundance and diversity extent and rugosity (the link between rugosity and fish abundance has been seen in several studies). This will contribute to condition assessment of the reef feature in the Pen Llyn a'r Sarnau Special Area of Conservation (SAC). Holden's Reef is situated more than 10 km off shore north west of Barmouth and is approximately 40 m in diameter. " Holden's Reef is a small (approximately 40 m diameter), roughly circular reef situated in the northern sector of Cardigan Bay. It is unusual in being formed as a result of a chemical reaction between seawater and the escape of natural gas from the seabed, a phenomenon generally found in much deeper waters. Holden's Reef is one of an assemblage of methanogen-precipitated calcium carbonate reefs and was the first of the group to be discovered. This dataset contains the results of a programme of monitoring established to study fish populations and reef extent. It also includes data from an in-house survey carried out at Holden's Reef in 2003, initiated following the reef's discovery, and Marine Nature Conservation Review (MNCR) phase 2 surveys carried out in 2005 of \Little\" and \"Big\" Reef. Holden's Reef is monitored for fish abundance and diversity extent and rugosity (the link between rugosity and fish abundance has been seen in several studies). This will contribute to condition assessment of the reef feature in the Pen Llyn a'r Sarnau Special Area of Conservation (SAC). Holden's Reef is situated more than 10 km off shore north west of Barmouth and is approximately 40 m in diameter. " Natural Resources Wales (NRW) Maes-y-Ffynnon, Penrhosgarnedd, Bangor, Gwynedd, LL57 2DW, Wales enquiries@naturalresourceswales.gov.uk http://naturalresources.wales/?lang=en Natural Resources Wales (NRW) Maes-y-Ffynnon, Penrhosgarnedd, Bangor, Gwynedd, LL57 2DW, Wales enquiries@naturalresourceswales.gov.uk http://naturalresources.wales/?lang=en Various contractors See Natural Resources Wales, Maes y Ffynnon, Penrhosgarnedd, Bangor, Gwynedd, LL57 2DW, Wales enquiries@naturalresourceswales.gov.uk http://naturalresourceswales.gov.uk/splash?orig=/ CCW Marine Monitoring Team Countryside Council for Wales, Maes y Ffynnon, Penrhosgarnedd, Bangor, Gwynedd, LL57 2DW enquiries@naturalresourceswales.gov.uk http://naturalresourceswales.gov.uk/splash?orig=/ Natural Resources Wales (NRW) Maes-y-Ffynnon, Penrhosgarnedd, Bangor, Gwynedd, LL57 2DW, Wales enquiries@naturalresourceswales.gov.uk http://naturalresources.wales/?lang=en Habitats and biotopes Species distribution GEMET - INSPIRE themes, version 1.0 2008-06-01 geonetwork.thesaurus.external.theme.httpinspireeceuropaeutheme-theme carbonate reef methane Holden's Reef Special Area of Conservation Monitoring fish ecology marine monitoring (see also marine surveys) Pen Llyn a'r Sarnau cSAC NRW Thesaurus 2025-01-08 geonetwork.thesaurus.local.theme.converted_nrw-keywords Fish abundance in water bodies Habitat characterisation Habitat extent Fish taxonomy-related counts SeaDataNet Parameter Discovery Vocabulary 2024-06-19 geonetwork.thesaurus.external.theme.rdf+xml There are no access restrictions to this data. NRW may release, publish or disseminate it freely. no limitations © CNC/NRW 2010. There are no use restrictions on this data. Recipients may re-use, reproduce, disseminate this data free of charge in any format or medium, provided they do so accurately, acknowledging both the source and NRW's copyright, and do not use it in a misleading context. It is the recipient's responsibility to ensure the data is fit for the intended purpose, that dissemination or publishing does not result in duplication, and that it is fairly interpreted. Advice on interpretation should be sought where required. To avoid re-using old data, users should periodically obtain the latest version from the original source. English oceans biota Irish Sea Cardigan Bay Tremadog Bay GWYNEDD - GWYNEDD -4.140899 -4.129506 52.735966 52.741304 SeaVoX Vertical Co-ordinate Coverages 2013-02-05 creation water column 2004-05-18 2010-12-31 -6.2 -9.5 Detailed methodologies in use at Holden's Reef can be found in Marine Monitoring Report 25b (standard operating procedures 11 12 and 13) Database [A] Reports in word and pdf format. [B] Species and Abundance data available as excel spreadsheets and formatted for PRIMER. [C] Extent of reef available as MapInfo TAB files. [D] Transect length used to measure extent and rugosity values available as excel spreadsheet. [E] Incidental video available as mini dv tapes, incidental images available as JPGs. Survey data from initial survey in 2003 and phase 2 surveys in 2005 are on Marine Recorder (MRCCW30000000030, MRCCW3010000000D) Commission Regulation (EU) No 1089/2010 of 23 November 2010 implementing Directive 2007/2/EC of the European Parliament and of the Council as regards interoperability of spatial data sets and services 2010-12-08 This dataset has not been assessed for conformance to the referenced INSPIRE regulation. false A site marker (a single road pin) has been established at the centre of the reef in its shallowest part. An additional marker is located due north from the centre at the edge of the reef. The extent of the reef is measured by divers who swim along a bearing from the central marker to the edge of the reef and the distance recorded. The distance from the centre to the edge of the reef is measured at 12 bearings 30 degrees apart. The rugosity of the reef is measured by splitting the reef into an inner and outer zone radiating from the central marker. The distance between the centre and zone boundary is measured using a taut tape measure. This distance is then repeated allowing the tape measure to rest along the contours if the reef. The rugosity is determined to be the ratio between the two measurements. Fish abundance and diversity is measured by a pair of divers undertaking 3 consecutive timed counts of 5 minute duration. These counts take place in up to 4 zones that represent different topography and depths on the reef. To aid recording slates are pre-marked with species likely to be observed. A site marker (a single road pin) has been established at the centre of the reef in its shallowest part. An additional marker is located due north from the centre at the edge of the reef. The extent of the reef is measured by divers who swim along a bearing from the central marker to the edge of the reef and the distance recorded. The distance from the centre to the edge of the reef is measured at 12 bearings 30 degrees apart. The rugosity of the reef is measured by splitting the reef into an inner and outer zone radiating from the central marker. The distance between the centre and zone boundary is measured using a taut tape measure. This distance is then repeated allowing the tape measure to rest along the contours if the reef. The rugosity is determined to be the ratio between the two measurements. Fish abundance and diversity is measured by a pair of divers undertaking 3 consecutive timed counts of 5 minute duration. These counts take place in up to 4 zones that represent different topography and depths on the reef. To aid recording slates are pre-marked with species likely to be observed.