Atlas of the Marine Mammals of Wales dataset https://portal.medin.org.uk/portal/start.php?tpc=007_NRW_DS111813 Welsh water support nationally and internationally important populations of marine mammals (whales, dolphins and seals) which use the area at various stages of their life histories such as feeding and breeding. Since the early 1990s the Countryside Council for Wales (CCW) has been a major supporter of studies that have recorded the distribution and abundance of marine mammals in Wales. These studies, often undertaken using dedicated networks of volunteers, have helped to inform conservation objectives for Special Areas of Conservation (SAC), monitoring targets and the assessment of proposed developments and impacts. This dataset brings together the best available effort-related survey data on marine mammals in Welsh waters and adjacent parts of the Irish Sea, to provide the highest resolution analysis carried out to date of the spatial and temporal variation in their distribution. The Atlas contains distribution maps, including variations both seasonally and over the long terms of the five most common species (harbour porpoise, bottlenose dolphin, short-beaked common dolphin, Risso's dolphin and minke whale). The purpose of this data capture is to describe the temporal distribution, spatial distribution and relative abundance of all marine mammal species sighted, detected or stranded in Welsh waters. This dataset is concentrated on species regularly sighted and is designed to help meet obligations under the EU Habitats Directive. In addition is help former Countryside Council for Wales (CCW) now Natural Resources Wales (NRW) advise on marine activities including renewable energy exploitation, fisheries, waterborne recreation and oil and gas exploitation. The study area consists primarily of Welsh territorial seas out to the median line with Ireland and the Isle of Man, and English territorial seas. It was assumed that data contributed were reliable with respect to species identification and had been checked for errors in date, time and position fields; however factors affecting sight ability, survey characteristics and ecological factors may have affected sighting rates. Survey effort remains inadequate in all but a few small areas. 20120630