Abstract: The Northern Gannet Morus bossanus is an avian sentinel; the largest breeding seabird in Ireland and an obligate piscivore. Gannet eggs were collected from two island colonies off the east coast of Ireland, approximately 150km from each other, in locations with divergent history of industrialization (n = 10-20). Levels of potentially harmful contaminants including Polychlorinated biphenyls (PCBs), Polybrominated diphenyl ethers (PBDEs), Organochlorine pesticides (OCs), heavy metals and mercury were measured and differences of contaminant concentrations between different colonies compared. This is the first such study of contaminant levels in Gannet, or in any seabird egg in Ireland. Stable isotopes of carbon (d13C) and nitrogen (d15N) were measured in each egg to understand the influence of diet in contaminant levels detected. Significantly higher levels of PCBs, PBDEs and mercury were detected near Dublin (Ireland's industrialized capital city and location of its largest port) compared to Wexford. No differences were observed in levels of OCs and heavy metals between the two colonies. Stable isotope analysis demonstrated that Gannets in both locations occupy the same dietary niche excluding a difference in diet as the driver of differing contaminant levels in the two feeding areas. Though Gannets travel significant distances when foraging for food (~200km) tracking studies have shown that Gannets colonies maintain exclusive feeding areas with little overlap between neighbouring colonies. Differences between colonies within the feeding range of Gannets can therefore be detected despite Gannet's high dispersal ability. These results are in concurrence with elevated levels of contaminants in lower trophic level organisms that have been found in Dublin Bay compared to the rest of Ireland, indicating potential for Gannets as a higher trophic level indicator - though variability in their diet, including feeding on fishing discard, may lead to unacceptable levels of variability for an indicator species. Authors: Andrew Power��, Philip White��, Brendan McHugh��, Sinead Murphy��, Simon Berrow��, Moira Schlingermann��, Stephen Newton��, Linda O'Hea��, Brian Boyle��, Marissa Tannian��, Denis Crowley��, Evin McGovern��, Ian O'Connor�� ��Galway Mayo Institute of Technology, ��Marine Institute, ��BirdWatch Ireland