North Bull Island, a key feature of the Dublin Bay Biosphere Reserve, is an actively accreting dune system, providing habitats for a rich diversity of protected species and a popular natural space for recreation. The northern end of North Bull Island is a well-known haul out site for grey and harbour seals, Ireland’s two breeding pinniped species, both protected under the EU Habitats Directive.
Sites like these are vital for seals to haul out of the water, especially during breeding and moulting seasons, and to rest and regulate their body temperature. IWDG Consulting have carried out monthly seal surveys of North Bull Island since May 2016, as part of the Marine Mammal Mitigation Plan for the Alexandra Basin Redevelopment and ongoing MasterPlan2 projects for Dublin Port Company. During these surveys, grey and harbour seals were present on 58% and 96% of the time, respectively. Typically, groups of up to 25-30 grey seals are present from May to October, while 10-20 harbour seals haul out year-round. Although not an established breeding site, the area has an important role for seals resting between foraging trips and moving between breeding and moulting sites in Dublin Bay. Over the past eight years, the numbers of grey and harbour seals hauled out on North Bull Island have significantly increased, highlighting its increased importance as a haul out site. Following a wider seal survey from July 2023 to January 2024 funded by Dublin Port it showed that North Bull Island is the second most important haul out site for harbour seals in Dublin Bay and adjacent waters.
Having access to haul out sites with minimal human disturbance is important for supporting healthy seal populations, especially when faced with other anthropogenic pressures, such as elevated noise levels from construction activities in the nearby Dublin Port. In 80% of the surveys, there was at least one incident of disturbance, predominantly from people and dogs. In light of these pressures, the increase in seal numbers is particularly noteworthy. Since 2022, a zoning plan has been in place, which designates the northern end of the island as a “Wildlife Only Area”. If complied with, this could help reduce disturbance to the increasing numbers of hauled out seals and maintain North Bull Island as an important haul out site for the grey and harbour seals of Dublin Bay.
Jo Green
Marine Mammal Ecologist
